Comet C/2020 P4 (SOHO): a Fragmented Sunskirter!

Comet C/2020 P4 (SOHO) was a unique non-group comet discovered in SOHO/LASCO data on August 6th, 2020. Not only was it brighter than most of SOHO’s non-group comets, but it could be resolved into three (if not four) individual fragments! This is unlike any previously observed SOHO/STEREO non-group comet. My guess is that C/2020 P4 was an intrinsically faint comet that underwent a series of fragmentation events soon before perihelion. This blog post is mainly focused on the comet’s discovery and appearance in SOHO/LASCO.

SOHO comet hunter Worachate Boonplod (Thailand) initially reported the object as one “single” comet when he first discovered it in real time SOHO/LASCO C3 images of August 5th – 6th, 2020. Indeed, although the comet displayed an elongated morphology, there was not yet any evidence of its multiplicity (see Figure 1). However, later that same day, more recent SOHO/LASCO C3 images revealed a second condensation, thus showing that the elongated nature of the object was due to (at least) two fragments. Worachate was the first to report the presence of a second companion. As of that point, it seemed that the object consisted of only two fragments: A moderately bright object (C/2020 P4-A) followed closely by a slightly fainter condensation (C/2020 P4-B) (see Figure 1).SOHO_fragments_Aug2020Figure 1: Images of C/2020 P4 (SOHO) as seen at different points when present in the SOHO/LASCO C3, prior to entering the C2 FOV. Left: C/2020 P4 as seen in C3 when discovered by Worachate Boonplod. Although elongated, the resolution of the image is not high enough to distinguish more than one object. Right: One of the discovery images revealing the object’s multiplicity for the first time. The resolution is still insufficient to reveal any additional fragments. Image credit: ESA/NASA SOHO/LASCO C3.

Finally, on August 7th at ~12:00 UT, the fragments entered the SOHO/LASCO C2 FOV. To everyone’s surprise, these images revealed two additional fragments! The leading fragment (C/2020 P4-A) was accompanied by a very faint object moving alongside it (C/2020 P4-C), while the second condensation itself (C/2020 P4-B) consisted of two distinct ones. Unfortunately, the proximity of the latter two makes it impossible to confirm them as separate objects. Both P4-A and P4-B displayed faint tails as seen in C2. In fact, P4-C appears to be located within the tail of P4-A, at least from SOHO’s perspective. As mentioned previously, Worachate Boonplod discovered P4-A and P4-B, while Masanori Uchina was the first to report P4-C (see Table 1). Worachate was also the first to report the double condensation of P4-B. Below is an annotated image and an animation showing C/2020 P4 (and all its fragments) as seen in C2.

Figure 2: Annotated SOHO/LASCO C2 image extract showing SOHO-4046 (P4-A), -4047 (P4-B) (and its two condensations), -4049 (P4-C). Notice how the three former condensations form a trail of fragments, while SOHO-4049 appears as an elongated condensation alongside SOHO-4046, within its tail. SOHO-4047 is not officially recognized as two seperate fragments, however it is very likely based on the above image (I arbitrarily designated them “a” and “b”). Image credit: ESA/NASA SOHO/LASCO C2.

output_x6NxBB(1)Figure 3: Animation of C/2020 P4 transiting the SOHO/LASCO C2 FOV. Notice the faint tails associated with the two brightest fragments (P4-A and B). The brightness of the tail of P4-A possibly appears enhanced by its convolution with P4-C. Image credit: ESA/NASA SOHO/LASCO C2.

P4-C and the double condensation of P4-B were only observed in C2 images. Hence, they were never seen again once the comet exited the C2 FOV in the late hours of August 7th (UT). The overall brightness of C/2020 P4 reached its peak on August 8th (when around perihelion), before slowly fading by August 9th. C/2020 P4-A and B were no longer distinguishable after Aug 9th, partially due to SOHO’s observing geometry. C/2020 P4 was last detectable in C3 images of August 11th. It was never recovered from Earth. This was hence the last time any of the fragments could be resolved. A C3 animation showing C/2020 P4 receding from the C2 FOV is shown below (Figure 4). Notice how it continues to brighten and then slowly fade, while appearing progressively more condensed.output_ekpanTFigure 4: Animation of C/2020 P4 as seen in SOHO/LASCO C3 after having transited the C2 FOV (images from August 7th 18:00 UT – Aug 9th 12:00 UT). Notice how the elongation decreases, partially as a result of the spacecraft’s observing geometry. This makes it impossible to distinguish the two main condensations, P4-A and P4-B, after August 8th. Faint tails are detectable. Image credit: ESA/NASA SOHO/LASCO C2.

Amateur astronomer Alan Watson (Australia) recovered the comet in STEREO/SECCHI HI1-A data of August 5th, 2020. In those images, the resolution was insufficient to resolve the individual components, although it did appear somewhat elongated on August 8th and 9th. The comet appeared to brighten as it arced around the Sun, before it left the HI1-A FOV on August 9th. It was even bright enough to have been detected in real beacon HI1-A images.  The comet was quite obvious and showed a several-degree long tail. This is much longer than observed by SOHO, which may be due to the observing geometry of STEREO-A and the lower limiting magnitude of the HI1-A instrument. Michael Mattiazzo was able to recover the comet in SOHO/SWAN images taken only a week before its apparition in SOHO/LASCO.output_chw1p7Figure 5: Animation of C/2020 P4 transiting the STEREO/SECCHI HI1-A FOV (August 5th -9th). Unlike in SOHO/LASCO, the comet shows an obvious tail in these images. The resolution however is insufficient to resolve any individual fragments, however the elongation increases just before it leaves the FOV. Notice the Pleiades a couple of degrees W and NW of the comet. Image credit: ESA/NASA SOHO/LASCO C2.

Table 1: Summary Information

Official designationInternal SOHO designationTelescopesMaximum apparent  magnitudeDiscoverer
C/2020 P4-ASOHO-4046SWAN,HI1-A,C3,C2~+5W.Boonplod
C/2020 P4-CSOHO-4049C2~+5-6 (fainter than A and C)M.Uchina
C/2020 P4-BSOHO-4047C3,C2~+5-6W.Boonplod
Table summarizing the various designations of the C/2020 P4 (SOHO) fragments and their discoverers. Magnitudes are from Seiichi Yoshida’s website. The resolution of the STEREO/SECCHI HI1-A images were insufficient to distinguish the fragments. C/2020 P4-B was too faint to appear in C3. Note that C/2020 P4-C likely consisted of two very close condensations of comparable brightness.

Other than being a relatively bright non-group comet, C/2020 P4 was unique as it could be resolved into three or four fragments. While it is common to observe close clusters of Kreutz-group comets in SOHO/LASCO (see Figure 6), this is rarely the case of non-group comets. The only other comparable case was that of SOHO-277 and 278 on December 20th, 2000. These were a close pair of comets discovered by German comet hunters Maik Meyer and Sebastian Hönig, respectively. Sebastian reported a possible third component that same day, but it was too faint to have been confirmed.

Figure 6: Animation of the Kreutz-group sungrazing comet SOHO-3478, and its four tiny fragments (marked by the crosshairs)!  These were some the last images of these comets before their demise. Image credit: ESA/NASA SOHO/LASCO C2.

Other than the potential SOHO-277/278 triplet, there have also been some cases of non-group comet pairs. For example, Zhijian Xu (China) and Karl Battams (US/UK) reported a pair non-group comets in C2 images of March 17th, 2012 and March 13th, 2015, respectively (see Figure 7). Unlike C/2020 P4 however, all three groups were much fainter and rapidly disintegrated. C/2020 P4 is (or was) most likely a significantly larger comet.  output_E78KKDFigure 7: Two non-group comet pairs (SOHO-2252, –2253 and SOHO-2890, -2891) seen entering the SOHO/LASCO C2. Ulike the recent sungrazer triplet, these pairs were fainter and appear to have disentigrated only hours after these images were taken. Image credit: ESA/NASA SOHO/LASCO C2.

References: Other than my own observations/interpretations, the information present in this post are from the Sungrazer Project website, the Comet Mailing list forum and Seiichi Yoshida’s website. Images were acquired from NASA’s SOHO website and the STEREO Science Center.

SOHO and STEREO Observe the Demise of a Bright Sungrazer!

On August 25th, the SOHO and STEREO spacecraft observed a previously undiscovered sungrazing comet. Although thousands of “sungrazers” have been observed by the spacecraft’s coronagraphs, this one was significantly brighter than most. In fact, it reached magnitude +3 on August 27th, before fully disintegrating. The object was a Kreutz-group member, part of a vast family that includes many great comets, most notably C/1843 D1, C/1965 S1 and C/2011 W3.

SOHO_comet_Aug272020_C2_extractFigure 1: SOHO/LASCO C2 image extract showing the comet only hours before it vanished. The apparent “tail” is actually more likely a trail of debris, a result of the comet’s disintegration. Image credit: ESA/NASA SOHO/LASCO C2.

In the early hours of August 26th (UT), amateur astronomers Junda Liu and Peiyuan Sun reported the presence of an unknown comet entering the lower left edge of the SOHO/LASCO C3 FOV (Figure 2). Based on its trajectory, it was obvious that it was a Kreutz-group comet on its way to perihelion. Both the discoverers are members of the Sungrazer Project, a citizen science project dedicated to discovering near-sun comets in SOHO/LASCO and STEREO/SECCHI data. In fact, this was the first comet discovered by Junda Liu.SOHO_Discovery_comet2020082527Figure 2: A SOHO/LASCO C3 image extract showing the comet as it appeared only hours after the discovery images. Although apparently small, it is much brighter than most Kreutz-group comets would appear at that distance from the Sun. Image credit: ESA/NASA SOHO/LASCO C3.

It is rare that a Kreutz-group comet is bright enough to be observed at the very edge of the SOHO/LASCO C3 FOV. Most sungrazers of this family are only visible much closer to the Sun, when they reach their very brightest (10-15 solar radii. Knight and Battams, 2014). These tiny comets tend to disappear after only a few hours, as they rapidly disintegrate and fade below the instrument’s threshold (Figure 3). According to Karl Battams (NRL): “It’s maybe once every 3 or 4 years we discover a Sungrazer at the edge of the C3 field of view like this!” Consequently, as the comet was already easily detectable when entering the FOV, it was evidently going to become a very bright object. output_vZhmwcFigure 3: A “typical” Kreutz-group comet as seen in SOHO/LASCO C3. The comet is designated SOHO-2572 and the images are from August 18th, 2013. Although having been present in the FOV for more than a day, this object was only detectable (for only a few hours) when at its brightest, before it rapidly faded below the instrument’s limiting magnitude. The low image resolution, along with the comet’s faint nature, made it impossible to detect any tail. Image credit: ESA/NASA SOHO/LASCO C3.

As expected, the comet brightened significantly over the course of the next couple of days (Figure 4). By August 27th, it had reached magnitude +3. Its brightness was such that it slightly saturated the SOHO/LASCO detectors. However, by 08:00 UT that same day, the object started fading dramatically, clearly indicating that the nucleus was disintegrating. This was sadly expected, as this tends to be the case of all Kreutz-group comets discovered by SOHO. Indeed, although relatively bright, the sungrazers detected by SOHO (and STEREO) are generally intrinsically faint (small). Based on the values provided by Knight et al. (2010), one can assume that the comet probably had an initial diameter of a only few tens of meters. The object was never recovered after it passed behind SOHO/LASCO’s occulting disk.output_sSGqUzFigure 4: SOHO/LASCO animation showing the comet’s inbound journey towards perihelion. Note how the comet rapidly brightened, before suddenly fading. The apparent “tail” is actually more likely to be a trail of debris, a result of the comet’s disintegration. Image credit: ESA/NASA SOHO/LASCO C2.

In addition to having been observed by SOHO, the comet was sufficiently bright to clearly appear in STEREO/SECCHI’s real time (beacon) COR2-A images (Figure 5). Unlike SOHO/LASCO, real time SECCHI data are of very low resolution. These are, however, exchanged by higher quality data a few days later. Figure 6 shows the comet as seen in a fully processed COR2-A image taken on August 26th. Note the difference in the comet’s trajectory as seen from STEREO, relative to SOHO. This is due to their different in locations in space, hence perspective.output_11WbIiFigure 5: Animation of COR2-A real time beacon images showing the demise of the new Kreutz-group comet. Note the difference in its trajectory relative to that observed by SOHO. This is due to STEREO-A’s location ~60° behind the Earth and SOHO. The images were taken on August 27th. Image credit: NASA/SSC STEREO/SECCHI COR2-A.

Perhaps the comet’s most obvious feature was its prominent tail, which grew progressively longer as it neared the Sun. Rather than being an “ordinary” tail, it was probably a debris trail – remnants from the comet’s vanishing nucleus. The “tail” was also clearly visible in COR2-A, however its morphology differed significantly from the “streak-like” appearance observed by SOHO (Figure 1 and 4). This was [again] due to the spacecraft’s differing perspectives.STEREO_COR2A_Kreutz_comet_20200827Figure 6: A full-resolution STEREO/SECCHI COR2-A image extract showing the comet. Notice the tail’s (debris trail’s) curved nature, compared to the “streak-like” morphology observed by SOHO/LASCO. Image credit: NASA/SSC STEREO/SECCHI COR2-A

Although now-gone, the comet is still being studied. As Karl Battams posted on Twitter: “I might actually resurrect this subject next week, pending some additional analyses. There might have been something slightly different about this particular Kreutz compared to most other Kreutz, but I need to look at some extra data first”. In addition, I believe that it is important to study the full resolution STEREO/SECCHI COR1-A images from August 27th, once those are made available. It is possible that the comet may have been faintly detectable in those.

References

Images acquired via sohowww.nascom.nasa.gov and the Stereo Science Center.

Information from various Twitter posts, formal articles (references included in the article), and via the Sungrazer Project website.

 

Copyright (c) 2020 Trygve Prestgard. All rights reserved.

Studying Long-Period Variables Present in the French Planetary Nebula Lists

While optical spectroscopy is generally considered the most reliable method for confirming planetary nebulae (PNe), multi-epoch photometry (i.e. light curve morphology) can be a good tool to rule-out and classify compact PN “impostors”. This section discusses five objects with light curves displaying long-period sinusoidal variability (i.e. Long-Period Variable stars, or LPVs). These objects are currently listed as potential PNe in the French PN list.

Broadly, the long-period sinusoidal variations are the signatures of ancient pulsating giant stars, and thus are unlikely to represent PNe. Recent ASAS-SN photometry of the featured objects (Pa 200, Mul-IR 33, DeGaPe 15, 34 and 55*) are presented in Figure 1. Note that this blog post is not an in-depth analysis of these objects, but an overall compilation of information I could gather on them (sufficiently so in order to discuss their classification). A more rigorous study will be required before any submissions can be made to AAVSO’s Variable Star index.LPV_PN_5_objects_French_PN_listFigure 1: ASAS-SN Light curves of various LPVs present in the French PN list (time period: from 2019-05-27 to 2020-04-11). Offset has been added for the purpose of clarity. The overall sinusoidal variations of DeGaPe 15, 34 and 55 are not very obvious due to the short time interval included in this figure. 2000 day ASAS-SN curves can be found here, here and here respectively. Data from: ASAS-SN (Kochanek et al., 2017).

The five candidates can be divided into two separate groups: Pa 200, Mul-IR 33, and DeGaPe 15, 34, 55. The two members of the first group have small, rapid Semi Regular (SR)-like variations (Period P <100 days) with apparently weak (if not absent) Hα emissions (based on SHS Short Red- Hα image comparison, see figure 5). The objects of the second group appear to strongly emit in Hα and display Mira-like variations. The second groups is also much redder than the former (see Table 1).DeGaPe_Pa_MUL_LPVs_DECaPS_Panstarrs1Figure 2: DECaPS and Pan-STARRS1 image extracts of Pa 200, Mul-IR 33 and DeGaPe 15, 34 and 55. Notice how the latter three are very red (an appearance typical of red giant stars (e.g. Mira-type variables).  Image credit: Aladin Lite.

Pa 200 was discovered by Dana Patchick while searching for PNe using WISE and optical survey imagery. Mul-IR 33 was catalogued by Lionel Mulato and Pascal Le Dû (Acker et al., 2016) during a systematic search for compact PNe using IRAS and WISE data. Both are relatively bright in the optical. The light curve morphology (i.e. SR-like variations), the Mid-IR excess, and lack of strong Hα emissions are consistent with Proto-PNe. They also appear to be of spectral class G and K, which is also common for proto-PNe (from what I understand). Their appearance is similar to Mul-IR 64, another likely Proto-PN from the French PN list. The colours and amplitude are also comparable to Pre 30. Proto-PNe are post-AGB objects – once-giant stars now shedding their outer layers. Indeed, the Gaia DR2 astrometry and magnitudes are consistent with stars of intrinsically bright (giant) nature (overall photometric properties of all five objects are summarized in Table 1).

Object Amplitude BP-RP (Gaia DR2) Plx (milli-“)
Period (days)
Pa 200 g= 13.0 – 13.3 1.16 0.092 ~20 
Mul-IR 33 g= 12.7 -13.0 1.75 0.555 68
DeGaPe 15 R= 14.5: – 17.0: 4.95 -0.176 221
DeGaPe 34 V= 15.0 – 17.0 6.14 0.187 125
DeGaPe 55 V= 14.7 – <17.8 5.89 -0.03 231
Mul-IR 64 g= 13.6 – 13.8 2.38 0.291 ~20
Pre 30 V= 14.9 – 15.2 1.9 1.928 340

Table 1: Summary of the photometric properties of the five objects studied here, as well as Mul-IR 64 and Pre 30 (possible Proto-PNe). The PPNe candidates are yellow-orange (spectral classes G-K), while the others are closer to the M and C spectral classes.  Note that this is not an in-depth photometric analysis, but an overall insight into the properties of these objects. Magnitude extinction and blending have not been taken into account. The values from DeGaPe 15 are based on the DSS Red plates***. The range is likely to be higher. Sources: ASAS-SN, AAVSO-VSX, Gaia DR2.

DeGaPe 15, 34 and 55 were detected as a result of their peculiar PN-like colours in SHO imagery (by the APO team). Indeed, the “green” appearance of these objects in Figure 3 suggest strong Hα emissions. As mentioned previously, this is also apparent when inspecting SHS images (see figure 5). At least at first glance, DeGaPe 34 and 55 display typical Mira-like variations. In fact, DeGaPe 55 has been classified as a Mira variable star by Jayasinghe et al. (2018).** DeGaPe 34 has been classified as a SR star in the same paper. Although, personally, I find the variations to be rather “Mira-like”.DeGaPe15_34_55_APO_SHOFigure 3: Extracts from the original APO/SHO images that lead to the discovery of DeGaPe 15, 34 and 55. Their relatively green appearance is a result of their strong Hα emissions. The exposures are colour-coded red ([SII]), green (Hα) and blue ([OIII]). However, it is possible that the invidiual exposures may have been taken months apart, thus their high amplitude variability may also affect their colours. (c) APO team.

The amplitude of DeGaPe 15 appears to be significantly smaller than that of the other two. Rather than being intrinsic to the object, the shallower variations are likely due to the contamination of a bright nearby star (Gaia DR2 5937601972472672128: G= 14.7 mag, angular sep: 5.4″). Hence, the blending has caused DeGaPe 15 to appear brighter and display shallower variations in ASAS-SN data, assuming that the contaminating star is constant. The DSS plates confirm the high amplitude variations of the object (see figure 4 and Table 1)***. To conclude, DeGaPe 15 is more likely associated with Mira-like variations rather than semi-regular.DeGaPe15_DSS_RedFigure 4: Two DSS-Red image extracts centered on DeGaPe 15, taken a decade apart. Notice the object’s drastic difference in luminosity, consistent with a Mira-type variable rather than a semi-regular star. Image credit: SuperCosmos Sky Survey.

DECaPS imagery further supports the red giant nature of these stars (their highly red nature, see figure 2). Moreover, Hα signals are common in Mira stars (Yao et al., 2017). However, let us note that strong Hα signatures are a trait of symbiotic stars (Miszalski et al., 2013) (i.e. Z Andromedae variables, or ZANDs). Thus, in my opinion, it could be possible that DeGaPe 15, 34 and/or 55 are in fact symbiotic stars in which the colder components are Mira-type giants. In support of this, in my opinion (at first glance), I find it can occasionally be difficult to distinguish certain mira star light curves from ZANDs with Mira components (judging based on the light curves presented in Gromadzki et al. (2009)). Moreover, they may also appear very red in survey imagery. Consequently, it could be interesting to study the DeGaPe objects through spectra.LPV_PN_SHS_Blog_postFigure 5: Comparison between SHS Short red and Hα image extracts. While not presented in this figure, both DeGaPe 34 and 55 are similar to DeGaPe 15. Note however that the SHS plates are often taken months or years apart, thus high amplitude variations must also be taken into account. Image credit: SuperCosmos Hα Survey.

References

Kochanek, C. S.; et al., 2017, The All-Sky Automated Survey for Supernovae
(ASAS-SN) Light Curve Server v1.0

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

Various Vizier catalogues: Gaia DR2 (Gaia Collaboration, 2018), USNO-A2.0 (Monet et al., 1998), USNO-B1.0 (Monet et al., 2003).

The AAVSO Variable Star Index (VSX) and PlanetaryNebulae.net.

Gromadzki et al. (2009): Light-curves of symbiotic stars in massive photometric surveys I: D-type systems.

Miszalski et al. (2013): Symbiotic stars and other Halpha emission line stars towards the Galactic Bulge.

Yao et al. (2017): Mira Variable Stars from LAMOST DR4 Data: Emission Features, Temperature Types, and Candidate Selection.

Gaia Collaboration (2018): Gaia Data Release 2: Observational Hertzsprung-Russell diagrams.

Footnotes:

*Each one of these objects have previously been reported as variable by the ASAS-SN team, with the exception of Pa 200.

**Note that I briefly mention DeGaPe 55 in an older blog post of mine, which also includes discussion on a similar object – DeGaPe 57 (which likely also belongs to the same category as DeGaPe 15, 34 and 55). Similarly, I have also mentioned Mul-IR 64 previously on this website.

***Due to the small angular seperation between DeGaPe 15 and Gaia DR2 5937601972472672128, both appear as a singular source (blended) in various catalogues (e.g. APASS, USNO-A2.0 and USNOB1.0). The magnitude variations I presented in Table 1 are based on estimations from the DSS Red plates themselves (calibrated with the GSC2.3 R magnitudes).

The Brightest Meyer-group Comets!

A significant fraction of SOHO’s comet discoveries belong to a vast, yet poorly understood family: the Meyer-group. This blog post will be focusing on this family of comets, more specifically, its brightest members. The comets featured here were chosen based on my own personal assessment of their appearance (brightness) in SOHO/LASCO images, and thus do not represent a choice based on any published work. Moreover, note that this list is not exhaustive, but simply includes five objects that clearly “stick out” from the ordinary Meyer-group comet population.

Background: What are Meyer-group Comets?

The Meyer-group is a populous family of sunskirting comets, first recognized by amateur astronomer Maik Meyer (Germany) in 2002. To date, the group contains about 300 known members, making it the most abundant known family of comets after the Kreutz-group. However, contrary to the Kreutz-group, Meyer-group comets have solely been observed in SOHO/LASCO data. There are no recorded ground based observations of any Meyer-group members. In addition, none have ever been recovered in STEREO/SECCHI images, including the brightest members. This is likely due to their intrinsically faint nature, and the photometric properties of the STEREO/SECCHI instruments. Consequently, the lack of observations (e.g. short observation arc) make it difficult to constrain any orbits with high certainty. In fact, data lacks in order to establish (or estimate) reliable orbital periods. It is thought however that Meyer-group comets are likely intermediate or long periodic comets (Battams and Knight, 2015, and references therein). Interestingly, Rainer Kracht suggested the possibility of a ~10 year orbital period, although this was never confirmed.Meyer_comet_composite_imageFigure 1: Composite SOHO/LASCO C2 image extracts showing nine different Meyer-group comets. Notice their stellar and/or elongated morphology. The orientation of the elongation depends on the time of the year (i.e. viewing geometry). Image credit: ESA/NASA SOHO/LASCO C2.

In regards to their appearance, Meyer-group comets all share a very common morphology. Nearly all Meyer-group comets are very condensed in appearance, although generally slightly elongated as seen in SOHO/LASCO C2. Their condensed appearance suggests that they are relatively “inactive” in nature (Battams and Knight, 2015). A composite image of various “typical” Meyer-group members is shown in Figure 1. Indeed, other than their varying brightness, their morphologies are strikingly similar. For comparison, Kreutz-group comets are generally somewhat fuzzy, (although highly condensed members have been observed, e.g. SOHO-3847). Marsden and Kracht comets also tend display “stellar” morphologies.output_rpQrwRFigure 2: Animation of SOHO/LASCO C2 image extracts showing Meyer-group comets SOHO-3850, SOHO-3851 and SOHO-3852. SOHO-3850 is the brightest member, while SOHO-3851 is the first to leave the FOV. SOHO-3852 is the trailing fragment of SOHO-3852. This one was discovered via an archival search for non-kreutz comets. This is the tightest cluster of Meyer-group comets ever recorded. The images were taken on 2019-11-04 and 05. Image credit: ESANASA SOHO/LASCO C2.

Unlike the Kreutz, Marsden, Kracht-I and Kracht-II groups, that appear to be dynamically evolving, the Meyer-group is most likely a highly evolved comet family (Lamy et al., 2013; Battams and Knight, 2015). Indeed, Battams and Knight (2015) suggest that the precursor of the Meyer-group may potential have first fragmented as long as ~10.000 years ago. Moreover, unlike the latter comet groups, Meyer comets rarely arrive in tight clusters. However, there have been exceptions: on November 4-5th, 2019, a group of three Meyer-group comets were seen over a span of only eight hours (Figure 2). Note that, in average, one Meyer-group comet is observed per month.  This is the “tightest” known Meyer-group cluster discovered to date.

July 3rd, 1996: C/1996 N3 (SOHO)

Also known as SOHO-931, this bright Meyer-group comet was discovered by Rainer Kracht (Germany) in 2005 during search for overlooked comets in the SOHO/LASCO archives. This is the earliest recorded Meyer-group comet, and perhaps one of the brighter members of this family. In fact, Karl Battams described the comet as “one of the brightest Meyer-group comets that he has seen […] it didn’t fade noticeably until well past perihelion“. Despite its brightness, it initially went overlooked due to the low image cadence. At the earliest stages of the SOHO mission, SOHO/LASCO images were taken at a much lower rate than currently. Hence, one could expect many undiscovered Meyer-group comets to be lurking in ancient SOHO/LASCO data, but the low amount of images and their stellar appearance would make these impossible to distinguish from cosmic ray hits. C/1996 N3 (SOHO) is a special case as it was sufficiently bright to be detected in several C2 (Figure 3), and C3 (Figure 4), images.  output_Yos4yEFigure 4: Animation of SOHO/LASCO C2 image extracts showing C/1996 N3 (SOHO). Due to the low and irregular cadence of images this comet went initially unnoticed. The object was relatively bright when entering the C2-FOV, unlike most Meyer-group comets that aren’t detectable in that region of the FOV at that part of the year. Image credit: ESA/NASA SOHO/LASCO C2.

Figure 4 is an animation of the comet in C2. Notice how the comet was already obvious when entering the C2 FOV. At that time of the year (June-July), most Meyer-group comets would be too faint to have been detected in this portion of the FOV. Moreover, the comet was sufficiently bright to appear in at least twelve C3 frames, despite the low image cadence. Below (Figure 5) is a C3 image extract showing the comet. SOHO_Meyer_C3_19960704Figure 5: SOHO/LASCO C3 image extract showing C/1996 N3 (SOHO). Although a relatively bright object, the image compression makes this comet fairly difficult to spot. Image credit: ESA/NASA SOHO/LASCO C3.

June 10th, 1997: C/1997 L2 (SOHO)

Also known as SOHO-11, this is the first Meyer-group comet to have been discovered. It was found by Shane Stezelberger (USA), a former member of the SOHO/LASCO team, in June of 1997. This comet was among those studied by Maik Meyer which allowed him to discover the Meyer comet group in 2002. Note that, in addition to being the first Meyer-group comet, it is also only the second “non-Kreutz” comet discovered by the SOHO/LASCO team (the first being SOHO-8, a non-group comet). Meyer_1997L2_SOHO_C2Figure 3: Composite image extract showing C/1997 L2 (SOHO) in SOHO/LASCO C2. This is the last image of the comet prior to it leaving the C2 FOV. Moreover, if not taking into account the image compression, this image is also the highest resolution frame showing the comet. Notice its typically elongated, typical of Meyer-group comets. Image credit: ESA/NASA SOHO/LASCO C2.

Similarly to C/1996 N3 (SOHO), the comet appeared during the early stages of the SOHO mission, thus a period of low SOHO/LASCO image cadence. Despite this however, C/1997 L2 was visible in several SOHO/LASCO C2 images, as well as being obvious in numerous C3 frames (Figure 5). Moreover, as seen in Figure 4, the elongated nature of C/1997 L2 (SOHO) is clearly visible. The official report lists this object to have reached a magnitude of around +5. However, by comparing C/1997 L2 with the other Meyer-group comets listed here, I estimate that it may have reached maximum magnitude of about +6.5 mag. The differences are significant, and it may partially be due to the method I apply.* Despite this discrepancy, it is obvious that C/1997 L2 (SOHO) was among the brighter subset of Meyer-group comets.output_OOu2vdFigure 4: Animation of SOHO/LASCO C3 image extracts showing C/1997 L2 (SOHO). The comet may initially be difficult to spot because of the high image compression as well as variable image exposure(?). It’s apparent in the left portion of this figure, progressively moving away from the Sun and fading. Image credit: ESA/NASA SOHO/LASCO C3.

February 27th, 2011: SOHO-2030

Amateur astronomer Masanori Uchina (Japan) was the first to report this bright Meyer-group comet. He found it in real time (half resolution B/W) SOHO/LASCO C2 images of 2011-02-27, but quickly recovered it in C3 images, where it was simultaneously visible. Due to a data gap of several hours, the comet was already obvious and near its maximum brigthness by the time was discovered.  Personally, I was unable to recover any traces of the comet prior to the gap. It would have been observable in C3, to the lower-left of the Sun (similarly to SOHO-2247, see below). The brightest moments of SOHO-2030 are shown in Figure 5 and 6.output_jYCSa7Figure 5: Animation of SOHO/LASCO C2 image extracts showing the transit of SOHO-2030 throught the C2 FOV. These are the highest resolution images of the comet. Image credit: ESA/NASA SOHO/LASCO C2.

In my opinion, SOHO-2030 is easily one of the brightest Meyer-group comets observed by SOHO/LASCO.  It could be detected long after it had passed perihelion (more than a day). As Karl Battams stated “Masanori’s Meyer comet was very impressive! It has to be one of the brightest we’ve seen. A very nice find!”. By crudely comparing the object to various field stars*, I estimate that the comet reached a maximum magnitude of about +6.5.  output_laN5tkFigure 6: Animation of SOHO/LASCO C2 image extracts showing  SOHO-2030 arcing around the Sun (perihelion and post-perihelion). These frames show the comet at its brightness. Notice how it slightly fades at the end of the animation. Image credit: ESA/NASA SOHO/LASCO C2.

March 1st, 2012: SOHO-2247

SOHO-2247 was first reported by amteur astronomer Krzysztof Kida (Poland) in real time SOHO/LASCO C3 images of 2012-02-29. At the time the comet was discovered, the object was a couple of degrees south-east of the Sun, overlapping the Kreutz-group comet track (Figure 7). Due to its brightness, it was detected much earlier in its path than most Meyer-group comets would. Consequently, having been discovered in a region where Meyer-group comets are uncommonly observed, it was initially assumed to be a Non-group comet. It was only as the comet kept approaching the Sun that it become clear that it was a Meyer-group member. Sergei Schmalz (Germany) was the first to suggest its classification.Meyer_20120229_C3_precovery_imageFigure 7: Composite image of SOHO/LASCO C3 image extracts showing one of the earliest images in which SOHO-2247 was detectable. The comet was about twleve hours from entering the C2 FOV. It is rare that a Meyer-group comet is bright enough to be detected so early prior to perihelion (in C3). The faint “streak” near the box is a Kreutz-group comet (SOHO-2246). Image credit: ESA/NASA SOHO/LASCO C3.

As the object kept approaching the Sun, it gradually brightened, becoming obvious by the time it had reached the C2 FOV. Again, by crudely comparing the object to various field stars, I estimate that the comet reached a maximum magnitude of about +7.5, thus somewhat fainter than SOHO-2030. Despite this, it is definitely among the brighter members, in my opnion. As Sergei wrote on the SOHOHunters yahoo forum: “Well, it’s evident now – yes, it is a Meyer-group comet. And a beautiful one!“. Coincidentally, a coronal mass ejection took place close to the time the comet reached perihelion. The eruption can be seen in both in Figure 8 and 9, along with the comet.  output_DPZKX2Figure 8: Animation showing SOHO-2247 briefly transiting the SOHO/LASCO C2 FOV on March 1st, 2012. These are the highest resolution images of the comet. Notice the coronal mass ejection (CME) to the west of the comet. Image credit: ESA/NASA SOHO/LASCO C2.

output_NGSD5vFigure 9: Animation of SOHO/LASCO C3 image extracts showing  SOHO-2247 arcing around the Sun (perihelion and post-perihelion). Notice the obvious coronal mass ejection that coincidentally took place during the comet’s passage. Image credit: ESA/NASA SOHO/LASCO C2.

June 17th, 2020: Currently Undesignated

Amateur astronomer Worachate Boonplod (Thailand) discovered this bright Meyer-group comet in SOHO/LASCO C2 images of 2020-06-16. Unlike most Meyer-group comets, this one was already (easily) detectable when enterering the C2 FOV. Most Meyer-group comets aren’t detectable until hours just before they leave C2, just around perihelion. The comet entered the FOV only hours after Kreutz-group comets SOHO-3999 and SOHO-4000, the latter still visible in the animation below (Figure 10). As of the time of this blog post, this is the most recent (real time) Meyer-group comet discovery.output_210wXcFigure 10: Animation of SOHO/LASCO C2 image extracts showing the Meyer-group comet from 2020-06-16/17 transiting the FOV. Notice how the comet was already apparent as it entered the FOV. Most Meyer-group comets are only detected hours before leaving the FOV (which is close to the time they reach their brightest). Notice also the Kreutz-group comet SOHO-3999, apparent early in the animation. Image credit; ESA/NASA SOHO/LASCO C2.

Meyer_C2_1997_2020Figure 11: Image comparison between C/1997 L2 (SOHO) and Worachate Boonplod’s recent Meyer-group comet. Notice how both comets are of comparable brightness, if not the latter being slightly brighter (taking into account the data compression in the former case). Image credit: ESA/NASA SOHO/LASCO C2.

Similarly to SOHO-2030, I estimate this comet may have reached a maximum magnitude of about +6.5 based on field star comparisons. In fact, the brightness is comparable, if not slightly brighter, than C/1997 L2 (SOHO) (Figure 11). Consequently, this makes it one of the brightest Meyer-group comets observed by SOHO. As Karl Battams stated in a response on Twitter “[…] that must be one of the brightest Meyer’s we’ve ever seen!”. The STEREO-A spacecraft was favourably placed to observe the comet in its HI1-A imager. Unfortunately, I was unable to recover it. It must have faded too much by the time it reached the HI1-A FOV.output_KP3RrvFigure 12: Animation of SOHO/LASCO C3 image extracts showing the Meyer-group comet from 2020-06-16/17. These images were taken just as the comet exited the C2 FOV (Figure 10). Note how the comet gradually fades. At the time of these images, the comet had just passed perihelion. ESA/NASA SOHO/LASCO C3.

Acknowledgments

I wish to thank amateur astronomer Peter Berrett (Australia) for providing the coordinates of SOHO-3786 and SOHO-3852 (Figure 1 and 2).

Footnotes

*The magnitude estimations I provide in this work are based on a rough comparison between the comets and visual magnitudes (Johnson V filter) of field stars (provided by the SIMBAD database). Perhaps it is possible that this band is not the most adapted when making such comparisons?

C/2020 F8: A New Comet Discovered in SOHO/SWAN!

On April 10th, amateur astronomer Michael Mattiazzo (Australia) reported the discovery of an unknown comet in images obtained by the SOHO/SWAN instrument. Now designated C/2020 F8 (SWAN), the object is currently located in Piscis Austrinus, slowly moving north-east and brightening. Follow-up observations have shown the comet to display a particularly condensed coma, as well as a faint ion tail (several arcminutes long).  Based on his own ground-based images, Michael estimated the comet to be V= +9.7 mag on April 10th. However, more recent estimates suggest the object to currently be around V= +8 mag! Due to the brightness of the comet, in addition to its condensed appearance, it is possible that it might be undergoing an outburst.

SWAN2, 11 Apr 2020 textFigure 1: Comet C/2020 F8 (SWAN) comet as imaged by Rob Kaufman (cropped). Notice the obvious green colour, as well as the highly condensed nature, of the coma! The image was taken from Bright (Victoria, Australia) and was acquired using  a Canon DSLR (EOS 800D) along with a 55mm F/5.6 lens. (c) Rob Kaufman.

The available observations suggest that the comet is still on its inbound journey towards perihelion, and will hence (most likely) keep brightening, while progressively moving towards less favourable skies (poorer solar elongation). According to Seiichi Yoshida’s website, the object could reach V= +4 mag near perihelion on May 31st (q= 0.43 AU), but will be at relatively poor solar elongation. Other sources suggest that it may reach a peak brightness within the range of V= +2 to +3 mag! As of currently, it is difficult to make any reliable estimations regarding the comet’s peak magnitude, and hence these values should be taken with caution.

92874449_3372617609433329_365951284001374208_oFigure 2: C/2020 F8 (SWAN) as imaged by Rolando Ligustri using the Siding Spring Observatory (Australia) facilities. Notice the comet’s vast green coma and faint ion tail! (c) Rolando Ligustri

Due to the poor resolution of SOHO/SWAN images, in addition to the numerous automated all-sky surveys (e.g. ATLAS, CSS and PANSTARRS), SWAN comet discoveries are rare. Indeed, over the past 25 years, only 14 comets carry the name “SWAN” (including co-discoveries)*. In many cases, these comets were located at poor solar elongation at the time of discovery (a “blind spot” for most surveys).

SWAN_COMET_20208_Discovery_FrameFigure 3: Cropped view of C/2020 F8 (SWAN) as seen in one of the discovery SOHO/SWAN (“comet tracker map”) frames. The poor resolution of the images makes it impossible to discern any of the obvious cometary features captured by ground-based images. Image credit: ESA/NASA/LATMOS SOHO/SWAN.

Note that C/2020 F8 is not the first SOHO/SWAN discovery made by Michael Mattiazzo. Indeed, Michael is credited for the (co)discovery of seven other SWAN comets, his first being C/2004 H6 (SWAN). His others include: C/2004 V13, C/2005 P3, P/2005 T4, C/2006 M4, C/2015 C2, and C/2015 P3 (SWAN). Among these, C/2006 M4 reached mag +4.5  a month after it had passed perihelion (August 21st, 2006), making it the only SWAN comet to be visible to the naked-eye!**

Mattiazzo_SWAN_CometsFigure 4: Composite image showing: A SOHO/LASCO C3 image extract of C/2004 V13, and cropped image extracts of C/2006 M4, C/2015 C2 and C/2015 P3, from photographs taken by Doug Neal (Kennewick, WA, USA) and Rob Kaufman (Bright, Victoria, Australia). (c) Doug Neal, Rob Kaufman.

To conclude,  C/2020 F8 (SWAN) is a relatively bright (V= +8 mag) comet, possibly in outburst, that is currently on its way to perihelion. Based on its orbit and current brightness, the comet is estimated to reach a peak brightness between 2 – 4 mag in late-May or June, but will likely be somewhat difficult to observe due to poor solar elongation. However, estimating the peak brightness of any comet is very difficult, and hence these values are to be taken with alot of caution. It is still a very new discovery, with relatively few observations currently available. Indeed, it is equally possible that the brightening may eventually stagnate, or that the comet may even start disintegrating. This would not be extremely surprising knowing that perihelion distance is only of q= 0.43 AU. As of currently, we can only wait!

Footnotes:

*This does not include 273P/Pons-Gambart, which was temporarily designated C/2012 V4 (SWAN) before it was found to be the return of lost comet D/1827 M1 (Pons-Gambart).

**Comet C/2012 E2 (SWAN) reached mag +1 before it disintegrated on March 14th, 2012. However, it was too close to the Sun to be observed from Earth. More information on this comet can be found here.

DeGaPe 71 and PN G326.9+08.2: Cataclysmic Variables Disguised as Planetary Nebulae!

Planetary Nebulae (PNe) can be difficult to distinguish from many other types of objects, including in the optical domain. However, in the case of compact objects, multi-epoch photometry may prove to be helpful in order to distinguish them from variable “PN-mimics” (e.g. proto-stars or symbiotic stars), and may also allow to provide a more accurate re-classification of impostors (thanks to the morphology of light curves, if available), especially when optical spectra lack.  In this post I discuss the cases of DeGaPe 71 and PN G326.9+08.2. These two objects were catalogued as PN candidates based on optical data (narrow-band imagery and spectra, respectively), but however display colours and luminosity variations typically associated with cataclysmic variable stars. 

DeGaPe 71 – A Nova-like Star?

Only recently added to the French Planetary Nebula (PN.net) database, DeGaPe 71 is a star-like object that displays peculiar blue “fluorescent” colours in SHO ([SII]+Hα+[OIII]) images obtained by the Atacama Photographic Observatory (APO) team (see figure 1).  The “SHO” colours suggest that the object might display [OIII] and Hα emission lines (especially the former), typical of planetary nebulae (PNe). Indeed, DeGaPe 71 resembles both KnDeGaPe 1 and DeGaPe 64 (also discovered by the APO team), two objects that are rather likely to be true PNe (see figure 2). More information regarding the APO team and their discoveries can be found here and here.

degape 71Figure 1: APO SHO image extract centered DeGaPe 71 (MGAB-V202), an object classified as a potential Planetary Nebula by the French PN database based on its appearance in this colour-combined image. (c) APO team.

Interestingly, unlike KnDeGaPe 1 and DeGaPe 64 (and compact PNe in general), DeGaPe 71 does not display any obvious Mid-IR emissions, nor does it appear to display obvious Hα emissions (based on the comparison between the SHS Short red and Hα plates, see figure 5).

DeGaPe_64_KnDeGaPe_1_APO_SHOFigure 2: APO/SHO discovery images of DeGaPe 64 (left) and KnDeGaPe 1 (right). Note the similar blue colours, typical of planetary nebulae and highly similar to DeGaPe 71. Note that KnDeGaPe 1 was also independently discovered by amateur astronomer Matthias Kronberger. (c) APO team.

However, despite its PN designation, amateur astronomer Gabriel Murawski was the first to note the object’s interesting nature. Though, rather than by its possible PN-like emissions, Gabriel found that the object displayed colours (highly blue: B-V= 0.07 mag) and high amplitude variations (more than a couple of magnitudes) typical of cataclysmic variable (CV) stars. He reported his findings to the AAVSO Variable Star Index (VSX) in July of 2019, and it has since been designated MGAB-V202; a potential Nova-like VY Sculptoris (NL/VY) star candidate. These objects are a class of CVs that undergo strong fadings rather than outbursts (e.g. Novae or Dwarf Novae). The highly blue nature these stars (as well as most CVs) are caused by the fact that they contain a hot and luminous white dwarf (WD) star component.

DSS_plate_R_comparison_DeGaPe71Figure 3: Digitalized Sky Survey (DSS) Red plate comparison showing the highly variable nature of DeGaPe 71/MGAB-V202. Image credit: DSS Plate Finder.

At the time of discovery, the available ASAS-SN data displayed only low-amplitude variations, though archival Digitalized Sky Survey (DSS) plates showed the object to have varied significantly in the past decades (see figure 3). However, since the object’s discovery, more recent data shows a strong fading event, typical of a NL/VY stars (see figure 4). It appears that the event started around the start of December, 2019, and might be ongoing. Perhaps this fading event may be sufficient to confirm the NL/VY classification of MGAB-V202?

light_curve_f9f61a36-b5ed-4577-9573-fe02d86a9594Figure 4: ASAS-SN light curve (last updated on March 24th, 2020) showing a high-amplitude (g ~2 mag), long-lasting fading event that appears to have lasted more than 100 days. Image credit: ASAS-SN.

As a result of the object’s high amplitude variations and highly blue continuum, it can be difficult to know if DeGaPe 71 intrinsically displays [OIII] and/or Hα emissions, or if the observed APO/SHO colours are artefacts from the above two properties (especially knowing that the individual APO/SHO filters may have been taken months apart). As mentioned previously, based on the comparison of SHS Short Red and Hα plates (see figure 5), there does not appear to be obvious signs of Hα emissions (at least at first glance, to the naked-eye), however these plates were taken over one year apart, thus the same problem applies here too….

DeGaPe71_SHSFigure 5: SHS plate comparison: Short Red (r) plate (left) and Hα (right), showing no obvious difference in brightness. This might suggest that the object does not display obvious Hα emissions, or it may be an artefact due to its highly variable nature. Image credit: SuperCosmos Halpha Sky Survey (SHS).

PN G326.9+08.2 – Nova Remnant?

PN G326.9+08.2 is a compact blue source (BP-RP= 0.55, Gaia DR2) in Lupus that was catalogued as a probable PN candidate based on the appearance of PN-like emission lines in MASH-II spectra (Miszalski et al., 2008) measured in 2007 (spectra are not publically available). However, similarly to DeGaPe 71, PN G326.9+08.2 does not display any obvious Mid-IR excess, typically observed in compact PNe. In addition, the coloured DSS2 images from Aladin Lite show the object to appear bright and extremely red (see figure 5), appearing much brighter than the reported Gaia DR2 magnitudes (G= +18.3 mag).

Nova_PN_DSS2Figure 6: Colour-combined DSS2 (Red + Blue plate) showing PN G326.9+08.2/USNO-B1.0 0423-0551514. Notice the object’s unusually red colour (much brighter than in the blue band). The colours are not a result of the object’s intrinsic colours, but however result from the combination between a blue plate (measured during its quiescent phase) and a red plate (measured during outburst). Notice the weak saturation “spikes”. This is the image that lead to the discovery of its high amplitude nature.

As can be seen when comparing archival DSS plates, it is obvious that the unusually red colour in figure 6 is the result of the object’s highly variable nature rather than its intrinsic colours (see figure 7). Indeed, the coloured DSS2 images from Aladin Lite consist of a DSS Blue plate taken in the 1970s or 80s, combined with Red plate taken on June 1993 in which the object appears very bright (R= +9.8 mag). No other eruptions were detected in ASAS-3 or ASAS-SN data, however the object appears brighter in the 1997 DSS plates compared to the those taken in 1992 (see figure 7).

NOva_1993_June_DSSFigure 7: DSS Red plates showing the variability of  PN G326.9+08.2/USNO-B1.0 0423-0551514. The object appears extremely bright (R= +9.8 mag) on the plate taken from June 1993. The object also appears relatively bright in the plate taken on April 1997 compared to that of 1992. Image credit: DSS Plate Finder.

Once the variability was first reported (official name AAVSO/VSX name: USNO-B1.0 0423-0551514), it was initially classified as a possible dwarf nova (UG type CV), with thoughts that it could be of the WZ-Sagittae subtype (UGWZs). These are UGs that have rare eruptions (up to decade intervals), but that can brighten up to 7 – 8 magnitudes (visual) during outburst. Indeed, this would explain the lack of outbursts detected in ASAS-SN and ASAS-3, as well as the high amplitude of the event (R~ 8 mag). Note that the amplitude is very similar to V1838 Aql a UGWZ that last had an outburst in May/June 2013 (CR = 8.5 mag – r’ = 18.5 mag).

Nova_ASASSN_CurvesFigure 8: ASAS-SN light curves showing the brightness of Nova Musca 2018 and Nova Lupi 2018 since their outbursts. Note the very progressive fading of both these events, especially the former. The “dip” in brightness at HJD 2458200 (Nova Musca) is due to dust temporarily obstructing good portions of the optical light. Image credit: ASAS-SN.

However, based on the large distance of the object (~ 15.000 ly according to Gaia DR2 astrometry), Taichi Kato suggested that the eruption from 1993 was possibly an overlooked nova rather than a UG. Indeed, novae are generally much less recurrent than UGWZs, which would also be coherent with the lack of outbursts detected in ASAS-SN and ASAS-3. In addition, novae tend to take years before they fully reach their pre-outburst (minimum) magnitude (see figure 8), which would explain the object’s brightness in the 1997 DSS red plates, compared to the ones from 1992.  This may equally explain the PN-like optical spectrum obtained by the MASH-II survey, as well as the apparently strong Hα in the SHS plates (see figure 9).

Nova_1993_Juna_HalphaFigure 9: SHS plates centered on PN G326.9+08.2/USNO-B1.0 0423-0551514. The images were taken in 2000 and are indicative of strong Hα emissions, at least at the time. Image credit: SuperCosmos Halpha Sky Survey.

Indeed, over the course of a nova outburst, the optical properties are dominated by the ejected matter (i.e. gas and dust) from the eruption (the progenitor remains obscured by this material). At the initial stages, the heat of the material gives it the appearance of a “star”. However, as time progresses (weeks or months?), the hot gas and dust (remnant) continues to expand and cool, in addition to being ionized by the central WD. This eventually gives it the appearance of a PN (and/or Wolf-Rayet star), most notably due to [OIII] and Hα emissions, as well as in regards to its continuum. This is referred to as the “nebular phase”. A better and more detailed explanation can be found here. A current example of a nova undergoing its “nebular phase” is most notably Nova Musca 2018 (V0357 Mus). Its discoverer, Rob Kaufman, imaged the object in late-February and found that it displayed “turqouise” colours in his optical images (see figure 10), giving it a similar appearance to many compact PNe. More information on this nova can be found in an old blog post I wrote around the time of the outburst.

87370938_2745849458825611_5823915845956403200_nFigure 10: Nova Musca 2018 as imaged by Rob Kaufman on February 26th, 2020. The turquoise colours of this object are due to the surrounding (now ionized) gas and dust ejected from the 2018 Nova eruption (rather than intrinsic to the progenitor itself). This gives it a similar appearance compact PNe. The main difference is however that the origin of the dust/gas in PNe are due to the progressive loss of outer layers from ancient red giant stars, rather than “brief” cataclysmic eruptions. (c) Rob Kaufman.

Consequently, in regards to PN G326.9+08.2, it is possible that the PN-like MASH-II spectra from 2007 are signatures of the remnants ejected from an undiscovered nova outburst that occured in June of 1993. Note however that the MASH team mentions the emissions-lines to be relatively weak (i.e. low S/N). This is interesting knowing the Hα response in the SHS Hα plate appears relativy strong (see figure 9). While the quality of the spectra may possibly be due to the observing conditions(?), it could perhaps suggest that the emissions may have faded over time (note that the SHS plates were taken in 2000), maybe as a result of the expansion and dissolution of the remnant? (I am quite unsure regarding the latter statements, hence please correct me if my reasoning is wrong! 🙂 )

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