Mul-IR 56, Mul-IR 58 and Mul-IR 62: True Planetary Nebulae?

Over the course of these past three years, efforts have been made to better classify the Mid-IR sources selected by Acker et al. (2016) (e.g. optical spectra). These objects were selected based on their Planetary Nebula (PN)-like colours in IRAS and WISE data, and are meant to represent a preliminary method to detect compact PNe. However, as mentioned in their work, further photometric and spectroscopic studies are needed to better assess the nature of these objects. This is because many “mimics” display similar colours in IRAS and WISE, most notably AGNs, HII regions, Post-AGB stars and YSOs (Acker et al., 2016). The article was also published in the popular French astronomy magazine Astronomie. (February 2016 edition). Based on photometric data and imagery from various other [modern] surveys (e.g. DECaPS, SHS and Herschel), this article shows that the PN-IR objects Mul-IR 56, Mul-IR 58 and Mul-IR 62 may have good chances of being true Planetary Nebulae. I also feature several other PN-IR objects at the end of this work that I am currently in the process of studying further.

cover_IR_MulatoFigure 1: Cover image of the article published in the February 2016 edition of the Astronomie magazine (written by Lionel Mulato). Image credit: Astronomie/SAF

1. IRAS and WISE Properties of PNe

At wavelengths inferior to 50 microns, compact PNe, YSOs (Class 0 and I), HII regions and Proto-PNe  may appear quite similar (see Acker et al., 2016). However, at higher wavelengths, these often tend to differ (especially YSOs and HII regions). Indeed, these often display Far-IR excess (caused by their dusty envelope/environment) while PNe generally do not. Indeed, PNe tend to have peak emissions in the Mid-IR, around 20-50 microns (e.g. figure 2a) while YSOs and HII regions peak in the Far-IR (e.g. figure 2b). Spectral Energy Distribution (SED) plots can thus be good ways of classifying objects, as can be seen on the Zooniverse/Disk Detective website. As a result, one can simplify the morphology these SEDs into a set photometric criteria (e.g. flux ratio) in order to classify such objects. Such work has notably been done using the various IRAS IR filters (see figure 2).

IRAS_PN_ColoursFigure 2: Plot comparing IRAS photometric properties (flux density ratios) of numerous confirmed PNe, and PN-IR objects from (Acker et al., 2016). Despite the dispersion, note how most PNe are defined within a single “photometric zone”. However, a very large amount do not meet these criteria.  Image credit: Acker et al. (2016)/Astronomie Feb 2016.

Indeed, Pottasch et al. (1988) and Acker et al. (2016) based their selection on the following predefined Flux Density ratio values: 12/25 > 0.5 and 25/60 < 0.35. Thus, these criteria describe objects with a strong slope between 12 and 25 microns, and relatively weak or descreasing slopes between 25 and 60 microns, hence reflecting the photometric properties of most PNe as shown by their SEDs (e.g. figure 3).  The small IRAS 12/25 Flux ratio displayed by most PNe can be translated by their low WISE W3/W4 (12/22 micron) ratio. However, based on the SED morphology of PNe (e.g. figure 3), one could also use the WISE W1/W4 ratio as a substitute, as this is roughly/graphically proportional to the WISE W3/W4. Consequently, in AllWISE W1+W2+W4 images (Colour-coded Blue-Green-Red), most PNe appear as large “red spots” reflecting their Mid-IR excess, in comparison to their relatively weak Near-IR signal (see figure 4).

HOPS288_SEDFigure 3: SED of MR 22 (a bright PN) as derived from on various astronomical catalogues calculated by CDS/SIMBAD. Notice how the continuum peaks around 70 microns, with a steep slope from the Near-IR to the Mid-IR. Image credit: CDS/SIMBAD.

PN_IR_WISE_ComparisonFigure 4: AllWISE Image extracts comparing various PN-IR objects. All the objects presented in this image are of the List-I, with the exception of Mul-LDû-IR 16. Mul-IR 9 and Mul-IR 14 are confirmed PNe. Image credit: Aladin Lite/AllWISE.

Note: The coordinates for all the PN-IR objects can be found here, as well as in the article by Lionel Mulato.

2. The Herschel-WISE Diagram

In regards to the work by Acker et al. (2016), all PN-IR objects (List I and II) were initially selected based on their relatively strong response in the W4 filter, with those of the List I all ressembling the objects shown in figure 4 (with the exception of Mul-LDû-IR 16, a list II object). Mul-IR 56, Mul-IR 58 and Mul-IR 62 are all members of the List I. Some object of the List II however have a significant response in the W1 and W2 bands, relative to W3 and W4, such as Mul-LDû-IR 16. The team then sorted/retained the WISE-selected objects according to their IRAS properties. However, in comparison to many modern IR surveys (e.g. Herschel and WISE), IRAS data is of relatively low resolution. Consequently, I initially decided to recreate the IRAS diagram (Figure 7) according to photometric bands from more modern surveys. Firstly, I exchanged the IRAS 12/25 ratio by the WISE W1/W4 (see explanation above), and I chose the PACS-Blue/W4 ratio as a [mathematically inverse] substitution for the IRAS 25/60. The PACS-Blue filter corresponds to the 70 micron band used by the Herschel telescope. These values were provided by the Herschel Point Source Catalogue.

HPPSC_All-skyFigure 5: A projection of the Herschel FOV. Notice how this is focused at the very center of the galactic plane and other areas rich in star formation. Image credit: ESA/Herschel.

In this search I focused on finding known objects in AllWISE images that dispayed the same colours as those in figure 4, located within the Herschel FOV. I also selected YSOs with similar properties from the HOPS catalogue and Class 0 YSOs from Savadoy et al. (2014). As a result, my preliminary results contain 44 PNe (including PN candidates), 41 YSOs, 12 HII compact regions and 5 Proto-PNe (PPNe). Since the Herchel FOV mostly focused on obscured starforming regions, AGNs were excluded as these were relatively unlikely to be present. PPNe were difficult to find in the Herschel FOV, hence the small number of subjects presented here. In the case of HII regions however, it is possible that future work could include numerous more of these. Furthermore, note that corrections due to magnitude/flux extinction have not been made. While interstellar reddening is unlikely to significantly affect the PACS70 and the W4 band flux values, the W1 band fluxes may be significantly underestimated in the case of highly obscured sources. Moreover, it is possible that the Herschel flux values do not correspond to flux density, as I initially thought. WISE Flux density values were calculated according to the following protocol. In order to further verify the accuracy of the WISE calculations, I made comparisons with other works (e.g. figure 6 and 9).

SEDs_known_YSOsFigure 6: Test results in the case of well known edge-on SEDs, MY Lup. Note how both studies yield similar results. Note the difference in data in at optical wavelengths is most likely due to the variable nature of these objects.  Data: 2MASS, Spitzer, GSC2.3, IRAS, AKARI, UCAC4, WISE.

Despite the statistically small number of samples, it seems possible to distinguish certain “areas” that correspond more to PNe rather than YSOs and HII regions (despite many PNe displaying properties that overlap with the latter two classes). Too few PPNe are available to draw any definite conclusion in regards to any “photometric zones”, especially as the dispersion is large, and three or four of the samples display properties that fall within the “PN region”. Interestingly, the HII regions selected here display relatively similar properties (poor dispersion), but more subjects are required to define a definite “zone”, if possible. As expected, YSOs of PN-like WISE colours have an average PACS70/W4 slope that is higher than for PNe (at least the ones chosen here). Visually, I find that PNe tend to roughly have PACS70/W4 values inferior to 0.5. The HII regions calculated here interestingly all appear to have a PACS/W4 < 0.5, which is coherent with the IRAS 25/60 ratio for these objects (see figure 2), considering the PACS-Blue values are measured in flux density. In addition, despite not being present in figure 7, the object Mul-IR 9 (now a confirmed PN) is also located in the “PN region”.YSO_PN_HII_PPN_MUL_IR_56_58Figure 7: Plots displaying the Herschel-WISE photometric properties of Mul-IR 56 and Mul-IR 58 compared to 44 Planetary nebula (PN) candidates, 41 confirmed Young Stellar Objects (YSOs), 12 HII regions and 5 PPNe of the Herschel Orion Protostar (HOPS) catalogue, and likely Class 0 YSOs from Sadavoy et al. (2014). Most HII regions and YSOs were selected based on their WISE colours being similar to those commonly observed PNe (and hence the IR objects of the Acker et al., 2016 database).

In the case of Mul-IR 56 and Mul-IR 58, these display photometric properties that best match those of PNe, especially Mul-IR 58. However, Mul-IR 56 is close to the “area” where both PNe and YSOs overlap. The few HII regions studied here have a significantly higher PACS70/W4 ratio than Mul-IR 56 and Mul-IR 58. Mul-IR 62 is not part of the Herschel Point Source Catalogue, but does however appear in the Herschel FOV (see below).

However, there might be a trend for which the PACS70/W4 ratio of YSOs decreases with decreasing W1/W4 ratio. This would be coherent with the decrease in the IR slopes as the YSOs progressively accrete/loose their outer envelopes (see figure 7). Interestingly, this property appears to cause YSOs to photometrically distinguish themselves from PNe, despite more evolved YSOs having similar PACS70/W4 ratios to the PNe chosen here. In support of this hypothesis, most of the Class 0 YSO candidates (from Savadoy et al, 2014) selected here have the highest PACS70/W4 ratios and overall the smallest W1/W4 ratios. On the other hand, more evolved YSOs, such as Mul-LDû-IR 16 (see future work), are located in the zone with the lowest IR slopes.

YSO_ColoursFigure 8: Composite image explaining the link between the photometric properties of YSOs, and their evolution. Indeed, as the YSO and its circumstellar matter accretes material, the outer envelope progressively vanishes, leading to a progressive decrease in the mid-IR excess. This leads to a decrease in the IR slopes. Image credit: Lada et al. (2002).

However, it is important to note that the Infrared flux values in the Near-IR/Mid-IR region may be significantly influenced by the inclination of the YSO according to our perspective (Robitaille et al., 2006). More specifically, as seen edge-on, YSOs may be relatively obscured between 1 – 15 microns (e.g. HOPS 136 [Fischer et al., 2014], MY Lupi [see figure 6]), in comparison to a YSO observed nearly pole-on (e.g. HOPS 329 [Manoj et al., 2013]). To demonstrate this effect, see figure 9. HOPS 329 flux density values were compared/verified to those of Manoj et al. (2013).

SED_HOPS_diskFigure 9: Comparison between the SED of a nearly pole-on YSO (HOPS 329, incorrectly labelled HOPS 392) and an edge-on YSO (HOPS 136). Note how the latter SED shows a strong and broad anomaly (dip) around 10 μm, while the former shows a smooth trend in this region. Image credit: Université Grenoble Alpes – Trygve Prestgard

3. Follow-Up Infrared Observations

For a more comprehensive view of the studied PN-IR candidates, I chose to calculate/extract the flux density values from various other Infrared catalogues listed in Vizier. As can be seen in figure 10, Mul-IR 56 appears to have a SED that displays peak Infrared emissions between 30 and 60 microns, very much like a PN. On the other hand, despite the PN-like WISE-Herschel colours for Mul-IR 58 (see figure 7), AKARI and IRAS give values that differ significantly from Herschel, making it difficult to say whether the peak IR emissions lay at wavelengths smaller or larger than 100 microns.  In the case of Mul-IR 62, no values were provided by the Herschel Point Source Catalogue. The only flux density values beyond 50 microns are provided by AKARI and IRAS (60 and 90 microns). With only two values (and IRAS being of relatively low resolution), it is difficult to say whether or not the peak IR emissions of Mul-IR 62 lie below 100 microns or not. However, if the values are considered accurate, the plot may be suggestive of a peak near 50 microns.

MUL_IR_56_SED

MUL_IR_58_SED

SED_MULIR62Figure 10: SEDs plotted for Mul-IR 56 (top), Mul-IR 58 (middle) and Mul-IR 62 (bottom), based on flux values extracted/calculated from various public survey data available via Vizier. While Mul-IR 56 has a SED typically expected of a PN, Mul-IR 58 and Mul-IR 62 are a bit more difficult to interpret based on these data alone, but may be suggestive of PNe too.

Fortunately, The SIMBAD/CDS portal allows access to numerous different combinations of images, including a coloured combination of the three Herschel/PACS filters (70 + 100 +160 microns). With the images being colour-coded Blue (70 microns), Green (100 microns) and Red (160 microns), PNe should appear distinctively blue due to the morphology of their SED. These images can hence serve as a visual substitue for the lack of values beyond 50 microns. Interestingly, all three objects display a distinctively blue colour in these images, with Mul-IR 62 even appearing somewhat elongated. This is hence further evidence in regards to their PN nature.

Herschel_colour_imagesFigure 11: Herschel colour images showing Mul-IR 56, Mul-IR 58 and Mul-IR 62 compared to PNe and some YSOs. Notice how the former display a distinctive blue colour (reflecting the decrease in the emissions beyond 70 microns), contrary to the YSOs. Image credit: SIMBAD/Herschel_RGB.

 

4. Follow-Up Optical Observations

Since early 2018, high-resolution images from the DECaPS survey were released to the public. These images allowed the discovery of numerous new PNe that were otherwise too faint, or too stellar, to “stick out” in optical images (LDû et al., 2019). Various examples of PNe and candidates (as seen in DECaPS) are shown in figure 12. Notice how the apparent colours of these nebulae vary depending on the intensity interstellar reddening.

PNe_DECaPS_reddeningFigure 12: DECaPS image extracts of six PNe and candidates suffering various degrees of interstellar reddening. Notice how Mul-IR 56 and Mul-IR 58 display the colours of highly reddened PNe. Image credit: Aladin Lite/DECaPS.

As can be seen in figure 13, all three Mul-IR objects display traces of nebulosity and colours typical of highly reddned PNe. Moreover, notice how Mul-IR 58 and Mul-IR 62 appear to display a possible bipolar morphology. The nebulous morphology shows that these are hence unlikely to be YSOs.

Mul_IR_objects_PNeFigure 13: DECaPS Image extracts showing Mul-IR 56, Mul-IR 58, and Mul-IR 62 too scale. Notice how Mul-IR 56 and Mul-IR 62 appear to possibly be bipolar, while Mul-IR 58 appears much more compact and elliptical in morphology. Note also that the colours match those of the highly reddened PN candidates displayed in figure 2. Image credit: Aladin Lite/DECaPS.

In addition to their nebulous signature in DECaPS, traces of the nebulosity can be found in SHS (SuperCosmos Halpha Sky Survey) images, in the case of Mul-IR 56 and Mul-IR 62. More importantly however, these two nebulae appear brighter in the Halpha plates in comparison to those taken in the r filter (see figure 14). This suggests that the nebulae display Halpha emissions, very typical of PNe. One could expect that the apparent emissions in Halpha may be stronger had the nebulae not been obscured by Interstellar matter. Mul-IR 58 is likely too obscured and/or too faint to display any significant Halpha emissions, if indeed a true PN. However, let us note that PPNe do not emit significantly in Halpha (these are reflection nebulae, unlike PNe), hence one could technically consider the latter to be a possible Post-AGB/PPN object. Note however that HII regions and YSOs may also display Halpha emissions, but the IR colours explained earlier do not appear in favour of this classification.

SHS_Halpha_Mul_IR_56_62Figure 14: SHS Short Red (left) and SHS Hα (right) image extracts showing Mul-IR 56 and Mul-IR 62. Notice the nebulous nature of these objecst in Hα, while they appears to be poorly resovled and alot fainter in SHS Short Red. Image credit: SuperCosmos Halpha Survey.

Conclusion

Based on this small study of Optical and IR photometric data/imagery, it is possible to conclude that Mul-IR 56, Mul-IR 58 and Mul-IR 62 may in fact be true PNe. Firstly, the Herschel-WISE colours of Mul-IR 56 and Mul-IR 58 fall within the “photometric region” of PNe, rather than YSOs or HII regions. Furthermore, the SEDs of Mul-IR 56 and Mul-IR 62 are rather suggestive of a PN (especially the former). The SED of Mul-IR 58 is difficult to interpret beyond 60 microns due to the high dispersion of flux values. However, the Herschel Image colours seem suggestive of PN-like continuum emissions beyond 70 microns.

DECaPS images show all three objects to be nebulous in nature, which is strong evidence against these being YSOs. The nebulous nature of Mul-IR 56 and 62 is also apparent in SHS Halpha images, with the latter also being nebulous in Herschel too. The SHS images show that Mul-IR 56 and Mul-IR 62 are very likely to be emission nebulae, as expected of PNe. However, if Mul-IR 58 is indeed a true PN, the object is likely too obscured for the Halpha signals to be detected. Hence, the lack of apparent Halpha emissions may also technically include the nebula as a PPN candidate (as WISE-Herschel PPN colours seem to overlap with those of PNe).

However, one must note that the study is preliminary and the number of reference samples are statistically low. Only detailed spectra can reveal the true nature of these three objects. Unfortunately, with these nebulae being faint and heavily obscured (and Mul-IR 62 partially eclipsed by a foreground star), optical spectra may be difficult to obtain for these objects, hence the importance of finding reliable IR diagnostic criteria.

Future and Ongoing Work

Studying the variability of sources using DSS and Pan-STARRS1 image plate comparison, as well as ASAS-SN can “weed out” some YSOs and Late-type stars from our list of PN candidates. Results: Mul-LDû-IR 16, Mul-IR 20, Mul-IR 64 and Mul-IR 92 are all clearly variable. Mul-LDû-IR 16 is most likely a YSO. Mul-IR 21 may be some evolved star (Gomez et al., 2015), while Mul-IR 64 displays Long Period Variations (LPV) typical of PPNe. In fact, Mul-IR 64 has already been identified as variable by the Bochum survey, and is listed as an unclassified variable according to the AAVSO/VSX (GDS_J1633219-461042). Mul-IR 92 is currently listed as a possible YSO by the AAVSO/VSX.

Mul_IR_64_light_curve_ASAS_SNFigure 15: ASAS-SN light curve showing the variable nature of Mul-IR 64. The variations are typical to those of a Semi-regular star, with a period of ~25 days. Based on the object’s optical and IR colours (“yellow” star with Mid-IR excess), in addition to the light curve, it is most likely a PPN.Image credit: ASAS-SN.

Mul-LDû-IR 3: A likely an elliptical Planetary Nebula, and has independently been listed as a PN by Gomez et al. (2015). The object is bright and clearly elliptical in DECaPS images (see figure 16), with apparently strong Halpha emissions.

Mul_LDu_IR_3_Mul_IR_56_62_ComparisonFigure 16: DECaPS image extracts of Mul-IR 56, Mul-IR 62 and Mul-LDû-IR 3 showing their nebulous nature. In comparison to the Mul-IR objects, Mul-LDû-IR 3 is much less reddened and elliptical in morphology.Image exctract: Aladin Lite/DECaPS.

A few other sources (e.g. Mul-IR 31 and Mul-IR 65) may perhaps be stellar PNe based on their optical colours (Optically blue with likely Halpha emissions, as seen in Pan-STARRS, DECaPS and SHS) and their IR properties. If I remember correctly, ASAS-SN data did not reveal any variability in the case of Mul-IR 31, hence another factor that increases the chances of this object being a true PN! 🙂

Mul_IR_31_65_PANSTARRS_DECaPSFigure 17: Pan-STARRS1 and DECaPS image extract of Mul-IR 31 and Mul-IR 65. Both objects are stellar in appearance, but their otical and IR colours may be in favour of them being true PNe. Work in progress! Image credit: Aladin Lite/DECaPS and Pan-STARRS1.

Future work should also include a better study of Spitzer data, as PNe tend to display particular colours in these images, that most often distinguish them from HII regions and many YSOs. In the Aladin/lite Spitzer/IRAC images these tend to appear “green” (best emissions in the IRAC2 filter(?)). Note that Mul-IR 56, Mul-IR 58 and Mul-IR 62 all display this green colorimetry. Mul-LDû-IR 3 is located outside the Spitzer/IRAC FOV.

Mul_IR_objects_SpitzerFigure 18: Spitzer Image extracts showing Mul-IR 56, Mul-IR 58 and Mul-IR 62. Notice how Mul-IR 56 and Mul-IR 62 are both clearly nebulous in these images, however the reoslution is likely to small to display their bipolar nature. The reolution is likely also too low to display the nebulous nature of Mul-IR 58. Image credit: Aladin Lite/Spitzer.

 

References

CDS: Simbad, Vizier (Université de Strasbourg).

Fischer, W. J. et al. (2014) ‘HOPS 136: AN EDGE-ON ORION PROTOSTAR NEAR THE END OF ENVELOPE INFALL’, The Astrophysical Journal, 781(123), pp. 11.

Gómez, J. F.; Rizzo, J. R.; Suárez, O.; Palau, A.; Miranda, L. F.; Guerrero, M. A.; Ramos-Larios, G.; Torrelles, J. M (2015): ‘A search for water maser emission toward obscured post-AGB star and planetary nebula candidates’ Astronomy & Astrophysics, Volume 578, id.A119, 15 pp.

Le Dû, P. (2019) ‘Nebuleuses planétaires, Découvertes & Confirmations ‘, Astronomie

Manoj, P. et al. (2013) ‘HERSCHEL/PACS SPECTROSCOPIC SURVEY OF PROTOSTARS IN ORION: THE ORIGIN OF FAR-INFRARED CO EMISSION’, The Astrophysical Journal, 763(2)

Mulato, L. (2016) ‘Nébuleuses dans L’Infrarouge Moyen’, Astronomie, Feb 2016

Pottasch, S. R., Bignell, C., Olling, R., Zijlstra, A. A (1988) ‘Planetary nebulae near the galactic center’

Robitaille, T., Whitney, B., Wood., K. (2006) ‘Interpreting Spectral Energy Distributions from Young Stellar Objects. I. A Grid of 200,000 YSO Model SEDs’

Sadavoy S. I. et al. (2014) ‘CLASS 0 PROTOSTARS IN THE PERSEUS MOLECULAR CLOUD: A CORRELATION BETWEENTHE YOUNGEST PROTOSTARS AND THE DENSE GAS DISTRIBUTION’, The Astrophysical Journal, 787(L18), pp. 6.

Spitzer Science Center (2019) Magnitude/Flux Density Converter: Point Sources, Available at: http://ssc.spitzer.caltech.edu/warmmission/propkit/pet/magtojy/

Planetary Nebula Candidates Discovered by the APO team: Update!

Last year I wrote a blog post on the numerous Planetary Nebula (PN) candidates discovered by the Atacama Photographic Observatory (APO), a French trio of amateur astronomers/astrophotographers (Thierry Demange, Richard Galli and Thomas Petit). The article can be found here. Since the time that post was published (February 2018), the team has added an extra 18 candidates to their catalogue, with a current total of 71 PN candidates! In this article I describe several of these new finds in detail. The work includes my personal analysis of the objects according to public survey imagery. Hope you enjoy! 🙂

DeGaPe_finds_recent_2018_2019_beautiful_imageFigure 1: Image extract of [SII] +Halpha +[OIII] (SHO) image of the RCW 19 and RCW 20 star forming regions. This is also the disocvery image of several Planetary Nebula candidates, including DeGaPe 52, DeGaPe 54, DeGaPe 58 and DeGaPe 59. Planetary Nebulae can easily be spotted in SHO images due to their strong emission in [OIII] and Halpha, making them appear fluorescent in such imagery! (c) APO Team.

The Atacama Photographic Observatory (APO) is a remotely controlled observory in the Atacama desert run by French amateur astronomers Thierry Demange, Richard Galli and Thomas Petit (the APO team). Its chosen location makes it ideal to observe the wonders of the Southern Sky, with stunning images having been taken of the Carina and Prawn Nebula, among many others (see gallery)! Their results are regularly featured in French popular astronomy magazines, such as Astrosurf and Ciel & Espace. Many of their images, (e.g. figure 1) are a combination of individual [SII], Halpha and [OIII] exposures. Such combinations are referred to as SHO images. Planetary Nebula (PNe) are often easy to spot in such images do to their generally strong emissions in [OIII] and Halpha. Consequently, in SHO imagery, their emissions causes them to appear unusually green, turqouise or blue in comparison to the surrounding star field. Figure 2 shows some examples of known PNe as seen in the APO Team’s SHO images.

APO_known_PNeFigure 2: Known Planetary Nebulae as seen in SHO imagery from the APO team. Notice their unusually blue or green colours in comparison to the background star field. This is due to their significant [OIII] and Halpha emissions. (c) APO team.

However, in addition to the known PNe, the APO team detects a large number of uncatalogued objects displaying PN-like colours in their SHO images. These are regularly reported to the French PN database and are given the designation “DeGaPe” (Demange – Galli – Petit). Their latest find (as of currently) is DeGaPe 69, the team’s 69th PN candidate (not including KnDeGaPe 1 and KnDeGaPe 2). This makes them one of the leading discoverers of PN candidates in France! Their discoveries can be found on their website and on the PlanetaryNebulae.net website. Figure 3 displays some of their recent candidates that display hints of nebulosity.

DeGaPe_nebulous_finds_2018_2019Figure 3: Nebulous APO Planetary Nebula candidates as seen their [discovery] images. Notice that DeGaPe 53 and DeGaPe 63 are apparently large (>50.’) and very diffuse, while DeGaPe 55 and DeGaPe 64 appear to be much more compact in comparison (~10″). (c) APO team.

Perhaps one of the more remarkable objects recently discovered by the APO team is DeGaPe 64. Indeed, this small compact nebula (~10″ in diameter) displays all the characteristics expected of a PN, both in survey imagery and in narrow-band images (likely dominated by [OIII] emissions, see figure 3). Firstly, the Mid-IR signal matches that typically displayed by many compact PNe, both in WISE (Acker et al., 2016) and Spitzer (see here) . Secondly, DECaPS imagery confirms the object’s nebulous nature (see figure 4), and the SHS Halpha plates also show the nebula to significantly emit in Halpha.

DeGaPe_64_DECaPSFigure 4: DECaPS image extract showing DeGaPe 64 (center). The object appears to be a compact elliptical nebula, well within a dense star field. Image credit: Aladin Lite.

DeGaPe 55 is also quite interesting. It shares some characteristics with DeGaPe 64, including its size and morphology according to the APO images. Unlike DeGaPe 64 however, this object appears to be dominated by Halpha emissions rather than [OIII]. This is the case of many PNe (e.g. EM* VRMF 90 [figure 2]). Strangely, DeGaPe 55 appears to be stellar in survey imagery, including in the Halpha SHS survey plates (see figure 5)! Moreover, the object’s photometric properties are typical of a red giant star (see figure 5). The latter is confirmed by its variable nature, and has already been officially classified as a Mira star by Jayasinghe et al. (2018). Perhaps the nebulous appearance in the APO team’s SHO image is an artefact?

DeGaPe_55_DECaPSFigure 5: DECaPS image extract showing DeGaPe 55. The object is stellar with colours typical of a red giant star. The star is recognized by the Variable Star Index (VSX) under the designation ASASSN-V J081826.63-344332.1. Image credit: Aladin Lite.

DeGaPe 53 and DeGaPe 63 show hints of being highly evolved PNe. Firstly, these nebulae appear better in [OIII] images than in optical survey images (e.g. DECaPS and DSS). Furthermore, DeGaPe 53 displays a PN-like Mid-IR excess according to WISE, while DeGaPe 63 appears as an arc in DECaPS (see figure 7), which is a morphology commonly observed in many PNe DeGaPe 53 also appears circular in DECaPS (see figure 6). However, it is also possible that these objects may be reflection nebulosity associated with nearby star forming regions, rather than true PNe. Only spectra can confirm the true nature of these objects!

Degape_53_true_decapsFigure 6: Enhanced DECaPS image extract showing DeGaPe 53. Notice the relatively circular nature of the nebula. Note also that it surrounds several bright white stars (possibly young OB stars), suggesting that it may be a reflection nebula rather than a true PN.  Image credit: Aladin Lite.

Degape_53_decapsFigure 7: DECaPS image extract showing DeGaPe 63. Notice the nebulous arc positioned southward that appears to be associated with the nebula. While this might be a sign of a highly evolved PN, the nebulosity may be linked to surrounding star forming regions. Image credit: Aladin Lite.

However, only a minority of the APO team’s finds are actually nebulous. In fact, most of their candidates are actually stellar in appearance, even in high resolution survey imagery! Figure 1and Figure 8 show many such objects.

DeGaPe_54_57_61_68Figure 8: Stellar APO Planetary Nebula candidates as seen their [discovery] images. DeGaPe 54 and DeGaPe 68 are located in the field of dark nebulae (c) APO team.

Similarly to DeGaPe 64, DeGaPe 61 also show several signs in favour of a true PN. Firstly, the object displays a WISE signal similar to many PNe (see figure 9). Secondly, no evident variability was detected in ASAS-SN data and in the DSS plates. The object’s Halpha emissions are clearly visible in the APO team’s SHO image (see figure 8) and in SHS Halpha plates. Coincidentally, the objects is only ~30′ of another known PN candidate, PaMo 1 (see figure 9), a probable PN that was only discovered in 2018.

PaMo1_DeGaPe61_AllWISEFigure 9: AllWISE (W1 + W2 +W4 filters) image extract showing Planetary Nebula candidates PaMo 1 and DeGaPe 61. Their colours in this image are typical of PNe, being much brighter in the W4, in comparison to W1 and W2 (indicating Mid-IR excess). Image credit: Aladin Lite.

Similarly to DeGaPe 55, DeGaPe 57 also appears to be a possible Mira star. Indeed, the object’s photometric properties ressemble those of red giant stars, and the DSS plates show the object to significant fluctuate in brightness (see figure 10). Unlike DeGaPe 55, this star has not yet been recognized as a variable star. A report was just recently been submitted to the Variable Star Index (VSX) by myself.

DeGaPe 57Figure 10: DSS image plate comparison (Red filter) used to discover the variability of DeGaPe 57. Image credit: SuperCosmos Sky Survey.

DeGaPe_57_DECaPSFigure 11: DECaPS image extract showing DeGaPe 57. The object is stellar with colours typical of a red giant star. Image credit: Aladin Lite.

DeGaPe 54 and DeGaPe 68 appear to be located infront (or within) dark nebulae, deep within the active star forming region RCW 19. This suggests that these two objects may be Young Stellar Objects (YSOs) rather than true PNe. Indeed, as visible in the SHO images and the SHS Halpha plates, the objects appear to display strong Halpha emissions, typical of young YSOs (e.g. Classic TT Tauri stars). The Mid-IR signals also appear to be quite similar to many known YSOs (see figure 12). Furthermore, DeGaPe 68 appears to be associated with a faint nebula (likely reflection nebulosity, see figure 13) similar to that surrounding the YSO IRAS 17079-4032 (see figure 13) However, unlike many YSOs, I was unable to find traces of variability in ASAS-SN and in the DSS plates. Perhaps this is an indication that DeGaPe 68 may be a highly reddened PN?

YSO_DEGAPE_objectsFigure 12: AllWISE image extracts showing DeGaPe 54, DeGaPe 68 and two confirmed YSOs. Notice their similar colours, suggesting that the two DeGaPe finds may also be YSOs rather than true PNe. Image credit: Aladin Lite.

DeGaPe_68_Decaps_comparsion_YSOFigure 13: Comparison between DeGaPe 68 and IRAS 17079-4032 as seen in DECaPS. IRAS 17079-4032 is a known YSO with a reflection nebula, located within the Barnard 58 dark nebula. DeGaPe 68 is also located within a dark nebula, and shows a small tail-like object similar in colour to IRAS 17079-4032. This suggesting that DeGaPe 68 may also be a YSO, rather than a true PN.

Summary

Since February 2018, the APO team has discovered many new awesome objects! While it is evident that DeGaPe 64 is likely a true PN, many other of their finds appear to be more difficult to classify, despite their PN-like colours in SHO images. Indeed, some objects (e.g. DeGaPe 54 and DeGaPe 68) show strong evidence in favour of YSOs, while others appear to likely be Red giant stars (e.g. DeGaPe 55 and DeGaPe 57) based on public survey imagery. Spectra are required to formally confirm the nature of these objects! 🙂

The SOHO and STEREO Comets of 2019!(1)

So far, 2019 has been a productive year in terms of sungrazing comet discoveries, especially these last couple of months! In this post I will be discussing many of the SOHO and STEREO comets discovered, so far, this year. Despite the numerous sungrazers discovered in 2019, there were significant fluctuations in the number of sungrazers observed for a given month. Explanations of the factors that might explain this variability will be given in this work.

This article will be one of two blog posts that describe this year’s sungrazers. In this post I will be focusing solely on January, February and March. Enjoy! 🙂

January

Very few sungrazers were discovered in SOHO/LASCO images during January. This was due to presence of the Occulting Pylon at the area of the Kreutz stream (see figure 1). Indeed, this had  a significant impact on the number of SOHO Kreutz comet discoveries, as the Kreutz-group comets contribute to the majority of sungrazing comets (~90%). In fact, no Kreutz comets were observed between Jan 01 and Jan 20! As we will later see, STEREO/SECCHI images picked up the Kreutz comets that SOHO missed.

C3_janFigure 1: The average location of the Kreutz stream track as seen from SOHO/LASCO in January. Note that a significant portion of this zone is covered by the Occulting Pylon (black diagolonal feature). The presence of the pylon can completely mask the presence of comets that typically would be detectable in SOHO/LASCO, especially the fainter ones. Image credit: Sungrazer Project.

While SOHO/LASCO was having a difficult time detecting Kreutz-group comets, SOHO was however able to observe one “C2-only” Meyer-group comet and a “C3-only” Non-group comet between Jan 01 and Jan 20. This is especially due to their apparent trajectory being unaffected by the SOHO/LASCO pylon. The Meyer-group comet (SOHO-3678) was the first SOHO comet find of 2019 (see figure 2). Neither of these comets were recovered in STEREO/SECCHI. In fact, as of currently, no Meyer-group comet has ever been in recovered in SECCHI data! The non-group comet is designated SOHO-3679 (see figure 3).

meyer_comet_Jan5_2019Figure 2: One of the several discovery images Meyer-group comet SOHO-3678., The comet was only visible in SOHO/LASCO C2 images. The image was taken on Jan 5th. Image credit: ESA/NASA SOHO/LASCO C2.

GIFMaker.org_vyI5Ov(1)Figure 3: The faint stellar Non-group comet SOHO-3679 as seen in SOHO/LASCO C3 images of 2019-01-18/19. Notice how the trajectory of this small comet is opposite the SOHO/LASCO occulting Pylon. Had the pylon been at the location of the comet it would likely have been undetectable and hence gone undiscovered. Image credit: ESA/NASA SOHO/LASCO C3.

The first two SOHO Kreutz-group comets of 2019 were found  in C3 images of Jan 21 and Jan 22 (SOHO-3680 and SOHO-3681 respectively, see figure 4). Unlike most faint Kreutz sungrazers, these were relatively bright (especially SOHO-3681) and their trajectories were somewhat outside the pylon, which is what made them detectable. As we will see below, most Kreutz comets that appeared in January were relatively faint (some likely fainter than the C3 limiting magnitude).

GIFMaker.org_yoZVjZFigure 4: SOHO/LASCO C3 animation of SOHO-3680 (fainter comet) and SOHO-3681 (brighter comet). The moderate brightness of these comets allow them to “stick out” among the noise related to the Occulting Pylon. Fainter comets (even those that one would expect to be visible in C3) might have been nearly impossible to detect at their location. Image credit: ESA/NASA SOHO/LASCO C3.

Fortunately, despite SOHO/LASCO having a hard time detecting Kreutz-group comets, the STEREO-A spacecraft had a good view of the Kreutz stream. Indeed, the spacecraft was located at a part in its orbit where the  STEREO/SECCHI HI1-A camera could easily observe Kreutz-group comets. Not only is HI1-A more sensitive to Kreutz-group comets than any other STEREO/SECCHI instrument, but it has a lower limiting magnitude than the SOHO/LASCO imagers. Hence, not only did STEREO-A have an unobstructed view of the Kreutz stream, but it was also picking up comets fainter Kreutz objects than what possibly could be observed by SOHO/LASCO C3. As a result, eight Kreutz-group comets were discovered in HI1-A. In fact, the first sungrazer of 2019 was a Kreutz-group comet found in HI1-A images of Jan 01 (STEREO-112, see figure 5). This period is occasionally (and informally) referred to as the “STEREO Kreutz season”.

GIFMaker.org_Khe2mBFigure 5: Animation of STEREO/SECCHI HI1-A image extracts showing STEREO-112 just before it left the HI1-A FOV. These were the last images of this comet, as it did not appear in SOHO/LASCO and was too faint to appear in STEREO/SECCHI COR2-A. Image credit: NASA/SSC STEREO/SECCHI HI1-A.

STEREO-112, was possibly a Subgroup-II Kreutz comet, based on its trajectory (slightly further south than that of “typical” Kreutz comets). Interestingly, STEREO-112 was quickly followed by two additional sungrazers of very similar trajectory on Jan 02 (STEREO-113) and Jan 03. The one from Jan 03 was signficantly fainter than the former two. Perhaps these were all direct fragments of each other. Kreutz-II group comets are significantly less common than the “typical” Kreutz-I comets, but among the Kreutz comet population, most of the known brightest [great comet] members are/were of this sub-group. Indeed, C/1887 R1 (Great Comet), C/1887 B1 (Great Southern Comet)C/1965 (Ikeya-Seki), and C/1970 K1 (White-Ortiz-Bolleli) were all Kreutz-II objects. The The Eclipse comet of 1882 (X/1882 K1) might also have been a Kreutz-II member (see figure 6).

eclipse comet 1882 WesleyFigure 6: The Eclipse Comet of 1882 as drawn by W. H. Whesley based on photographs taken of this comet during the 1882 eclipse of May 17th. Image credit: Extracted from the work by CECIL G. DOLMAGE, M.A., LL.D., D.C.L: ASTRONOMY OF TO-DAY A POPULAR INTRODUCTION IN NON-TECHNICAL LANGUAGE (http://www.drugfreereading.com/interest_novels/popular%20astronomy.htm#Page_95). I apologize in advance if there are more specific references or if the original image is copyrighted. If so, please let me know and I’ll take this down!

In addition to STEREO-112, a very faint Kreutz-I comet was also later found in HI1-A images of Jan 01 (STEREO-111). The next HI1-A Kreutz comets appeared in images of Jan 07. These consisted of a pair of Kreutz fragments: a tiny comet (STEREO-114) leading a signficantly brighter member (STEREO-115). Personally, I believe STEREO-115 might have been bright enough to be faintly detectable in SOHO/LASCO had the pylon not been obstructing the Kreutz path. STEREO-114 was probably an object that only SOHO/LASCO C2 could detect during certain parts of the year (the “C2 Kreutz Season”, see below). Both STEREO-114 and STEREO-115 comets are shown in figure 7.

STEREO_114_115_HI1A_Jan07_2019

Figure 7: STEREO/SECCHI HI1-A image extracts showing Kreutz-group comet fragments STEREO-114 and STEREO-115. STEREO-114 was significantly fainter than the latter (most likely something that only SOHO/LASCO C2 would be able to detect). STEREO-115 might have been bright enough to detect in SOHO/LASCO C3 had it not been for the obstruction of the occulting pylon at its path. Image credit: NASA/SSC STEREO/SECCHI HI1-A.

A few other interesting STEREO Kreutz comets were those observed just before SOHO-3680 and SOHO-3681. These were STEREO-119 (Jan 19) and STEREO-118 (Jan 20) Notice how these (as well as the other STEREO comets mentioned here) are significantly fainter than the SOHO Kreutz comets (see figure 8). STEREO-118 might have been bright enough to have been faintly detectable in SOHO/LASO C3 in my opinion, had it not been for the occulting pylon. STEREO-119 was probably something that only SOHO/LASCO C2 could detect during the “C2 Kreutz season” (see below).

STEREO_118_119_HI1A_Jan19_22_2019Figure 8: STEREO/SECCHI HI1-A image extracts showing Kreutz-group comets STEREO-118, STEREO-119, SOHO-3860 and SOHO-3681. Notice how the STEREO comets are significantly fainter than those found in SOHO. It is because of their faint nature that the STEREO comets weren’t observed by SOHO/LASCO. Image credit: NASA/SSC STEREO/SECCHI HI1-A.

February

As of February the location of the STEREO-A spacecraft was progressively becoming less adapted for its HI1 imager to observe Kreutz-group comets. As a consequence, no new STEREO Kreutz comets were discovered. Fortunately, on… the occulting pylon changed location, hence no longer partially masking the Kreutz path. Indeed, February, March and April are also affected by the Pylon location in regards to Kreutz comet hunting.

pylon_rotation_2019020506Figure 9: Rotation of the Occulting Pyon in SOHO/LASCO. The images are consecutive, hence there is a data-gap of nearly 8.5 hours resulting from this manuver. Image credit: ESA/NASA SOHO/LASCO C3.

This meant, despite comets such as STEREO-112, STEREO-113, STEREO-114 and STEREO-119 being no longer possible to detect, very faint C3 Kreutz comets were now detectable in SOHO/LASCO, unlike January. Indeed, several comets were discovered during the first half of February that would otherwise have been hidden by the occulting pylon. Furthermore,  the faintest of these were not detected by STEREO/SECCHI HI1-A (due to the less favourable location of the STEREO-A spacecraft) and hence could have gone completely undiscovered had the Pylon not changed location. In February a total of four Kreutz-group comets were observed, with only two being detectable in HI1-A. The brightest among them was SOHO-3682, which was observed in SOHO/LASCO C3 images of Feb 06 abd 07 (see figure 10).

soho_comet_Feb_06_2019Figure 10: SOHO/LASCO C3 image extract showing comet SOHO-3682. Image credit: ESA/NASA SOHO/LASCO C3.

In addition to the Kreutz comets, two small Meyer-group comets were observed transiting the corner of the C2 FOV on Feb 20 and Feb 23. The first one (SOHO-3686) was quite easy to spot (see figure 11). It displayed the typical condensed and elongated morphology commonly observed in Meyer comets. The one on Feb 23rd (SOHO-3687) was much fainter (see figure 12), perhaps one of the faintest Meyer-group comets on record!

GIFMaker.org_538Y3BFigure 11: Animation showing SOHO-3686 transiting the corner of the SOHO/LASCO C2 FOV. These were the only images in which it was visible. Image credit: ESA/NASA SOHO/LASCO C2.

Meyer_20190223_C2_single_image_captionFigure 12: Composite image extract showing SOHO-3687 as seen in SOHO/LASCO C2. Image credit: ESA/NASA SOHO/LASCO C3.

March

The last comet to be detected in HI1-A data was SOHO-3688, in frames from Feb 27 and 28. In fact, the comet was bright enough to be clearly obvious in these images (see figure 13), despite it being a period where the STEREO-A spacecraft was capable of only detecting the brightest Kreutz comets in its HI1-A intrument (due to the spacecraft’s angle of observation). Only weeks later the Kreutz-stream passed outside the FOV of the HI1-A imager, meaing that only COR2-A could detect Kreutz-group comets. This marked the end of the “STEREO Kreutz season”.

GIFMaker.org_6DEKA4

Figure 13: SOHO-3688 as seen in STEREO/SECCHI HI1-A images of 2019-02-27/28: days before perihelion.This was the last Kreutz-group comet of 2019 to be observed in HI1-A images. Image credit: NASA/SSC STEREO/SECCHI HI1-A.

SOHO-3688 was discovered in C3 images of March 1st, at the very edge of the FOV. Indeed, despite the HI1-A images being taken earlier than SOHO/LASCO, the STEREO/SECCHI data is not quite as real time as SOHO/LASCO. As the comet continued to approach the Sun, it reached about mag 4-5 at its brightest before disentigrating, showing a faint narrow tail at near peak brightness. Consequently, at the time of discovery, this was the brightest comet of 2019.

SOHO_20190302_0318_C3_tailFigure 14: SOHO/LASCO C3 image extract showing SOHO-3688  (centered) close to its maximum brightness. Notice the comet’s faint and narrow tail. Image credit: ESA/NASA SOHO/LASCO C3.

GIFMaker.org_wzhTwBFigure 15: comet SOHO-3688 rapidly fading as it enters the SOHO/LASCO C2 FOV. Notice the long tail partially reulting from the comet’s disentigration. Image credit: ESA/NASA SOHO/LASCO C2.

SOHO-3688 was also well visible in COR2-A images. In these images the comet appeared headless with a long tail (see figut, which is the common appearance of Kreutz-group comets in these images. only in rare cases do Kreutz-group comets appear completely stellar in COR2.

SOHO_March_Kreutz_20190302_COR2AFigure 16: COR2-A Image extract of SOHO-3688. While the COR2-A coronagraph is poorly sensitive to comets, this one was sufficiently bright to appear obvious in the images. The long-tailed, “headless” appearance of the comet in this image is the typical appearance of Kreutz-group comets in COR2. Image credit: NASA/NRL STEREO/SECCHI COR2-A.

Over the course of March, the SOHO spacecraft had reached a place in its orbit where the SOHO/LASCO C2 camera was placed at such an angle that it became progressively better at observing Kreutz-group comets than the C3 imager. Indeed, an increasing amount of Kreutz-group comets started becoming visible in these images over the course of March. This eventually ledd to the first “C2-only” detectable Kreutz comet of 2019, which appeared in images of March 9th (designated SOHO-3691, see figure 17). This marked the beginning of the “C2 Kreutz season”. In other words, the period of the year when SOHO can detect Kreutz comets fainter than what can be observed by the C3 instrument. This is because the C2 images are of higher resolution than C3. As a consequence, the number of Kreutz comet discoveries generally increase significantly during these periods. Basically, C2 took the same role as HI1-A had in January, when it comes to observing Kreutz-group comets.

GIFMaker.org_Tj5iAVFigure 17: Animation of cropped SOHO/LASCO C2 images showing SOHO-3691, the first Kreutz-group comet of 2019 to be solely detactable in C2 images. This is because the comet was too faint to be detectable in C3. In my opinion, it might be a comet similar to STEREO-112 and STEREO-113, in terms of brightness and orbit (all three comets are likely of the Kreutz-II subgroup). Image credit: ESA/NASA SOHO/LASCO C2.

This small comet, SOHO-3691, was likely a Subgroup-II Kreutz based on its trajectory (slightly south of the average Kreutz group track). The trajectory of these comets can often cause them to be detectable in C2 earlier than “ordinary” Kreutz comets (comets of the Kreutz Subgroup-I). The first [likely] Kreutz-I comet was found in C2 images of March 21st (SOHO-3694), and was followed by yet another C2-only Kreutz only a couple of days later (SOHO-3697). SOHO-3691 is perhaps comparable to other objects such as STEREO-112 and STEREO-113, which were also Kreutz-II comets fainter than the C3 threshold. SOHO-3695 and SOHO-3697 might have been similar to very faint Kreutz-I comets such as STEREO-114 and STEREO-119, if not fainter (in my opinion).

soho_c2_faint_comet_c2_only_20190321Figure 18: SOHO/LASCO C2 image extract of SOHO-3695, the first “ordinary” C2-only Kreutz-group comet. It is due to the comet’s very faint nature that it did not appear in C3. Image credit: ESA/NASA SOHO/LASCO C2.

soho_comet_march__c2_only_comet_2019Figure 19: SOHO/LASCO C2 image extract of SOHO-3697, the second third C2-only Kreutz-group comet. It is due to the comet’s faint nature that it did not appear in C3. Image credit: ESA/NASA SOHO/LASCO C2.

Between the extremely faint C2-only Kreutz comets, and  fairly bright comets such as SOHO-3688, there were many other Kreutz members that appeared quite nicely in SOHO/LASCO, despite being too faint to appear in COR2-A. For example, this was the case of comet SOHO-3692 as seen in SOHO/LASCO C3 images of March 12th. Note the comet’s slightly elongated nature (figure 19).

GIFMaker.org_gt2oB7Figure 19: Animation of SOHO/LASCO C3 image extracts showing comet SOHO-3692. Notice how this comet is generally condensed but occasionally elongated and slightly fuzzy. Image credit: ESA/NASA SOHO/LASCO C3.

Another example were SOHO-3702 and SOHO-3703. These two Kreutz-group comets appeared on March 31st, seperated within only eight hours of each other. Both were of similar brightness and of very similar [condensed] appearance. They were seen in C3 and C2 (figure 20).

SOHO_3702_3703_C2_20190331Figure 20: Comet SOHO-3703 (left) and SOHO-3702 (right) as seen in SOHO/LASCO C2 image extracts. Notice their similarly bright and condensed appearance. Image credit: ESA/NASA SOHO/LASCO C2.

Two Meyer-group comets were also found in March. The brightest of them was SOHO-3693. This comet was moderately bright (relative to the Meyer-group members). Like SOHO-3686, it displayed the typical condensed in elongated morphology commonly observed in Meyer comets (see figure 21). The second one (SOHO-3700) was significantly fainter, but still displayed a fairly condensed and elongated appearance.

Meyer_group_comet_Peter_Berrett_Mar11_C2Figure 21: SOHO/LASCO C2 image extract showing the moderately bright Meyer-group comet SOHO-3693. Notice the comet’s condensed and elongated appearance, very typical of Meyer comets. Image credit: ESA/NASA SOHO/LASCO C2.

References

CECIL G. DOLMAGE, M.A., LL.D., D.C.L: ASTRONOMY OF TO-DAY A POPULAR INTRODUCTION IN NON-TECHNICAL LANGUAGE (http://www.drugfreereading.com/interest_novels/popular%20astronomy.htm#Page_95)

Sungrazer project Report pages and archives (https://sungrazer.nrl.navy.mil/index.php?p=recent).

Note: I only assume comets such as SOHO-3691, STEREO-112 and STEREO-114 are Kreutz-II comets, while the rest are of type I based on their trajectories in HI1-A and SOHO/LASCO. Only formally dervived orbit can confirm this. The maximum magnitude of SOHO-3688 is only a personal estimation, and is not based on any published values.

2018 Autumn-Winter discoveries made by amateur astronomers!

2018 has proven to be a fruitful year in terms of new astronomical discoveries, especially by amateur astronomers! This article describes only a minute fraction of the many excellent finds made during the last few months of 2018. The original article was expected to cover a much larger fraction of these discoveries. However, due to limited time on my end, the post had to be significantly shortened.  I expect to write about many more of these finds in later blog posts, in which I hope to include objects such as comet C/2018 V1, C/2018 Y1, Nova Normae 2018, “Finn’s Nebula” and many new planetary nebulae candidates recently added to the French database!

Note that the descriptions for of the each objects below are partially based on my opinions, which are based on my personal interpretation of each one these objects. Hence, please correct me if you I’ve made any errors!

Happy New Year to all readers! 🙂

Bright “long-tailed” Kreutz-group comet!

On November 22nd, Hanjie Tan (China) reported three previously unknown Kreutz-group comets within a period of only two hours! Among these comets was one of the brightest sungrazers observed over the past couple of years! Indeed, Hanjie found this sungrazer at the very edge of the SOHO/LASCO C3 FOV (see figure 1), where Kreutz-group comets are rarely brighter than the limiting detection magnitude of the C3 detetctors! The fact that a Kreutz-group comet had already reached such levels of brightness, so far away from the Sun, was a strong indication that it might brighten into a very nice sungrazer!

hanjie_kreutz_discovery_imageFigure 1: One of the several discovery images of Hanjie’s bright Kreutz-group comet. Hanjie found the comet near the edge of the SOHO/LASCO C3 FOV, where most Kreutz-group comets are still fainter than the limiting magnitude of the C3 detectors. Image credit: ESA/NASA SOHO/LASCO C3.

As expected, the comet quickly brightened as it kept approaching perihelion, to the point where its brightness had saturated the SOHO/LASCO C3 detectors! At that point, the comet had reached its peak apparent brightness (around mag +2 or +3, see figure 2). Unfortunately, only hours after it had started saturating the telescope’s detectors, the comet started declining in brightness. The drop in brightness indicated the onset of disentigration, and hence the comet’s upcoming demise.

Hanje_comet_C3Figure 2: Hanjie’s bright Kreutz-group comet as seen in SOHO/LASCO C3 image extracts from 2018-11-24. Notice how the comet’s brightness is such that is saturates the C3 image detectors in the 15:42 UT image. In that image the comet was likely around mag +2. Image credit: ESA/NASA SOHO/LASCO C3

The comet entered the SOHO/LASCO C2 FOV at about 19:30 UT, where it showed obvious saturation spikes too (see figure 3). These however quickly vanished as the head of the comet continued to drop in brightness. Only hours later the comet completely vanished, after its head passed behind the coronagraph. The tail (or its remnant) persevered a few hours longer, before being blown away by the Solar wind.

C2_Hanjie_KreutzFigure 3: Hanjie’s comet as seen in a SOHO/LASCO C2 image extract, a couple of hours before it entered the instrument’s FOV. Notice the saturation spkies, likely indicating that the comet was close to mag +2 in brightness! Image credit: ESA/NASA SOHO/LASCO C2.

SOHO/LASCO C2 images also reveal striations in the comet’s tail, only hours before it vanished. The image below (figure 4) shows these striations clearly!

soho comet striationsFigure 4: A contrasted SOHO/LASCO C2 image extract of Hanjie’s bright SOHO comet as seen only hours before it completely vanished. The images are meant to reveal the striations in the comet’s tail, which are best visible in the mid(left portion of the image. Image credit: ESA/NASA SOHO/LASCO C2.

Alongside the real time SOHO/LASCO data, the comet was also being tracked in the real time STEREO/SECCHI HI1-A and COR2-A images. Despite the real time STEREO/SECCHI images being of low resolution, the brightness of the comet was such that it appeared obvious in those images! This can be clearly seen in the animation below (figure 5) as well as in figure 8. In fact, even a faint tail can be detected in some of the low quality HI1-A frames, while the tail is appears quite obviously in the low quality COR2-A images!

output_GZowUZFigure 5: Hanjie’s bright Kreutz-group comet as seen in low quality STEREO/SECCHI HI1-A images from 2018-11-23 to 2018-11-24. Notice how the comet’s tail is apparent in some images. It’s rare that comets are bright enough to appear in these low resolution images. Image credit: NASA/SSC STEREO/SECCHI HI1-A.

As seen in the STEREO/SECCHI images, one of the most interesting features of this comet was the large apparent length of its tail. Indeed, the high resultion HI1-A images indicate a tail with an apparent length of ~10 solar radii. This is alot more apparent in substracted HI1-A frames (see figure 7). Furthermore, one can clearly see the interaction of the tail with the solar wind, causing it to “wiggle” and perhaps even disconnect in some cases, as seen in the animation below.

output_HI2FhXFigure 6: Animation of STEREO/SECCHI HI1-A image extract showing the comet’s bright coma, and the obvious tail dynamics caused by solar wind interaction. Image credit: NASA/NRL STEREO/SECCHI HI1-A.

Hanjie_Kreutz_comet_discovery_HI1-A_long_tailFigure 7: Difference HI1-A image extract showing the comet’s long tail. Its clumpy and slightly non-linear nature is the result of its interaction with the solar wind. Image credit: NASA/NRL STEREO/SECCHI HI1-A.

COR2A_Hanjie_KreutzFigure 8: Image extract from a low quality real time COR2-A image, shwing Hanjie’s bright Kreutz-group comet. Note the comet’s long and obvious tail! It is rare that comet’s are sufficiently bright to appear in these low resolution images. Image credit: NASA/SSC STEREO/SECCHI COR2-A.

In comparison to the HI1-A images, the full-resolution COR2-A frames show a maximal apparent tail length of about ~8 solar radii, instead of ~10 (see figure 9). This could be due to the lower sensivity of that instrument to detect comets in general. Despite some minor differences in the apparent tail length, note how spectacular it appears in these images! 🙂

Hanjie_kreutz_highres_COR2Figure 9: COR2-A image extract showing the comet. Note the length of the tail, with an apparent length of almost 10 solar radii! Image credit: STEREO/SECCHI COR2-A

The comet was also faintly detected in the full resultion COR1-A instrument images (see figure 10). As previously mentioned, the comet had significantly faded during its presence in the COR2-A FOV, making it hardly detectable by the time it had made it into the COR1-A FOV. In those images, one basically observes the remaining tail of a dead comet! Indeed, it is possible (if not likely) that the comet fully disentrigrated before entering the COR1 FOV.

COR1A_Hanjie_cometFigure 10: COR1-A images showing the comet just before perihelion, on 2018-11-25. The comet significantly faded before entering the FOV, making it hardly detectable in these images. The contrast has been significantly enhanced to better see to comet. Image credit: NASA/SSC STEREO/SECCHI COR1-A.

The comet was accompanied by a preceding fragment, also discovered by Hanjie Tan in SOHO/LASCO images! The comet was faint, and preceded the comet by only three hours. It was also detectable in the STEREO/SECCHI HI1-A images (see figure 11).

fragment-soho_20181124Figure 11: The leading fragment as seen in STEREO/SECCHI HI1-A images. In these images the comet was much too faint to show any particular traces of a tail. NASA/NRL STEREO/SECCHI HI1-A.

Bright Kreutz-group comet: December 12th, 2018

On 2018-12-10, amateur astronomer Worachate Boonplod (Thailand) reported a previously unknown Kreutz-group comet in SOHO/LASCO C3 images. As was the case of Hanjie’s bright sungrazer (see figure 1), this comet was well brighter than mag +10 at several degrees from the Sun (see figure 12).

20181210_comet_SOHOFigure 12: SOHO/LASCO C3 image extract showing Worachate’s bright comet, taken only hours after the discovery images. Notice a faint tail directed in the south direction. Image credit: ESA/NASA SOHO/LASCO C3.

During the morning hours (Universal time) of 2018-12-12, the comet entered the SOHO/LASCO C2 FOV (see figure 13). In those images the comet sported a tail over 1° long (see figure…). Unfortunately, the comet had already started fading hours before it entered the C2 FOV. At around this tie the comet had nearly completely disentigrated, eventually leaving behind a narrow remaining tail which eventually dissipated due to the Sun’s intense solar wind.

soho_comet_20181212Figure 13: SOHO/LASCO C2 image extract showing Worachate’s comet only a few hours after having entered the FOV. By the time this image was taken the comet had significantly faded (disentigrated). Image credit: ESA/NASA SOHO/LASCO C2.

The comet was also well visible in the STERE/SECCHI images, where it displayed an obvious tail (especially in COR2-A images, see figure 15). However, this tail appeared much fainter and much shorter than that of Hanjie’s Kreutz-group comet (see figure 6, 7 and 14). The most likely explanation of the generally shorter apparent length is rather due to the limiting magnitude of the HI1-A images, rather than the tail itself. Indeed, the tail is likely to be comparable in length to Hanjie’s comet (and this is possibly apparent in some images), but is only much fainter. I was not able to receover the comet in COR1-A.

HI1A_stereo_comet-20181211Figure 14: Enhanced STEREO/SECCHI HI1-A image of Worachate’s comet. Note the stubby tail, in comparison to Hanjie’s sungrazer. In some images one can see hints of a much longer tail, waving rapidly in accordance to the solar wind. Image credit: NASA/NRL STEREO/SECCHI HI1-A

soho_comet_20181212_COR2AFigure 15: Worachate’s comet as seen STREO/SECCHI COR2-A images. The tail of this comet is comparable to its length in the SOHO/LASCO images. Image credit: NASA/NRL STEREO/SECCHI COR2-A.

Worachate’s comet was followed by three smaller fragments, that vanished over the course of 2018-12-12 and 13 (see figure 16). The situation is quite similar to a bright Kreutz that appear almost exactly one year earlier: https://skyhuntblog.wordpress.com/2017/12/23/solar-observatories-observe-the-demise-of-a-bright-sungrazer-and-its-many-fragments/ The small fragments were discovered by Worachate and Masanori Uchina (Japan) in SOHO/LASCO C3 and C2 images.

fragments_soho_comets_20181212Figure 16: The three trailing fragments of Worachate’s comet as seen in STEREO/SECCHI HI1-A. Image credit: NASA/NRL STEREO/SECCHI HI1-A.

AT 2018 hfn – Dwarf Nova desguised as a Supernova!

In early October of 2018, Malhar Kendurkar of the Global Sunpernova Search team (GNSTS) detected a previously unknown transient near the nucleus of 2MFGC 2715, a edge-on spiral galaxy in Perseus (see figure 17). The event was designated AT 2018 hfn, and was measured to have a visual magnitude of +14.95 +/0.05, at discovery. Follow-up observations some days later indicated a Vmag of +14.80.  Preliminary spectroscopic observations were in favour of a Supernova, and the transient was hence designated SN 2018 hfn. It was initially believed to be the first Supernova discovery made by the GSNST team, until further follow-up spectra indicated the transient to be a dwarf nova in outburst! Indeed, the outburst stemmed from a progenitor located within our own galaxy,  at the line of sight of a background galaxy (2″ east of its nucleus)! What an odd coincidence!

thumbnail_tns_2018hfn_atrep_23441_GSNSTFigure 17: Discovery image of AT 2018hfn, taken by Malhar Kendurkar of the GSNST team. Note how the transients clearly outshines the background galaxy, 2MFGC 2715. (c) Malhar Kendurkar, GSNST.

The GSNST was only founded last August at the initiative of amateur astronomers Malhar Kendurkar and Cedric Raguenaud. Malhar is a graduate astrophysics student, and director of the Prince George Astronomical Observatory in Canada, while Cedric is a computer scientist with an interest in deep sky transients and variable stars. The project focuses on detecting and observing deep sky transients, as well as variable stars, with the objective of gaining more information on their true nature. As of November, 26th, 2018, the team has discovered five transients, including a rapid dwarf nova outburst in M31 (AT 2018 hvv). However, perhaps their most notable discovery is AT 2018 hfn, due to the odd coincidences explained previously! The work done by the GSNST team can be followed on their website: http://globalsupernovasearchteam.space

2MFGC 2715_UG_quietFigure 18: SDSS image showing the progenitor of AT 2018 hfn and the coincinding background galaxy, 2MFGC 2715. Note that AT 2018 hfn appears only 2″ from the nucleus of the galaxy! Image credit: SDSS Aladin Lite.

Photometric data from APASS (Henden et al., 2016) suggests that the progenitor of AT 2018 hfn shines at about B= 15.55 mag (APASS), with a colour index of B-V= 0.78 mag. Gaia and Pan-STARRS1 data indicate similar magnitude measurments. Indeed, it appears that photometric data from surveys give quite similar magnitudes to Malhar’s measurments during outburst. Rather than this being evidence against Malhar’s discovery (which has been confirmed spectroscopically), it’s more likely that survey photometric measurments are overestimated due to significant contamination from the background galaxy. Despite AT 2018 hfn being a genuine case, Malhar explains that the team has stumbled upon false positives in the past, including known variable stars, minor planets and even a globular cluster (see figure 19).

GSNST_GC_false_positiveFigure 19: Discovery image of the transient candidate AT 2018 fhy, which was later confirmed to be a faint globular cluster belonging to M31. The detection was done automatically via software. (c) Malhar Kendurkar, GSNST.

Further to the subject of transient detection,  Malhar states that “Finding new Astronomical Transients is not an easy task when limited equipment is available, time on telescopes is restricted, and weather proves challenging. It is a game of patience and perseverance”. This could not have been said better!

 

Hen 3-860 – Possible Symbiotic Variable

Gabriel Murawski (Poland) first noticed this object due to its signficant Halpha emissions, as such objects most often display such emissions. Indeed, Gabriel was hunting for Halpha-emitting stars with the intent of discovering new symbiotic variables. To do this he searched the WRAY catalogue for uncatalogued emission-line objects, where he found Hen 3-860 (WRAY 15-10622) and studied their ASAS-SN light curves in search for variations commonly observed in such variables. He found Hen 3-860 to display such variations (see figure 20), which consequently lead it to be classified as such a candidate in AAVSO’s Variable Star Index.

Hen 3-860 JD plotFigure 20: ASAS-SN light curve of Hen 3-860 as plotted by Gabriel Murawski. Note the significant irregular(?) variations in brightness, with a peak (outburst?) at around HJD 2457800. Image credit: ASAS-SN and Gabriel Murawski.

Unlike many symbiotic binaries, the light curve does not display clear evidence of LPV variations, which tend to be associated with red giant star companions (hence Mira or Semi-regular variables). For example, the symbiotic variable Vend 47, aka ASASSN-V J195442.95+172212.6 (object expected to appear in an upcoming blog post) shows LPV variations, in addition to outbursts (see figure. 21).

Vend_47_ASAS-SN.jpegFigure 21: ASAS-SN light curve of Vend 47 (aka ASASSN-V J195442.95+172212.6), clearly displaying the LPV nature of this object. This is due to the presence of a red giant star (a semi-regular variable more precisely, with a period of about 418 days). Image credit: Jayasinghe, T.; Kochanek, C. S.; Stanek, K. Z.; et al., 2018

Unlike ASAS-SN, the Digitalized Sky Survey (DSS) plates do not indicate any significant changes in brightness, regardless of the filter. The images also don’t indicate any bipolar jets, which are also commonly observed in the case of symbiotic variables. In other words, one would easily overlook the unusual nature of this object based on DSS plates alone (see figure 22)

DSS Hen symbiotic starFigure 22: Coloured DSS2 image extract showing Hen 3-860 and its neighbouring field stars. At first glance, based on these images alone, one would easily overlook the intereseting nature of this “star”. Image credit: DSS2 Aladin Lite

Mo Object 9 and Mo Object 11 – Nebulae Associated with Starforming Regions

Sankalp Mohan (India) recently discovered several new nebulae in online survey images. Among these were Mo Object 9 and 11, located in Circinus and Puppis respectfully. Despite their significantly different appearance in DECaPS imagery (see figure 23), both nebulae are likely of rather similar nature. Indeed, it’s possible that they might be reflection nebulae (at least partially), illuminated by young stars within active star forming regions. Both objects were added to the French database of new nebulae by Pascal Le Dû (France) in November.

Mo_object_9_11Figure 23: Mo Object 9 and Mo Object 11 as seen in DECaPS image extracts. Image credt: DECaPS Aladin Lite

Mo Object 9 is a spherical nebula surrounding a V= 14 mag white star. The nebula does not appear to display any significant Halpha emissions, according to the SupCosmos Survey plates at least. The lack of these indicate that the object shines due to reflection rather than emission. Said otherwise, the central star does not ionize the nebula. Consequently, Mo Object 9 is unlikely to be a Stroemgren sphere. The colour of the central star, as well as its location, are perhaps indicative of a young O or B-type star.

Contrary to the spherical nature of Mo Object 9, Mo Object 11 is a very narrow nebula, seeming to originate from a highly reddened YSO, deeply burried within the surrounding dark nebula. Indeed, the object displays a strong mid-IR component, typical of YSOs. It’s possible that Mo Object 11 is similar to another nebula, IRAS 17079-4032 (see figure 24), which was also [co-]discovered by Sankalp. Unlike IRAS 17079-4032 which is variable in nature, I wasn’t able to demonstrate any variability in the case of Mo Object 11.

comet_nebula_DECaPSFigure 24: DECaPS image extract of  the reflection nebula associated with IRAS 17079-4032. This nebula is perhaps a good analogue with Mo Object 11. Image credit: DECaPS Aladin Lite.

 

References:

Aladin Lite DSS2, DECaPS, SDSS; ESA/NASA SOHO/LASCO C2; NASA/NRL STERE/SECCHI and the Sungrazer Project.

AAVSO’s Variable Star Index: Hen 3-860.

Private communication with Sankalp Mohan, Malhar Kendurkar and Gabriel Muraswki.

Jayasinghe, T.; Kochanek, C. S.; Stanek, K. Z.; et al., 2018, The ASAS-SN
Catalog of Variable Stars I: The Serendipitous Survey

Three Accidental Discoveries Made By Amateur Astronomers!

In this blog post I describe the discovery of three astronomical objects that were discovered by amateur astronomers, accidentally! The three featured objects in this article are of very different nature. Indeed, one being an asterism, and the other two being a nebula and a variable star!

 

Calvet 1 – Asterism

During the spring of 2000, amateur astronomer Cyril Calvet (France) decided to scan the rich star fields located within Cygnus constellation using his monture, including an Achromatic lens. During this particular night, Cyril was hoping to observe some of the various catagloued asterisms located within this region of this sky. Among the asterisms he observed, he came across a vast (30′ long) alignment of stars (between V= +8 mag and +11 mag) in the shape of an inversed interrogation mark (see figure 1). Based on the object’s bright and evident nature, he assumed it was an already catalogued asterism. He made a sketch the object and added it to his catalogue of interesting targets. Since then, he and fellow observers had been continuing to observe it repetively during star parties, still assuming it was a known asterism.

Calvet_1Figure 1: Calvet 1 as imaged by its discoverer. Notice the interesting alignment of this group of stars, making it appear similar to an inversed question mark. (c) Cyril Calvet

It wasn’t until 2006, when Xavier Galliegue (France) independently spotted the asterism, that action was taken in order to identify the asterism. Indeed, after looking through Cyril’s comet sketches, Xavier scanned the sky in Cygnus and eventually stubmled upon the asterism, not realizing it corresponded to Cyril’s object found back in 2000. Once the identification was made, Cyril decided to contact experienced asterism discoverer Alexandre Renou (France) for his help. To his surprise, Alexandre could find no mention of this asterism! Philipp Teutch (Austria) confirmed the star group to be uncatalogued, and hence deicded to add it to the Deep Sky Hunters (DSH) database, under the designation DSH J2105.6+4639, or Calvet 1! Due to the morphology of the asterism, Calvet 1 has often been referred to as the “Question Mark”.

Calvet_1_sketch

Figure 2: Sketch of Calvet 1 made by Cyril in 2006. (c) Cyril Calvet

In images from the Digitalized Sky Survey (see figure 3), the asterism more difficult to distinguish from the background star field. However, once the asterism is spotted, it is difficult to see anything else! 🙂

Note that the most west-maying stars of this asterism are a group of three blue stars, between mag +8 and +10. It is perhaps possible that the two brighter stars are part of a true binary system (based on Gaia DR2 data astrometry), but it is difficult to be sure. Using Gaia DR2, Bruno Alessi (Brazil) found that Calvet 1 could be an open cluster. Note also that Calvet 1 is located only a few degrees north east of the NGC 7000, or the “North America Nebula”!

Calvet_1_DSS_skymapFigure 3: A coloured DSS extract showing Calvet 1. At first glance, the asterism is difficult to spot as it blends easily with the back ground stars in these images. However, when comparing this image with figure 2, it is easily noticeable. Image credit: Skymap.org.

Cyril is an experienced astrophotographer, regularly publishing images of all types of celestial objects, including comets, nebulae, star clusters. More information can be found on his blog: http://astronomiecyrille.over-blog.com/

 

Mul 1 – Planetary Nebula candidate, Possible Strömgren Sphere

Inspired by an image of the “Necklace nebula” (PN G054.2-03.4) captured by the Hubble Space Telescope (HST), amateur astronomer Lionel Mulato (France) decided to image the same object using his own equipment. He started with an Halpha exposure of PN G054.2-03.4, in November of 2013. In the resulting exposure he quickly noticed a faint and diffuse 2′ sized nebula, only 0.5° distant from PN G054.2-03.4. Despite the excitement, Lionel first reasoned that the nebula could be an artefact. However, this was ruled out when he was able to recover the object in plates from the Digitalized Sky Survey!

Mul_1_discovery_imageFigure 4: Extract of the original Halpha + OIII + OIII (HOO) Wide_field image enabling the discovery of Mul 1. Mul 1 is the red smudge to the left, while the Necklace nebula appears in the upper right quadrant. (c) Lionel Mulato.

Now that Lionel knew the nebula was real, he decided to image the object using an [OIII] filter, to test appearance at that wavelength. Interestingly, the object was completely absent in those (see figure 4 and 5)! As Lionel was unable to identify the object in any online catalogue, he contacted Pascal Le Dû (France), who then suggested he should contact Agnès Acker (France) as it could possibly be considered a planetary nebula candidate. Agnès confirmed the nebula to be unknown, and suggested that it might possibly be a Strömgren sphere rather than Planetary nebula, due to the lack of any obvious OIII signal in his images. Despite this, Agnès designated the nebula as Mul 1 and listed it as a possible Planetary Nebula candidate in the French Database of Planetary Nebula discoveries. Wether the nebula is a Strömgren sphere or a Planetary Nebula, no central star of Mul 1 has been discovered, so far.

Mul1 crop HOO 50Figure 5: Crop of the original discovery image centered on Mul 1. The object’s red colour is due to its strong Halpha emissions (while being absent in OIII). (c) Lionel Mulato

Mul 1 is also clearly detectable in Halpha images from the INT/WFC Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS), see figure 6. In fact, the nebula was independently spotted by Sabin et al. (2014) using the IPHAS Halpha exposures. Their team classified the nebula as interstellar matter, and designated the object as IPHASX J194422.5+165605. Mul 1 is also apparent in Mid-Infrared data from the WISE satellite, similarly to many Planetary Nebulae.

MUL_1_IPHASFigure 6: Mul 1 as seen in an Halpha image extract from IPHAS. Image credit: INT/WFC Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS).

Lionel is an experienced astrophotographer, and discoverer of several planetary nebula candidates. He is perhaps most famous for his work on Wolf-Rayet stars. More information on his work can be found on his website: https://sites.google.com/site/lionelmulato/home

 

IRAS 18322-1921 – Mira Variable Star

On June 19th, 2012, amateur astronomer Amar Sharma (India) decided to photograph Pluto to test its appearance in unfiltered images aqcuired using his new CCD camera (model: SBIG ST8XME).  In order to spot Pluto in his images, he compared them with archival Digitalized Sky Survey plates (Red filter), as Pluto is absent in those, at that location. Coincidentally, he noticed a bright transient in his image (CV= +11.5 – 12 mag) that he first mistakened for Pluto. However, once he compared his images with one he took a couple of days later (June 21st, 2012), he realized that what he had found was not Pluto. Indeed, Pluto was in fact 4′ south of this object!

Amar_pluto_mira_discoveryFigure 7: The two unfiltered CCD images taken by Amar Sharma, clearly showing IRAS 18322-1921 (red crosshairs) and the motion of Pluto against the background starfield (green crosshairs). It was these images, along with a Digitalized Sky Survey plate which enabled the discovery of the variability of IRAS 18322-1921. The bright star to the lower-right is HIP 91135 (c) Amar Sharma.

While Pluto had clearly moved during the time both Amar’s photographs were taken, the transient he first spotted had not (see figure 7). It remained just as bright in both his images. Amar hence concluded that he had stumbled upon a deep sky transient, possibly an unknown Nova. He was unable to find any mention of it on the American Association of Variable Star Observers (AAVSO) websites, neither did he find any match within the General Catalogue of Variable Stars (GCVS).

Amar_sharma_Mira_discoveryFigure 8: Image extracts demonstrating the variable nature of IRAS 18322-1921. By comparing an image Amar himself (far right), with one of the Digitalized Sky Survey (far left), Amar discovered the star’s high amplitude variability. (c) Amar Sharma, DSS Plate Finder and the SuperCosmos Halpha Sky Survey.

Amar soon reached out to amateur and professional astronomers worldwide for opinions and follow-up observations in regards to his potential Nova discovery. Alan Hale (USA) visually estimated the transient to be about V= +13.5 mag on June 26th, 2012. Amar also contacted Dan Green, of the The Central Bureau for Astronomical Telegrams, (CBAT), which eventually lead to a mention of the object on the Transient Objects Confirmation Page (TOCP).

The Himalayan Chandra Telescope (HCT) telescope obtained a spectrum by June 27th, in which G. C. Anupama interpeted the spectrum as that of a possibe a Red dwarf star. This  suggested Amar’s transient to be a flare star, undergoing an eruption. She added however that the faible presence of Hydrogen Balmer lines (Halpha emissions, for instance) in the spectra could be in favour of a Mira star. The latter classification was confirmed by Dan Green and Brian Skiff (CBAT) based on the ASAS-3 light curve of the object (see figure 2). They also confirmed the variable star to be unreported. The star has been designated IRAS 18322-1921, and is the first modern-time astronomical discovery made in India!ASAS-3_amar_starFigure 9: The light curve which enabled to confirm the Long Period Variable (LPV) nature of IRAS 18322-1921. Image credit; The All Sky Automated Survey III (ASAS-3).

Indeed, the data indicated that the star displayed periodic high-amplitude variations in brightness, with a period of 270 days. The data also indicated a maximum visual magnitude of V= +12.6 mag, and a minimum below V= +14.5 mag (see figure 2). Indeed, the object appeared between R= +16 mag and R= +17 mag on the DSS Red plate, probably meaning that the DSS plate shows the star near its minimum brightness.

Amar ASASSN lightcurveFigure 10: ASAS-SN light curve of IRAS 18322-1921. Image credit: Jayasinghe et al. 2018.

In 2018, the results from an automatic search for variable stars performed by the ASAS-SN team was published, in which IRAS 18322-1921 had been detected too. ASAS-SN data suggests that the star has a pulsation period of 280 days, rather than 270.

 

Acknowledgement

I wish to thank Amar Sharma and Cyril Calvet for the information they provided on IRAS 18322-1921 and Calvet 1, respetively.

 

References

Cyril Calvet, Nouvel astérisme dans le Cygne, Available at: http://www.astreschevelus.fr/articles.php?lng=fr&pg=67

Lionel Mulato, Découverte d’un nouvel objet inconnu : Mul 1, Available at: https://sites.google.com/site/lionelmulato/Imagerie-CCD/mu1

Amercan Association of Variable Star Observers (AAVSO) and The Variable Star Index (VSX).

Private communication with Amar Sharma and Cyrille Calvet.

More New Nebulae Discovered Online By Amateur Astronomers!

Over the course of 2018, amateur astronomers have continued scrutinizing the online survey images in search for previously unknown nebulae, in particular Planetary Nebulae (PNe). Many of these discoveries (if not most) were made possible due to the high-resolution DECam Plane Survey (DECaPS) imagery, which was only made available to the public this year. While many of these discoveries have been kept as Planetary Nebula candidates, others are more likely to be Reflection Nebulae, globules or HII regions. Furthermore, some even appear to not be nebulae at all. Indeed, galaxies and highly active Young Stellar Objects (YSOs) are also likely included in the mix!

In this post I feature only a fraction of the numerous objects that were just recently added to the French database of PN candidates (maintained by Pascal Le Dû, France). All the active YSOs have been added to AAVSO’s Variable Star Index (VSX). The database also includes a sublist dedicated specifically to other kinds of nebulae, rather than PNe.

Note: This blog post discusses and characterizes the appearance of Planetary Nebula candidates (and PN mimics) in online survey images (such as the SuperCosmos Halpha Survey images, WISE and Spitzer). Hence, I recommend reading the “Hunting for Planetary Nebulae guide” of this website, for better understanding of this article! 🙂  https://skyhuntblog.wordpress.com/planetary-nebulae/personal-method-and-discoveries/

 

Small And/Or Compact PN Candidates

These are examples of Planetary Nebula candidates with small apparent dimensions. None of the below examples objects exceed 25″ in diameter. The largest one among this selection is Mo 16, measuring about 20″ across. Many of these candidates are likely to be true PNe, but only spectroscopy can confirm their true nature.

Mo 16 is an interesting Bipolar Planetary Nebula candidate discovered by Sankalp Mohan (India) while searching Aladin Lite’s coloured Spitzer images. Indeed, he states “While observing an area of 22.1′ Fov in Spitzer, I noticed a small fuzzy-greenish patch. As I zoomed-in, I realized something about that patch which made my heart melt: A Bipolar morphology, similar to Mo 1 and Mo 3!” (see figure 1). Planetary Nebulae often appear to have a recurrent green tint in Aladin Lite’s colour-coded Spitzer images. Sankalp continues by saying “I took a deep breathe, copied the coordinates [of the nebula] and searched for it in Simbad. Fortunately the search came up negative, so I decided to […] send it to Mr. Pascal Le Dû for further analysis.” Mo 1 and 16 spitzerFigure 1: Planetary Nebula candidates Mo 1 and  Mo 16 as seen in colour-coded Mid-IR images from Spitzer. Note the obvious “green” tint, typical of PNe. One can also distinguish a possible bipolar morphology in both these objects (a bit more obvious in the case of Mo 16). Image credit: IRAC/Spitzer Aladin Lite.

Mo 16 also displays a Mid-IR WISE signature typical of most PNe (essentially emitting in the WISE W4 band) and appears faintly in the SHS Halpha plate, while being absent in all other optical survey images (likely due to Halpha emissions, which are typical of PNe). Mo 16 is a strongly obcured object, as it is hidden behind the optically opaque dust making up the Barnard 58 dark nebula.

Tan 5 (discovered by Hanjie Tan, China) is similar to Mo 16 as it seems to be a highly obscured bipolar PN (see figure 4). Furthermore, Tan 5 is practically invisible in optical survey images, but it does however emit well in Halpha (according to the SHS plates). Dana Patchick (USA) notes that Tan 5 is only located 18.7′ NW of Hadar (Beta Centauri).

Mur 1 is Gabriel Murawski’s (Poland) first PN candidate discovery. He spotted it in early July 2018 using DECaPS and WISE imagery. The object appears to have the typical Mid-IR signature of a PN in WISE (see figure 2), and displays an annular morphology in DECaPS (see figure 4). However, based on its colours in DECaPS, it could be possible that Mur 1 is a distant active galaxy rather than a true PN. Mur 1 wiseFigure 2: Individual WISE filter extracts centered on Mur 1. Notice how it appears much brighter in the W3 and W4 filters versus W1 and W2. This is a very typical characteristic of PNe. Image credit: WISE/IRSA.

Pre 42 is a small elliptical PN candidate I first spotted in DECaPS imagery. The object also seems to emit significantly in Halpha, according to plates from the SuperCosmos Halpha Sky Survey (SHS, see figure 3). Pre 42 appears only very faintly in WISE. Pre 48 is possibly an object similar to Pre 42, except that it displays a much more typical PN-like WISE signature and a bluer optical counterpart (as well as being a bit more compact, see figure 4).

Pre 42 shsFigure 3: Pre 42 as seen in the SuperCosmos Halpha Sky Survey (SHS) plates. Note how the nebula appears very clearly on the Halpha plate (very typical of PNe, which often are accompanied by Halpha emissions). Image credit: UKST/SHS.

SuFe 1 is another similar nebula, but appears to have a more defined spherical morphology. SuFe 1 was discovered independently by Guoyou Sun (China) and Laurent Ferrero (France). The nebula displays the typical WISE mid-IR signature, as well as an optical-NIR colorimetry similar to many (if not most) Planetary Nebulae (green colour in Pan-STARRS1 g+r+i images, as seen in figure 4).

Mur _1_et_alFigure 4: Six recently discovered Planetary Nebula candidates found in online survey images. Notice the obvious difference in colours and morphology. The Mo 16 and Tan 5 are both possibly bipolar and highly reddened due to  interstellar dust. Mur 1 appears annular, and might be a galaxy. Pre 42, Pre 48 and SuFe 1 all display typical optical colours. Image credit: DECaPS/Aladin Lite and the Pan-STARRS1 Science Consortium.

 

Large PN Candidates And Other Similar Nebulae

The nebulae listed here have much larger dimensions than the ones listed previously. Indeed, most have diameters superior to 1′. Despite most of these nebulae sharing many similarities to PNe (especially in the Mid-IR), many are likely to be reflection nebulae and/or associated to star forming regions.

Both Mo 10 and PreMo Object 1 (discovered by Sankalp Mohan and myself) possibly appear to be reflection nebulae. While Mo 10  is associated with a single star, PreMo Object 1 appears to be associated with an entire (young) Open Cluster (Prestgard 115, see figure 6). PreMo Object 1 is also likely linked to the possible HII region WISE G254.160-02.889, which displays a strong WISE signal similar to many Planetary Nebulae (making PreMo Object 1 a rather interesting object). Mo 10 is more difficult to rule out as a PN. Indeed, its blue colour in DECaPS, its central star and PN-like WISE signal are very much characteristics one would expect from a PN. Only a spectra can determine its true nature.WISE IRSA MO 13 18Figure 5: Individual WISE filter extracts centered on Mo 13 and Mo 18. Notice how both nebulae much brighter in the W3 and W4 filters versus W1 and W2. This is a very typical characteristic of PNe. Image credit: WISE/IRSA.

It seems possible that Mo 18 could be a reflection nebula too. Indeed, its brightness may have been significantly enhanced due to the presence of a central white star, which could potentially be the reason that Mo 18 is detectable in optical images at all! Mo Object 4 is an obvious dark nebula/globule, as one can clearly be seen in figure 6. Mo 13 appears to possibly be globule/dark nebula based on its DECaPS colours. However, its WISE signal is similar to many PNe (see figure 5), hence it cannot be ruled out as a PN. Mur Object 1 (another recent discovery by Gabriel Murawski) could potentially be an HII region, or nebulosity associated with a YSO (perhaps similar to IRAS 17079-4032 and/or Fe Object 1, see figure 10).

Mo18_PreMo1_MurObject1Figure 6: DECaPS image extracs of Mo 10, Mo 13, Mo 18, Mo object 4 and PreMo Object 1. While the former four objects are classified as possible PN candidates, it is also possible that they may actually be reflection nebulae and/or globules. Mo Object 4 is a beautiful dark nebula/globule. Mur Object 1 and is an interesting nebula that might be an HII region or something associated to YSO. Image credit: DECaPS Aladin Lite.

 

Young Stellar Objects and associated nebulae

IRAS 17079-4032 is a very interesting variable nebula in Scorpius, displaying a beautiful cometary morphology in DECaPS (see figure 10). Indeed, as Sankalp first comically told me over Facebook when spotting this object: “A Comet, Journey of a Star into Black Hole XD”. At the time, we didn’t realize that the object was uncatalogued. It wasn’t until I was composing this blog post that I realized the nebula had no actual reference! Furthemore, I discovered that it was a highly variable in nature (see figure 6). Indeed, the nebula is likely a reflection nebula (similar to Hind’s and McNeil’s variable nebula) surrounding a highly active YSO. IRAS 17079-4032 is located only 4′ west of Mo 16, which is how Sankalp Mohan first spotted it.

IRAS 17079-4032Figure 7: DSS Plate Comparison demonstrating the highly variable nature of IRAS 17079-4032. Image credit: DSS Plate Finder.

Fe Object 1 was discovered by Laurent Ferrero (France) in SDSS images. It is possible that it is an object similar to IRAS 17079-4032 and Mur Object 1. Indeed, The nebula seems to stem from a single star (likely a YSO). In the case of Fe Object 1, the nebula has a rather annular morphology (see figure 10), giving the visual impression of bipolar jets streaming from the core (proto-)star. Unlike IRAS 17079-4032 however, no obvious variability could be detected, even in ASAS-SN and NSVS data!

Patchick YSOFigure 8: DSS Plate Comparison demonstrating the highly variable nature of 2MASS J14553751-5728023. Image credit: DSS Plate Finder.

Unlike the two previous examples, both 2MASS J14553751-5728023 and IRAS 18044-2420 are not associated with any obvious nebulae. The former was first spotted by Dana Patchick (USA) due to its Mid-IR WISE signature similar to many PNe. At first glance it ressembles a stellar PN (see figure 10). However, when studying archive images from the Digitalized Sky Survey (DSS), one could clearly detect obvious variability in its brightness, typical of an active YSO (see figure 8).

IRAS 18 YSO trygveFigure 9: Individual WISE filter extracts centered on IRAS 18044-2420. Notice how it appears much brighter in the W3 and W4 filters versus W1 and W2. This is a very typical characteristic of PNe. Image credit: WISE/IRSA.

IRAS 18044-2420 appears to be a similar object to 2MASS J14553751-5728023. I first reported it in December, 2017 as a possible stellar PN candidate (based on its PN-like WISE signal and PN-like colours in Pan-STARRS1, see figure 9 and 10). It wasn’t until August, 2018 that I realized it displayed a high-amplitude variable nature, similar to a 2MASS J14553751-5728023.

YSO_Feobject1Figure 10: 2MASS J14553751-5728023, IRAS 17079-4032, IRAS 18044-2420, Fe Object 1, as seen in DECaPS, Pan-STARRS1 and SDSS respectively. Note the cometary morphology of IRAS 17079-4032 and the annular nature of Fe Object 1. Image credit: SDSS DECaPS Aladin Lite and the Pan-STARRS1 Science Consortium.

 

Acknowledgements

I wish to thank Sankalp Mohan for sharing details on his discovery of Mo 16 and IRAS 17079-4032, Dana Patchick for his help searching the HASH database, and (last but not least) Pascal Le Dû for his continous analysis of our discoveries.

 

References

AAVSO: The Variable Star Index (VSX).
Aladin Lite and SIMBAD – Université de Strasbourg
Ferrero, L. (2018) Mes découvertes officielles, Available at:
Private communication with Dana Patchick and Sankalp Mohan.

The story behind Nova Lupi 2018!

After only five months since his first discovery, Australian amateur astronomer Rob Kaufman discovered yet another Nova! As its name suggests, the eruption took (is taking) place in the Lupus constellation, located within a 10° radius of several naked-eye stars, including η Normae, ε Lupi and μ Lupi, as well as the bright Globular cluster NGC 5927! Rob discovered the Nova on the night of June 3rd, 2018, when it was near peak brightness (visual magnitude of +9 mag)! Within the following day, photometric and low resolution spectroscopic observations were collected, suggesting that Rob’s transient was most to likely be a Nova. The suspicions were confirmed only a couple of days later based on high resolution spectroscopy.

34383482_1683551908388710_3574476353023508480_nFigure 1: Nova Lupi 2018 as imaged by Rob Kaufman soon after its discovery. Notice how the object seems to display a slight orange tint. The redder appearance of Novae is a reliable way of distinguishing them from other more commonly observed Cataclysmic variables, such as Dwarf novae. The latter usually display much bluer colours. The bright star near the lower limit of the image is HD 139559. (c) Rob Kaufman.

Rob Kaufman discovered the Nova during his holidays at White Cliffs, a small and isolated outback town deep within the Australian mainland, known for its prosperous opal mining during the 1890s and early 20th century. The poorly light-polluted sky  makes it an ideal location for observations!

Rob has been a dedicated Nova hunter since 2010, but it was only last January that he made his first discovery, Nova Muscae 2018! I describe this Nova in an older blog post I wrote on January 28th, 2018: https://skyhuntblog.wordpress.com/2018/01/28/nova-muscae-2018-the-story/ ). His search method consists of imaging wide-field sections of the southern Milky Way (see figure 2), using  a Canon DSLR (EOS 650D) together with a 55mm F/5.6 lens, as well as a Vixen Polarie monture for star tracking. He later blinks these images with earlier ones taken of the same field using Adobe Photoshop, with the hope of spotting new “stars” that don’t appear in his earlier images. These are indicative of bright transients, such as a Novae.

NMus2018 widefield, 16 Jan 2018 textFigure 2: Stacked wide-field image centered on the Carina, Crux and Musca constellations. It was this image which enabled the discovery of Nova Musca 2018 on January 14th, 2018! (c) Rob Kaufman.

During his holidays on June 3rd, inbetween the presence of the full Moon, Rob imaged two wide-field sections, one centered on the Centaurus constellation, the other on Norma. Back inside, he compared his images with archival ones he took on May 20th. First he compared the one centered on Centaurus, finding no significative difference between the two. However, when comparing the one centered on Norma, he noticed a a potential candidate (see figure 3)! “Hmmm, I’ve got something here ” he told his partner once he noticed the object.

Rob explains that blinking objects are not uncommon to stumble upon during Nova hunting as many “impostors” may mimic the appearance of cataclysmic variables, at first glance. These “impostors” include Minor planets, periodic high-amplitude variable stars (Mira or Eclipsing Binaries for example), and even cosmic rays! The latter was ruled out, as the transient appeared in all the five exposures making up his image. Next he used the Guide planetarium program and the AAVSO/VSX catalog to try to identify the transient, but nothing showed up at the positions of the object. A search via the MPChecker revealed no known Minor planets at the location. Furthermore, he found that the object did not correspond to any known cataclysmic variable.

PNV J15384000-4744500 discovery imageFigure 3: Cropped version of the original stacked wide-field discovery image of Nova Lupii 2018. The nova is clearly visible, despite not appearing any different from the surrounding field stars. (c) Rob Kaufman

Excitement was growing as the realisation that this was a reportable object sunk in” Rob states. Unfortunaly however, the transient was too faint to be studied via his low resolution spectrometer, hence he could not confirm the nature of the object as being a true Nova. Nevertheless, the next step was to report the transient to the Central Bureau for Astronomical Telegrams (CBAT). Unfortunately again, he first encountered difficulties when trying to submit his report. “I carefully typed my report in the format required, taking great care with the spacings necessary, and hit the Submit button. And nothing happened!!” He continues by saying “A moment of panic, then I adjusted a few spacings, hit Submit again and a green banner saying “Report received” came up. Phew!

He then proceeded by posting a notification of this object on the PACA_Transients facebook group, where people such as Patrick Schmeer (Germany) and Seiichiro Kiyota (Japan) took immediate action! Indeed, Seiichiro was the first to take a confirmation image the transient. He also provided a low resolution spectra, along with a Visible-NIR photometric magnitudes. Unfortunately the spectra was inconclusive, but the colours determined by photometry indicated a relatively “red” appearence, typical of Novae. It is possible that the spectra showed very weak Halpha emissions, but these were too close to the background noise level to be confirmed. Rob took a few more images of the transient under Moon light, stating that it appeared “pinkish under high-saturation” which was also a good sign! Notice how the reddish colour is also apparent in figure 1. During this time the transient was designated PNV J15384000-4744500.

High resolution spectra obtained by Fred Walter (Stony Brook University) via the SMARTS and SALT telescopes confirmed the transient to indeed be a true Nova, only two days after Rob first spotted it! More precisely, the spectra indicated the object to be an optically thick classical Fe-II type Nova near peak brightness, at the time of discovery. Swift observations failed to detect the Nova in X-Rays, but it does however appear obvious in the ultraviolet. The confirmation of the Nova lead it to be officially designated as Nova Lupi 2018!  At the time of this post (June 13th, 2018), the Nova has a visual magnitude of about +10 according to AAVSO.

References

Private communication with Rob Kaufman, AAVSO VSX, AAVSO Light Curve, PACA_Transient facebook group, ATel #11681 and ATel #11693.

STEREO’s spectacular view of the sky!

Since mid-April, the STEREO-A spacecraft (more specifically its Heliopsheric imagers, HI) has been exposed to a spectacular view of the Sky! Indeed, not only is it observing a good portion of the Solar System’s planets, but the telescopes are pointed towards the heart of the Milky Way! In this blog post I dive into detail regarding the objects that STEREO-Ahead’s HI instruments are currently observing (planets and deep sky objects).

Remark: This blog post was originally intended to be published a week ago, at the time when STEREO/SECCHI images from May 6th were the most recent. Since then, many more images have appeared! To keep this post “up to date”, I’ve consequently added a few extra figures with STEREO/SECCHI images taken later than May 6th (the latest of them being from May 12th). These newer figures are marked with an asterisk “*”. 

Below are individual STEREO/SECCHI HI1-A and HI2-A images taken on May 6th, 2018. As one can see, the Milky way’s galactic plane, along with several planets are clearly visible, even prominent!

20180506_180901_tbh1AFigure 1: Beautiful view of the Solar System and the Milky Way as seen by the HI1-A telescope. The vertical streaks associated with the planets are caused by saturation (pixel bleading). Image credit: NASA/NRL STEREO/SECCHI HI1-A.

20180506_180921_tbh2AFigure 2 Beautiful view of the Solar System and the Milky Way as seen by the HI2-A telescope. The vertical streaks associated with the planets (and one star) are caused by saturation (pixel bleading). Image credit: NASA/NRL STEREO/SECCHI HI1-A.

The relative motion of the planets as seen by STEREO HI

Over the course of April until mid-May, six planets transited the HI FOV. These include all the inner planets (Mercury, Venus, Earth and Mars), as well as Jupiter and Saturn. This does not include Neptune, which was only briefly visible in rolled STEREO data (from May 8th). Their brightness and/or motions differ drastically due to their individual orbital periods, as well as their relative distance from the STEREO-A spacecraft. GIFMaker.org_FhJTCIFigure 3: Timelapse showing the relative motion of the planets as seen from the STEREO-A spacecraft, with the heart of the Milky Way laying in the background. Notice the solar corona on the left side of the images. Image credit: NASA/NRL STEREO/SECCHI HI1-A.20180506_002400_dbc2AFigure 4: STEREO/SECCHI COR2-A image showing Mercury. Pluto is also located in the FOV, but its brightness is way below the limiting magnitude of the COR2-A instrument to be detected. Notice the Solar corona radiating from behind the occulting disk. Image credit: NASA/NRL STEREO/SECCHI COR2-A.

As can be seen in figure 3, 5 and 6, Mercury rapidly arcs around the Sun (leaving the HI1-A FOV in late April) and heads into the COR2-A FOV by May (see figure 4). In comparison to the other five planets, Mercury has a rapid motion, while also moving in the opposite direction. This is caused by the planet’s significantly shorter orbital period in comparison to that of the STEREO-A spacecraft (88 days versus 346 days), as well as its location near the opposite side of the Sun relative to STEREO-A.

Despite Venus having a relatively similar velocity to STEREO-A (orbital period of 243 days), its apparent motion is very rapid due to the planet’s proximity with the spacecraft. It enters the HI1-A FOV in late May, and then the HI2-A FOV only a week later.

Earth only appears in the HI2-A FOV, with no apparent motion. This is due to the spacecraft having been purposefully placed in an orbit quasi-identical to it (orbital period: 365 days). Earth appears significantly fainter than Venus, due to its greater distance from STEREO-A.

Despite being larger than Mercury, Mars appears alot fainter due to its greater relative distance from the spacecraft. Its slow motion relative to the apparent motion of the other planets is due to its rather similar orbital period to STEREO-A (only about twice as long).output_OkvkCKFigure 5: The motion of the inner planets and the STEREO spacecraft relative to Earth. Knowing that STEREO-A and STEREO-B moves practically at the same velocity as Earth, one can interpret this as being the relative motion of the planets as seen from both STEREO-A and STEREO-B. Image credit: NASA/SSC STEREO Orbit tool

With Jupiter and Saturn being located much further away from the spacecraft relative to the inner planets, their apparent brightness does not [at all] reflect their intrinsic luminosity. Indeed, Jupiter appears similar to Earth, while Saturn‘s brightness is comparable to that of Mars! The rapid motion of both these planets reflects their great distance from the spacecraft, relative to the inner planets. In fact, Saturn has such a large distance from the spacecraft that its relative motion is comparable to that of the background star field! output_nsXy80Figure 6: The motion of the outer planets and the STEREO spacecraft relative to Earth. Knowing that STEREO-A and STEREO-B moves practically at the same velocity as Earth, one can interpret this as being the relative motion of the planets as seen from both STEREO-A and STEREO-B. Image credit: NASA/SSC STEREO Orbit tool

By combining the HI1 and HI2 images, one can create a “family portait” of the different planets visible in those images. My attempt of creating such an image is shown in figure 7a. Note that the planets all show vertical lines, due to saturation. The star Antares (in the HI2 FOV) is the only non-planetary body bright enough to saturate the HI cameras (which is currently visible in STEREO/SECCHI). An updated version of this “portrait” is shown in figure 7b.STEREO solar system may 2018Figure 7a: Image extract from the composite figure 8. It’s an HI “Family portait” of the Solar system, the the Milky Way‘s galactic plane in the background! Mercury is located in the COR2-A FOV (figure 4). Uranus and Neptune are located outside SECCHI FOV. Image credit: NASA/NRL STEREO/SECCHI HI-A and Trygve Prestgard.family portrait STEREO HI2_AFigure 7b*: Update: This is an HI2-A image extract from May 12th (latest available high-quality HI2 image at the time of this blog post). At this date all the five planets shown in figure 7a had entered the HI2-A FOV. Note that this is an exctract from figure 2b. Image credit: NASA/NRL STEREO/SECCHI HI2-A.

Neptune HI2AFigure 8*: The faint presence of Neptune in rolled STEREO data from May 8th. Image credit: NASA/NRL STEREO/SECCHI HI2-A.

 

 

STEREO’s current view of the Milky Way

Perhaps more spectacular than the six planets described above is the is Milky Way‘s star-rich galactic plane and dusty star-forming regions! Indeed, the heart of our home galaxy covers a significant portion of the HI FOV, as one can see in figure 9! Figure 10 is a combination of image extracts I put together from the Mellinger survey, meant for comparison with figure 9.stereo sky may 2018Figure 9: Combined and enhanced HI1-A and HI2-A images meant to bring out the Milky Way. Image credit: NASA/NRL STEREO/SECCHI HI-A and Trygve Prestgard

mellinger-milky-way-stereo-fov.pngFigure 10: Composite images from the Mellinger survey of the same region currently observed by STEREO/SECCHI HI. Image credit: Mellinger Aladin Lite.

Some Deep sky objects currently visible in STEREO

When looking closer at the Milky Way, other then the million stars, one can distinguish some quite obvious and interesting deep sky objects, including star clusters and nebulae. Figure 11 shows some examples of such objects.deep sky objects stereo april may 2018Figure 11*: Combined image showing several Deep Sky Objects, including Antares (and its neighbouring nebulae), the dark nebula complex that includes LDN 52, Dobashi 47 and 48 (among many other!), the active star forming regions M8 and M20, and the visual asterism Ferrero 2. Image credit: NASA/NRL STEREO/SECCHI HI and Trygve Prestgard.

Figure 11, 1 is an HI2-A image extract from May 12th showing the Rho Ophiuchus complex. This is an immense region of star formation, hence it includes numerous HII regions and dark nebulae. The latter are clearly visible in this HI2 image extract,  just north of Antares. Antares is one of the brightest objects in the night sky, with a visual magnitude +1 (but reaching negative magnitudes in the near-Infrared!). The star’s brightness is such that it saturates the STEREO HI detectors, causing vertical “spikes” to appear ( due to pixel bleeding). In figure 11, 1 one can also see the reflection nebula asociated to this star (faint “cloud” surrounding Antares).

Another complex system of dark nebulae, similar to the ones observed in Figure 11, 1, can be seen in Figure 11, 2. This vast cluster of dark nebulae include LDN 52, Dobashi 47, Dobashi 48, among many others!

M8 (Lagoon Nebula) and M20 (Trifid Nebula) are a couple of bright star forming regions in the Sagittarius constellation. Both of these nebulae have been known for centuries, and are faintly visible to the naked eye (best seen in the Southern Hemisphere). In STEREO HI their nebulous nature is obvious, as one can see in Figure 11, 3!

Ferrero 2 was discovered in the early 2000s by amateur astronomer Laurent Ferrero (France) using a 60mm refractor. The brightest stars of this asterism are just below naked eye visiblity, hence in STEREO-A HI they are very obvious! As one can see in figure 11, 4, it’s a very well defined group of stars, in the shape of an irregular polygon.

The Small Magellanic Clouds (SMC) and Large Magellanic Cloud (LMC) are the two dwarf satellite galaxies of the Milky Way. The former is an irregular galaxy while the latter is a disrupted barred spiral galaxy. Both these galaxies are visible to the naked eye, and hence, they popular visual targets (in the Southern Hemisphere). During the roll of the STEREO spacecraft on May 8th, the HI2 camera was briefly pointed towards these objects. STEREO (accidentally) caught two images of them (one of those being figure 12).

20180508_114720_tbh2AFigure 12*: HI2-A image taken during a STEREO “roll”. Note the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC). These correspond to the fuzzy patches on the right side of the FOV (SMC to the left, and LMC to the right). The galactic plane of the Milky Way  is partially visible at the very top of the FOV.

References

NASA/NRL STEREO/SECCHI (2018) Javascript movie tool, Available at: https://secchi.nrl.navy.mil/index.php?p=js_secchi

Wikipedia (2018) STEREO, Available at: https://en.wikipedia.org/wiki/STEREO

SIMBAD (2018) Antares, Available at: http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Antares&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id

Wikipedia (2018) Lagoon Nebula, Available at: https://en.wikipedia.org/wiki/Lagoon_Nebula

Wikipedia (2018) Trifid Nebula, Available at: https://en.wikipedia.org/wiki/Trifid_Nebula

Ferrero, L. (2018) Mon catalogue d’amas d’étoiles, Available at: http://splendeursducielprofond.eklablog.fr/mon-catalogue-d-amas-d-etoiles-p563496

STEREO spacecraft discovers its 100th comet, and many others!

On January 25th, 2018, Masanori Uchina (Japan) reported a small Kreutz-group comet in images from the STEREO-A spacecraft (see animation below). Not only was this Masanori’s first STEREO comet discovery, but it also coincidentally turned out to be STEREO’s 100th find! Masanori discovered his second STEREO comet only 17 days later, on February 9th, 2018 (STEREO-105)! STEREO-100 and STEREO-105 were both only two comets of a total of twenty-two recorded Kreutz-group sungrazing comets between January and mid-February 2018. Note that eight of them were observed over a period of only ten days, these all having been found (and solely observed) by STEREO!

output_waJXBcCropped animation of enhanced STEREO/SECCHI HI1-A images showing STEREO-100. Notice how this fuzzy comet brightens as it exits the animation (FOV). These were the last images of the comet before it disentigrated. Image credit: NASA/NRL STEREO/SECCHI HI1-A

 

STEREO-100: The story behind its discovery

Masanori Uchina spotted this comet on January 25th, 2018 when hunting the near-real time STEREO/SECCHI HI1-A images for Kreutz-group comets. Indeed, between November 2017 and March 2018 the STEREO-A spacecraft was well placed in space to observe the “Kreutz-group stream”, some coin this period the “[STEREO] Kreutz season”. Note that STEREO/SECCHI data is not quite real time (unlike SOHO/LASCO), as it takes a longer time to downlink the data from the spacecraft (2-3 days in average), hence the term “near-real time”. Indeed, Masanori found this comet in images taken on January 23rd. He reported his discovery to the STEREOHunters Yahoo Group forum shortly after spotting the comet (see his report below).

STEREO 100 report

Original report for STEREO-100 and my (unofficial) confirmation message posted some moments later. The official confirmation was done after the end of the “STEREO Kreutz season” in April 2018 by Karl Battams (USA NRL). Image credit: STEREOHunters Yahoo chat group forum.

Consequently, as STEREO/SECCHI data is not real time, this means that one must make sure that any possible “new” STEREO comets haven’t first been spotted in SOHO/LASCO. Luckily there were no prior mentions of any Kreutz-group comets that alligned with Masanori’s discovery, hence his find was a definite “STEREO discovery”! The comet most likely went unnoticed in SOHO/LASCO due to its apparent faintness and due to the SOHO/LASCO occulting pylon (see further below for more details).

Like all small Kreutz-group sungrazers, STEREO-100 most likely disentigrated before reaching perihelion. I say “most likely” as there are no actual images showing the comet disentigrating (even though it’s obvious in the case of such small comets). In fact, the last (and only) images of this comet show it to still be brightening! (see animation of STEREO-100 above). This is because the STEREO/SECCHI HI instruments do not observe the Sun and its immediate surroundings. In fact, the FOV focuses on the Solar Outflow beyond  a 15° offset from the Sun. Kreutz-group comets in average start disentigrating at about 10° (I think…). Hence, it’s the SOHO/LASCO and STEREO/SECCHI COR telescopes that usually get to observe the disentigration. However, as mentioned, STEREO-100 was too faint be observed by SOHO, thus making it even less likely to have been observed by the COR telescopes! Indeed, COR is poorly sensitive to comets, only brighter ones (that already appear obvious in SOHO) can be detected in these (see COR2-A animation of SOHO-3492 further below).

output_laVUlACropped animation of enhanced SOHO/LASCO C2 images showing the disentigration Kreutz-group comet SOHO-2465. These were the last images of the comet. Image credit: ESA/NASA/NRL SOHO/LASCO C2.

On April 16th, 2018, long after the “STEREO Kreutz season” was over, Karl Battams posted the official confirmations of the most recent STEREO comets, in which he stated that Masanori’s comet was STEREO’s 100th comet discovery! “Many congratulations to Masanori for finding STEREO’s 100th comet!“.

 

Backstory: STEREO’s comets

STEREO’s 100th comet discovery is the result of eleven years of intense scrutinizing of the STEREO/SECCHI images, by both professional and amateur astronomers. The first ever STEREO comet discovery was made by amateur astronomer Alan Watson (Australia) in STEREO/SECCHI HI1-B images of 2007-08-31 and 2007-09-01. The comet is unintuitively designated STEREO-65.

stereo 65STEREO-65 as seen in a strongly cropped STEREO/SECCHI HI1-B image from 2007-09-01. This was one of the last (and only) images of this comet. It was most likely too faint to be recovered in SOHO/LASCO. Image credit: NASA/NRL STEREO/SECCHI HI1-B.

This small Kreutz-group comet appeared very similar to STEREO-100, and was only officially confirmed a couple years ago, when re-inspecting the HI1-B images (hence its designation). At the time of its discovery, astronomers were still getting to know and “figuring out” STEREO/SECCHI. Consequently, because of the comet’s faintness, there was not enough evidence to support its existence (especially as the processing methods used back then were less adapted to the data). The first confirmed STEREO comet find, STEREO-1, was discovered quite a bit later, on 2008-02-19 (also by Alan Watson)! STEREO-1 was followed by STEREO-2, STEREO-3 and STEREO-4 only days later! All these four comets were of the Kreutz-group, like about 95% of STEREO’s comets! The remaining 5% are non-group comets, like STEREO-61 and STEREO-88 (see images below).

soulier C2014 C2Comet STEREO-61 imaged on 2014-03-02. (c) Jean-François Soulier

output_7GBRe9Animation of comet STEREO-88 in STEREO/SECCHI HI1-A images arcing around the Sun. The comet is seen in the upper-left quadrant of these images. These were the last images of this comet. Image credit: NASA/NRL STEREO/SECCHI HI1-A.

 

January 2018: A flurry of faint Kreutz-group sungrazing comets!

As mentioned previously, STEREO-100 was far from being the only STEREO comet discovery of 2018. In fact, a total of fourteen Kreutz-group STEREO comets were discovered between January 6th and February 9th, with eight of these (including STEREO-100) having appeared between January 13th and January 23rd! Below are animations of a couple comets that appeared during this mild “comet storm”. As one can see in these GIFs, the comets were indeed faint. It is mainly due to this reason that they went unnoticed SOHO/LASCO.

output_I1pw3RCropped animation of enhanced STEREO/SECCHI HI1-A images showing STEREO-94. These were the last images of the comet before it disentigrated. Image credit: NASA/NRL STEREO/SECCHI HI1-A

output_UYsCmwCropped animation of enhanced STEREO/SECCHI HI1-A images showing STEREO-96. These were the last images of the comet before it disentigrated. Image credit: NASA/NRL STEREO/SECCHI HI1-A

A couple weeks after this small flurry of Kreutz-group sungrazers, a very bright comet (about mag +2) of the same family transited the STEREO/SECCHI and SOHO/LASCO FOVs. More about this comet (SOHO-3492) can be found in a previous blog post I wrote back in February: https://skyhuntblog.wordpress.com/2018/02/19/bright-sungrazer-observed-by-stereo/

output_MXkQVJThe bright Kreutz-group comet SOHO-3492 as seen in STEREO/SECCHI COR2-A images of February 9th, 2018. Image credit: NASA/NRL STEREO/SECCHI COR2-A.Note that

 

January 2018: Why did STEREO suddenly discover so many Kreutz-group comets?

Interestingly, despite a similar number of Kreutz comets having been observed in December 2017 and January 2018, the proportion of these found either and SOHO or STEREO vary drastically! Indeed, only 13% of the 17 Kreutz comets observed in December were discovered in STEREO images. However, this proportion increased  to 75% in January!

December 2017 January 2018
SOHO Kreutz-group comets 15 4
STEREO Kreutz-group comets 2 12
Total  17 16

Table displaying the number of Kreutz-group comets discovered individually by SOHO and STEREO during December 2017 and January 2018.

This sudden and drastic increase of the STEREO discovery rate has nothing to do with the location or properties of the STEREO spacecraft. In fact, it is an effect almost enterily related to SOHO! The reason starts with the geometry of the Kreutz-group comet stream as seen from SOHO during December and January. Over the course of the year, as SOHO orbits the Sun, it observes the “Kreutz stream” from different angles, meaning that the geometry of the trajectories of Kreutz comets over the course of the year changes as seen from SOHO. These changes are already obvious from one month to another. In the case of December to January, the “Kreutz stream” moves East in the sky, further away from the SOHO/LASCO C2 FOV (see the schematic representation below). This impacts the amount of comets observed by SOHO, as the SOHO/LASCO C3 FOV has a much lower resolution than C2. Indeed, practically all Kreutz-group comet discovered in SOHO/LASCO in December are found in C2! This is because most Kreutz comets are infact too faint to show up in C3, due to the telescope’s resolution.  On top of all that, the Kreutz stream in January was mostly masked by the Occulting pylon (the device holding up the SOHO/LASCO Coronagraph). Unlike SOHO, the apparent geometry of the Kreutz stream as seen from STEREO practically didn’t change between December and January, hence it had a very good view of the comets that SOHO was missing out on! Especially as the STEREO/SECCHI HI1-A images can observe comets just as faint as those solely visible in C2!

Kreutz comets december januarySchematic representation of the trajectory of typical Kreutz-group comets as seen in SOHO/LASCO from mid-December to mid-January. The length of the curves correspond to the part of the trajectory in which the comet are usually detectable.  Notice how Kreutz-group comets observed in December [generally] survive long enough to appear in the SOHO/LASCO C2 FOV. In contrast, those observed in January rarely make it into the C2 FOV. Furthermore, notice how the trajectory of the January Kreutz comets are alot more obscured by the pylon, in comparison to the December comets. Image credit: ESA/NASA SOHO/LASCO and Trygve Prestgard.

With the reasons above, this explains why comets such as STEREO-100 went undetected by SOHO/LASCO in January, while comets of similar brightness such as SOHO-3481 and SOHO-3483 were first found and best seen in SOHO/LASCO*. Note that the latter two comets were only visible in HI1-A and C2 (see animations below).

output_s2BIxzCropped animation of enhanced STEREO/SECCHI HI1-A images showing SOHO-3478, SOHO-3481, SOHO-3483. Despite these comets being just as bright the STEREO comets of January 2018, they better appeared in SOHO/LASCO! Image credit: NASA/NRL STEREO/SECCHI HI1-A

output_zJVm5e

Animation of the bright Kreutz-group sungrazing comet SOHO-3478, and its four tiny fragments (marked by the black lines, in the floowing order): SOHO-3483, SOHO-3481, SOHO-3479 and SOHO-3480. These were some the last images of these comets before their demise. Had these comets appeared only a couple weeks later, it would have been unlikely that the four fragments would have been detected by SOHO at all! They would most likely solely have appeared in STEREO, similarly to STEREO-100.

 

*Note that I actually first discovered SOHO-3483 in STEREO/SECCHI HI1-A images of 2018-12-20 before reporting it in SOHO/LASCO C2. This is because I was very unsure about its appearance in STEREO. It appeared better in C2 images, hence it was easier to report it using those observations. More about this comet and it’s fragments can be found in this old blog post I wrote back in December 2017: https://skyhuntblog.wordpress.com/2017/12/23/solar-observatories-observe-the-demise-of-a-bright-sungrazer-and-its-many-fragments/

 

Newly discovered Planetary Nebula candidates by amateurs!

Now with more amateur astronomers being equipped with larger instruments, along with the increasing amount of High-resolution survey data being released to the public online (SDSS, Pan-STARRS; DECaPS, etc), amateurs are playing an important role in the total number of discovered Planetary Nebulae!

Below are just some of the numerous Planetary Nebulae that were discovered in late 2017 and early 2018 by amateur astronomers. These do not include the discoveries made by the Deep Sky Hunters group (they deserve a blog post of their own! ) Anyways, this blog post is already  longer than usual! 🙂

As can be seen when looking at these discoveries, it’s obvious that there are still many beautiful Planetary Nebulae still waiting to be discovered!

CaVa 1 – Vast and complex circular Planetary Nebula

This is a beautiful circular Planetary Nebula discovered by Jean-Paul Cales et Michael Vanhuysse (France) in an Halpha image they took via their observatory in Nerpio (Spain). The nebula has a large apparent size, almost 7′ in diameter! Its morphology is almost completely circular, displaying internal heterogeneities. The nebula is also very obvious in IPHAS survey data, and also faintly detectable in images from the Digitalized Sky Survey and Pan-STARRS1. Pascal Le Dû (France) confirmed the object to be a true Planetary Nebula via his own spectroscopic observations.

CaVa1 stephane zollImage extract of CaVa 1 as imaged by Stephane Zoll (France). The image is a combination of an Halpha and OIII exposure, colour-coded red and blue respectively. Notice how the nebula almost appears as a complete (heterogeneous) disk. (c) Stephane Zoll.

CaVa1 iphas halphaIPHAS Halpha Image extract partially showing CaVa 1. In these images too the nebula is very circular, displaying an inner “bar” structure dividing the nebula in two nearly-equal halves! Image credit: IPHAS Halpha

CaVa_dssredDSS Image extract showing *very faintly* CaVa 1. In these images only the most enhaced parts of this PN appear. One can partially see the nebula as a very faint arc covering the lower and lower-right part of this image.

 

Su 1 – Bipolar Planetary Nebula candidate with no optical counterpart

Guyou Sun (China) discovered this very strange nebula in images from the WISE satellite, where the object displayed a Mid-Infrared signal typical of a Planetary Nebula. Atypical of Planetary Nebulae however, Su 1 appears best in the Near-Infrared,  where PNe usually appear the faintest! Furthermore, the nebula doesn’t seem to appear in optical images at all. The nebula displays a remarkable bipolar nature, especially as seen in Pan-STARRS1 images. Su 1 is also faintly apparent in Infrared images from the Digitalized Sky Survey.

Su 1 ps1Pan-STARRS1 image extract showing Su 1. Notice the object’s very obvious bipolar nature, stemming from a bright Near-Infrared central object. The object lacks an optical counterpart in Pan-STARRS1, which is very atypical of Planetary Nebulae! Image credit: Pan-STARRS1 Science Consortium

dss2ir su 1Su 1 as seen in a Near-Infrared DSS frame. The lines mark the center of the object, and is quasi-perpendicular to the orientation of the bipolar nebula (faintly visible). Image credit: DSS Plate Finder.

 

Tan 2 – Possible condensed elliptical Planetary Nebula

This is a small Planetary Nebula candidate discovered by Hanie Tan (China) using survey images from the WISE satellite, the Digitalized Sky Survey (DSS), SuperCosmos Halpha Sky Survey (SHS) and Pan-STARRS1. The object was first flagged as an emission-star but Pan-STARR1 clearly indicated the object to be a nebula, with the morphology and colours typical of a Planetary Nebula! The SHS images also seem to faintly reveal the object’s non-stellar nature.

Tan2ps1Pan-STARRS1 image extract showing Tan 2 as an obvious compact elliptical nebula. It’s green colorimetry in these images is typical of Planetary Nebulae. Image credit: Pan-STARRS1 Science Consortium.

Tan2 shsSHS Image comparison showing Tan 2. Notice the object’s non-stellar nature, as well as its very strong Halpha signal, typical of Planetary Nebulae. Image credit: SuperCosmos Halpha Sky Survey.

 

St 4 – Possible quasi-stellar Planetary Nebula

This is a small compact Planetary Nebula candidate discovered by Xavier Strottner (France) online using the Digitalized Sky Survey Plates (DSS). In these images, the object appears best in the optical plates, rather than in the Infrared DSS images (which is a good sign of a Planetary Nebula!). Despite the object apearing alot more disk-like (with a central condensation) in Pan-STARRS1, the object has the colorimetry more typical of a galaxy in those images.

st 3 dss2 aladinliteSt 4 is the small, round and diffuse object located at the center of this coloured DSS2 image extract. Its morphology is typical of compact Planetary Nebulae. Image credit: DSS2 Aladin Lite.

St 3 ps1St 3 as seen in a Pan-STARRS1 image extract. These images seem to reveal St 3 to have a round morphology with a central condensation. However, Pan-STARRS1 data is also in favour of this being a small unresolved spiral galaxy too. Image credit: Pan-STARRS1 Science Consortium.

 

St 9 – Possible elongated Planetary Nebula

This is another PN candidate discovered by Xavier Strottner in images from the Digitalized Sky Survey (DSS). Unlike St 3, St 9 is a rather obvious and diffuse nebula (as can be seen in the image below). It appears best in bluer images, such as in the DSS blue plates. The object is also obvious in the Mid-Infrared, displaying a typical Mid-IR signature of a PN!

st 9St 9 as seen in a coloured image from the Digitalized Sky Survey (DSS). Image credit: DSS2 Aladin lite.

 

Mo 7 – Possible obscured compact Planetary Nebula

Sankalp Mohan (India) discovered this small Planetary Nebula candidate in WISE satellite data, where it diplayed the typical Mid-Infrared signature of a Planetary Nebula. It was initially rejected from being kept as a PN candidate, as it did not display any signs of nebulosity (more observations were needed). It wasn’t until the release of DECaPS in 2018 that Mo 7 was a confirmed to be a nebulous, and hence a Planetary Nebula candidate!

Mo7decapsDECaPS coloured image extract showing Mo 7 as a small round nebula. These were the images that confirmed the objects nebulous nature. Image credit: DECaPS Aladin Lite.

Mo 7 allwiseAllWISE Image extract showing Mo 7 (centered). The “red” colour is due to the colour coding, with the red band corresponding to the WISE W4 filter. PNe usually respond best in these images, hence the colour! Image credit: AllWISE Aladin Lite.

 

LDû 36 – Possible Round and faint Planetary Nebula

This nice Planetary Nebula candidate was discovered by Pascal Le Dû (France) in images from the SHS Halpha survey. In the visible wavelength, the nebula only seems to appear in these images, even in DECaPS and Pan-STARRS1! The nebula however emits significantly in the Mid-IR, with a signature in WISE images typical similar to many PNe. Its nebulosity also seems to be visible in Mid-IR images.

LDu 36SHS Halpha Image extract showing LDû 36. Notice its round morphology, similar to many Planetary Nebulae. This is the only current optical image available for this object, so far! Image credit: SuperCosmos Halpha Sky Survey. 

LDu 36 spitzerSpitzer image extract showing a faint nebulosity and poorly defined, likely corresponding to LDû 36. Its colour-coded green colorimetry in these images is very common in the case of Planetary Nebulae. Image credit: Spitzer Aladin Lite.

Pre 43 – Possible blue Planetary Nebula with a complex morphology

I first noticed this possible Planetary Nebula in images from the WISE and DECaPS surveys online, where it appears as a rather heterogeneous, clumpy and elongated blue nebula. In images from the Digitalized Sky Survey (DSS) the nebula mixes rather well with the surrounding nebulosity, but DECaPS, Narrow-band (OIII and Halpha) and WISE Mid-Infrared images clearly indicate it to be seperate and an independent nebula!

Pre 43 BertPre 43 as seen in a OIII+NII image extact taken by Bert Van Donkelaar. The nebula appears as a tiny blue patch of nebulosity at the center of this image. The image indicates that Pre 43 emits strongly in the OIII band, typical of PNe. (c) Bert Van Donkelaar.

Pre 43 decapsDECaPS Image extract showing Pre 43. In these images it appears as a complex, heterogeneous, clumpy and elongated blue nebula. The brightest part of this nebula is 0.5′ x 0.7′ in size. This is surrounded by a faint, vague and blue halo measuring perhaps 2′ x 1′ in size. Image credit: DECaPS Aladin Lite.

shs Pre 43 short red halphaImage comparison between a SHS Short Red and Halpha frame showing Pre 43. Notice how the nebula only seems to appear in Halpha. This is typical of Planetary Nebulae as they most often have hydrogen emissions. Image credit: SuperCosmos Halpha Sky Survey.

 

DeGaPe 51 – Possible vast circular Planetary Nebula

This is an apparently round nebula that appears best in OIII narrow band images, discovered by the APO team (Thierry Demange, Richard Galli and Thomas Petit) in combined narrow band images. The team photographs the sky regularly using OIII, Halpha and SII filters (then combining them them to create a coloured “SHO” image), to obtain images like the one below! Except for in OIII, DeGaPe 51 is optically very faint, being undetectable in most optical surveys, including Pan-STARRS1!

Discovery SHO image extract showing DeGaPe 51. Notice its rather round morhpology as seen in these images, appearing mainly in the OIII band (colour coded blue). (c) APO Team.

DeGaPe 51 SHS HalphaSHS Halpha Image extract of DeGaPe 51. Notice how the object is poorly detectable in this image. Other than these survey images, the nebula seems to be absent in other optical survey, such as the Digitalized Sky Survey and even Pan-STARRS1! Image credit: SuperCosmos Halpha Sky Survey

DeGaPe 54 – Possible Stellar Planetary Nebula

This is yet another Planetary Nebula candidate discovered by the APO team in their combined SHO narrow-band images. This object is very stellar as seen in survey data, not even DECaPS indicate any signs of a nebulous nature! Furthermore, the object displays an Infrared excess atypical of most PNe! Only narrow band images seem to indicate this object to have PN-like charactersistics.

Discovery SHO image extract showing DeGaPe 54, located at the center of this image. Despite its stellar morphology, its flashy green colour in this image clearly distinguishes it from the rest of the background stars! (c) APO Team

DeGaPe 54 DECAPSDeGaPe 54 as seen in the combined colour DECaPS images. Unlike the SHO image, the object is impossible to distinguish from the surrounding stars! This image also suggests the object could be slightly obscured by the nebulosity in this region. Image credit: DECaPS Aladin Lite.

 

DeGaPe Object 3 – Possible diffuse Planetary Nebula?

As of April 12th, 2018 this is the most recent discovery made by the APO team. It’s a rather faint and diffuse object that could be a Planetary Nebula. However, based on its designation, this status is still pending. The nebula is optically faint (not even detectable in DECaPS images!), essentially emitting in OIII. In the Mid-IR however the nebula is obvious, with a typical PN-like signature in WISE!

Discovery SHO image extract of DeGaPe Object 3. Notice its rather diffuse morhpology as seen in these images, appearing mainly in the OIII band (colour coded blue). (c) APO Team.

Ra 66 – Strongly obscured compact Planetary Nebula candidate

This Planetary Nebula candidate was discovered by Thierry Raffaeili (France) using the available SDSS images online, via Skymap.org. The nebula is round and apparently compact. It is optically faint, but it has a much brighter Infrared counterpart (with a Mid-Infrared signal typical of most Planetary Nebulae). Its faintness in optical images is most likely due to this object being strongly obscured behind thick dust of a star forming region.

PN Ra 66 sdssCombined colour SDSS image extract showing Ra 66 as a faint round nebulosity, laying next to a relatively bright star. The colorimetry of the nebula in these images is not typical of a Planetary Nebula, but this effect might just be due to this object being strongly obscured behind thick dust of a star forming region. Image credit: SDSS Aladin Lite.

Ra 66 allwiseCombined Mid-Infrared AllWISE image extract showing Ra 66. Its colorimetry in these images is the typical signature of a Planetary nebula (similar to Mo 7, further above). Image credit: AllWISE Aladin Lite