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!
Figure 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.
Figure 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.
Figure 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!
Figure 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!
Figure 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.
Figure 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.
Figure 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.
Figure 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! 🙂
Figure 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.
Figure 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).
Figure 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).
Figure 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.
Figure 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.
Figure 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
Figure 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.
Figure 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!
Figure 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
Figure 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).
Figure 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.
Figure 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).
Figure 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)
Figure 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.
Figure 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.
Figure 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.
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.