SOHO and STEREO’s View of a Recent Impressive CME!

On July 15th, at around 20:00 – 21:00 UT, the SOHO and STEREO-A spacecraft were treated to a spectacular Coronal Mass Ejection! Based on STEREO-A’s observations, the event occured on the backside of the Sun, as seen from Earth. This post is simply to report on the beautiful images that SOHO and STEREO caught of the eruption.

Fig. 1: The Coronal Mass Ejection (CME) from July 15th as seen from various SOHO/LASCO and STEREO/SECCHI telescopes. Quite a spectacular view! Image credit: ESA/NASA/NRL SOHO/LASCO and STEREO/SECCHI C3, C2, EUVI304-A and COR1-A.

Coincidentally, the event took place almost exactly 21 years after the Bastille day event (July 14th, 2000). In my opinion, this is among the most impressive CMEs observed by SOHO and STEREO during this solar cycle! Such events should further increase in frequency as the Sun continues to increases in activity (the peak of the current solar cycle should take place around 2025).

View from SOHO/LASCO

As seen from SOHO/LASCO (hence Earth’s perspective), the event appeared as a halo CME which included a prominent ejection to the SE of the Sun. This ejection could be tracked for days in SOHO/LASCO C3, as it slowly dissipated into space.

Fig. 2: The Coronal Mass Ejection (CME) observed on July 15-16th, 2021, is among the more impressive ones to have taken plce (so far) during this solar cycle (in my opinion). Image credit: ESA/NASA SOHO/LASCO C3.
Fig. 3: The Coronal Mass Ejection (CME) as observed by SOHO/LASCO C3 from July 15-17th, 2021. Notice how the matter slowly extends and fades as it dissipates into space. Image credit: ESA/NASA SOHO/LASCO C3.

View from STEREO/SECCHI

The STEREO-A spacecraft is located about 46° behind the Earth, and thus saw the CME from a significantly different perspective. Based on these images, it was obvious that the ejection was headed in the opposite direction from Earth. Hence, it will not enter the FOV of the heliospheric imagers.

Fig. 4: The Coronal Mass Ejection (CME) as observed by EUVI 304 on July 15, 2021. Image credit: NASA/NRL STEREO/SECCHI EUVI 304-A.
Fig. 5: The Coronal Mass Ejection (CME) as observed by EUVI 195-A and COR1-A on July 15, 2021. Image credit: NASA/NRL STEREO/SECCHI EUVI 195-A and COR1-A.
Fig. 6: The Coronal Mass Ejection (CME) as observed by COR2-A on July 15, 2021. Image credit: NASA/NRL STEREO/SECCHI COR2-A.

Copyright (c) 2021 Trygve Prestgard. All rights reserved.

SOHO and STEREO Observe the Demise of a Bright Sungrazer!

From May 7th to May 10th, the coronagraphs aboard the SOHO and STEREO-A spacecraft witnessed the rise and fall of a bright Kreutz-group comet. In fact, the comet reached +1 mag on May 10th, making it among the brightest sungrazers to have been discovered by SOHO, more specifically through the Sungrazer Project. Author of the discovery is Szymon Liwo, an amateur astronomer and long-time SOHO comet hunter from Świdnica, Poland.

Fig 1: The recently-discovered Kreutz-group comet plunging towards its demise as seen in SOHO/LASCO C3 (blue) and C2 (red) images. Notice the saturation “spikes”, due to the comet’s brightness. The intensity of the saturation spikes indicate that the comet probably reached mag +1, at its brightest. Image credit: ESA/NASA SOHO/LASCO.

On May 7th at 12:23 UT, amateur astronomer Szymon Liwo reported a previously unknown Kreutz-group comet entering the SOHO/LASCO C3 FOV. As it is rare that Kreutz comets are bright enough to be detectable near the edge of the SOHO/LASCO images, it was obvious that the object had the potential of becoming an impressive comet. Fortunately it lived up to expectations. On May 10th the comet reached mag +1, making it among the brightest sungrazing comets to have been discovered by SOHO. In fact, the comet’s brightness caused it to saturate the SOHO/LASCO detectors (see Fig. 1). Unfortunately, soon after entering the SOHO/LASCO C2 FOV, the comet started to rapidly fade. Eventually, it fully disintegrated. This is the fate of most sungrazers, especially those discovered by SOHO and STEREO. In fact, no SOHO-discovered Kreutz-group comet has ever survived perihelion.

Fig 2: Slowed-down animation showing the comet’s small trailing companion. It’s possible that this object was a small fragment that recently broke apart from the bright comet. Unfortunately, the fragment was too faint to be apparent in C3 or COR2-A images. Image credit: ESA/NASA SOHO/LASCO C2.

On May 10th, SOHO comet hunter Worachate Boonplod (Thailand) reported a faint object closely trailing the Szymon’s bright comet (Fig. 2). Due to its apparent proximity, it is possible that Worachate’s object may have been a fragment that recently split off from the comet. It is not unusual that Kreutz-group comets travel in close pairs, or in clusters (e.g. Fig. 3). This is unlike the Meyer-group, which is likely to be a much more evolved comet family.

Fig 3: An example of a close cluster of five Kreutz-group comets. The bright objects is SOHO-3478, another impressive discovery made by Szymon Liwo. Image credit: ESAN/ASA SOHO/LASCO C2.

On May 10th Szymon’s comet entered the FOV of STEREO-A’s SECCHI COR2 telescope. As can be seen in Fig. 4, the comet follows a significantly different trajectory in these images relative to SOHO’s observations (Fig. 1). This is due to the differing perpectives between both spacecraft. Indeed, STEREO-A is located 52° behind SOHO, meaning that it observed the sky as SOHO would in February. In Fig. 4, notice how the tail (debris trail) grows in length, as the comet progressively fades. This is due to the object rapidly disintegrating. Similarly to LASCO, the comet was not observed to re-emerge from behind the COR2-A solar occulter. Worachate’s faint fragment was too faint to be recovered in STEREO/SECCHI.

Fig 4: Animation showing the bright sungrazer as seen in COR2-A images. STEREO-A was located 52° behind Earth at the time of these images, hence explaining the difference in the comet’s trajectory in STEREO/SECCHI and SOHO/LASCO. The trailing companion was too faint to appear in these images. Image credit: NASA/NRL STEREO/SECCHI COR2-A.

Szymon Liwo has been an active member of the Sungrazer project for 10 years. As a result of years scrutinizing the SOHO/LASCO images, he has found a total of 110 sungrazing comets. In fact, among Szymon’s numerous discoveries are several objects similar to his recent Kreutz-group comet. Indeed, Szymon is also the [co-]discoverer of the following objects: SOHO-2143, SOHO-2505, SOHO-3069 and SOHO-3478 (Fig. 5). He also co-discovered one SWAN comet, C/2015 F5 (SWAN-Xingming). Moreover, Szymon is uncle of Rafal Biros, the current record holder for youngest SOHO comet discoverer.

Fig 5: Four other bright Kreutz-group comets that Szymon Liwo has also discovered. Image credit: ESA/NASA SOHO/LASCO C2, NASA/NRL STEREO/SECCHI COR2-A.

Note: coincidentally, this bright comet appeared almost exactly a decade after SOHO-2062, a similarly-bright Kreutz-group comet. More information about that comet can be found here. Szymon’s comet has yet to receive an official designation.

Acknowledgements: I wish to thank Szymon Liwo for the information that he provided in order to complete this article.

References: The information provided in this article is based on my personal analysis, the Sungrazer Comet Section facebook group, the Sungrazer Project website, and personal communication with Szymon Liwo. Additional sources have been incorporated into the text.

Copyright (c) 2021 Trygve Prestgard

C/2021 D1: A New Comet Discovered in SOHO/SWAN!

On February 26th, amateur astronomer Michael Mattiazzo (Australia) discovered a previously unknown comet using publicly available data from the SOHO/SWAN instrument! At the time of discovery, the object (C/2021 D1) was already near perihelion, and close to maximum brightness (mag +11). Due to the comet’s location (poor solar elongation), despite its brightness, it had escaped previous detection by ground-based minor planet surveys (e.g. ATLAS, CSS, Pan-STARRS). Fortunately, comet SWAN is moving east towards increasingly favorable skies. Sadly, however, it is slowly fading (currently ~ +12 mag), and is expected to have reached mag +15 by June of this year. The comet currently displays a somewhat condensed appearance, although a faint tail has been reported.

Fig. 1: Image extract showing the newly-discovered comet C/2021 D1 (SWAN) on March 1st. Notice the comet’s condensed morphology. (c) Michael Jäger

Michael Mattiazzo discovered C/2021 D1 (SWAN) on February 26th, using near-real-time Comet Tracker images from the SOHO/SWAN website. More specifically, he found the comet in data from February 19th – 23rd, where it was situated only tens of degrees NE of the Sun, in the Pegasus constellation. Located aboard the SOHO (Solar and Heliospheric Observatory) spacecraft, the SWAN (Solar Wind Anisotropies) cameras image the sky in the Lyman-α band, a wavelength that happens to be particularly sensitive to comets. In fact, although SWAN mainly focuses on the solar wind, it has also been used to study comets (e.g. Combi et al., 2000). In order to make this possible, the instrument must first block the overwhelming glare of the Sun and Earth. As a consequence of its build-up, SWAN is capable of imaging the sky at low solar elongation, a blind spot for most ground-based minor planet surveys. However, due to the very poor resolution of its images (of ~1°), only comets brighter than ~12 mag are possible to detect using this data. Despite this, SWAN has enabled the discovery of fourteen comets (not including two co-discoveries), as well as one recovery!  Most SWAN comets nowadays are found at low solar elongation, by amateur astronomers regularly studying the data. This was the case of C/2021 D1 (SWAN). Due to the faint nature of this comet in SWAN data (Fig. 2), it could not be confirmed using these images alone. Ground-based observations were hence necessary to confirm the comet.

Fig. 2: Comet C/2021 D1 (SWAN) as seen in SWAN Comet Tracker image extracts from February 20th to 27th, 2021. The animation inscludes many of the discovery images. At the time of these images were acquired, the comet was around +11 mag. Image credit: ESA/NASA/LATMOS SOHO/SWAN.

Following the comet’s discovery, attempts at constraining a rough orbit were made. This was particularly challenging, not only because of the short observation arc, but also due to the very low resolution of the SWAN Comet Tracker images. Consequently, preliminary estimations varied widely. Indeed, some proposed solutions suggested that it might have been a sunskirting comet, or that it was related to the Great Comet of 1686 (C/1686 R1). Other solutions suggested that it was headed for a close approach with Earth in early-March (which sadly did not happen). The uncertainty in the comet’s trajectory and location made it initially difficult to locate the comet from Earth. Astrophotographer Nicolas Lefaudeux (France) took a wide-field image of the region on February 27th, however early attempts at locating the comet in his image failed.

Fig. 3: Image extract of the recovery/confirmation image of C/2021 D1 (SWAN), taken on February 28th. (c) Michael Jäger.

On February 28th, after multiple attempts since the comet’s discovery, astrophotographer Michael Jäger (Austria) became the first to recover C/2021 D1 (Fig. 3). Jäger measured the comet to be ~10.5 mag in his confirmation image (Fig. 3). He further described the comet as displaying a condensed appearance, with an apparent diameter of 3.5′, and even showing a very faint tail. The comet was also independently recovered by Krisztián Sárneczky (Hungary). Following Jäger’s announcement, other astrophotographers (e.g. Luca Buzzi and Nick James) were also able image the comet that same day (Fig. 4). Based on these new observations, astronomer Alan Hale (U.S.A.) was able to locate the comet in Lefaudeux’s image.

Fig. 4: Extract of the image acquired of C/2021 D1 (SWAN) by Nick James on February 28th. This was among the first images taken after its recovery by Michael Jäger. (c) Nick James

Since the comet’s recovery, with the increasing observations and growing observation arc, the comet has been found to be periodic (P= 76.9 years), with a perihelion on that occurred on T= February 27th at q= 0.89 AU. Fortunately, the comet is slowly moving towards more favorable skies. However, it is also slowly fading. By June the comet will likely have faded below mag +15, assuming no outbursts occur.

Acknowledgements: I wish to thank Michael Jäger and Nick James for granting me permission to use their images of C/2021 D1 (SWAN).

References: The Comet_mailing_list.io group, Gideon van Buitenen’s website, the facebook ICQ Comet observations page, and the SOHO/SWAN website. Other sources are referred to in the text.

Copyright (c) 2021 Trygve Prestgard. All rights reserved.

SOHO and STEREO’s Late-December “Holiday Comets” of the 2000s!

In the past, many interesting and/or bright comets have coincidently appeared in SOHO’s LASCO FOV around the late-December holidays. For instance, December 2011 was marked by the spectacular passage of comet C/2011 W3 (Lovejoy); 2010 by SOHO’s 2000th comet discovery and a sungrazer comet “storm”, while SOHO’s first “bright” (mag +1) sungrazer was observed only a couple of days before Christmas day, 1996. In fact, currently, the bright comet C/2020 S3 (Erasmus) is slowly transiting SOHO/LASCO c3 (Figure 1). In this blog post I will be discussing the various late-December “holiday comets” that appeared throughout the 2000s.  

Figure 1: Comet C/2020 S3 (Erasmus) seen entering the SOHO/LASCO C3 FOV from December 18th to 21st, 2020. Note the presence of Mercury (below the solar occulter) and the Milky Way’s galactic plane. The bright streaks in the final frame of this animation are particles of debris, probably material that has flaked off from the spacecraft’s insulation layers. Image credit: ESA/NASA SOHO/LASCO C3.

2000 – Comet C/2000 W1 (Utsunomiya-Jones)

This small comet was discovered independently by Syugu Utsunomiya (Japan) and Albert Jones (New Zealand) in mid-November, 2000. At the time of discovery, the comet was already +8.5 mag, although decreasing in solar elongation. In fact, it was only about five weeks from perihelion, which took place on December 26th at only q= 0.32 AU from the Sun. A couple of days later, the comet entered the SOHO/LASCO C3 FOV. As can be seen in the animation below (Figure 2), the comet was quite obvious in SOHO/LASCO, shining at around mag +6, although slowly fading. It left the FOV on January 4th, 2001. Unfortunately, the comet eventually disintegrated on its journey outbound. Some ground-based images of this comet (before and after perihelion) can be found here.

Figure 2: Comet C/2000 W1 entering the SOHO/LASCO C3 FOV from December 28th to 30th, 2000. Notice the obvious tail dynamics, indicative of its interaction with the solar wind. Image credit: ESA/NASA SOHO/LASCO C3.

2003 – Several Kreutz-group Sungrazing Comets

From December 19th to 27th, 2003, a total of twelve Kreutz-group sungrazing comets were recorded transiting the SOHO/LASCO FOV. In fact, there may have been more, seen that this period was marked by numerous data gaps. As can be seen in the animation below (Figure 3), two of the observed sungrazers were relatively bright, displaying long and obvious tails (SOHO-714 and SOHO-721).  Three other bright (although significantly fainter) members were also included in the animation, these generally being fairly condensed or globular, with short tails (SOHO-713, 715, and 719). An even fainter member, SOHO-720, can also be seen (see details below). These objects were discovered by members of the Sungrazer citizen science project, more specifically by Michael Boschat, Rainer Kracht, Rob Matson, John Sachs and Xing-Ming Zhou. All the comets disintegrated soon after discovery, as most SOHO-discovered Kreutz-group comets tend to do.

Figure 3: Several Kreutz-group comets were seen in the last couple of weeks of 2003. Six of these are shown in the animation above. In order of apparition: SOHO-713 (disc: Michal Boschat), SOHO-714 (disc: John Sachs), SOHO-715 (disc: Rainer Kracht), SOHO-719 & 720 (disc: Rob Matson) and SOHO-721 (disc: Xing-Ming Zhou). Image credit: ESA/NASA SOHO/LASCO C2.

As mentioned above, in addition to the more obvious comets, a much fainter member is also visible in Figure 3. Unlike the other five, this one resembled a small, faint, diffuse cloud of debris, indicating that it had probably disintegrated prior to entering the SOHO/LASCO C2 FOV. This object trails SOHO-719. Below (Figure 4) are one of the frames best showing both comets together (contrast enhanced).

Figure 4: Contrasted image better showing both the faint diffuse (SOHO-720) and the condensed (SOHO-719) Kreutz-group sungrazing comets observed on December 27th, 2003. Image credit: ESA/NASA SOHO/LASCO C2.

Due to data gaps, the extent of the tail of SOHO-721 could not be seen in SOHO/LASCO C2 (Figure 3). Its long tail was however clearly visible in C3, despite the lower resolution, as can be seen in the animation below (Figure 5). Jumps in the animation are also due to data gaps.

Figure 5: SOHO-721 as seen in SOHO/LASCO C3 showing the extent of its tail (debris trail). During a portion of this animation it passed behind the occulting arm, making it undetectable in those images. Image credit: ESA/NASA SOHO/LASCO C3.

2004 – Comet C/2004 V13 (SWAN) and C/2004 Y4 (SOHO)

Also known as, SOHO-884, C/2004 V13 (SWAN) was a relatively bright sunskirting comet that transited the SOHO/LASCO C3 FOV from December 16th to 21st, 2004. The object was initially reported by Michael Mattiazzo (Australia) in SOHO/SWAN data from November 2004, however it was very faint in those images and hence difficult to confirm. Moreover, the object was at low solar elongation, hence further adding to the above difficulty. Fortunately, Michael noted that it was on a path towards the Sun and predicted that it might enter the SOHO/LASCO FOV in mid-December.

Figure 6: Comet C/2004 V13 (SWAN) entering the SOHO/LASCO C3 FOV (images from December 16th to 18th, 2004). Image credit: ESA/NASA SOHO/LASCO C3.

On December 16th, 2004, former SOHO comet hunter John Sachs (USA) reported a bright non-group comet entering SOHO/LASCO C3 FOV. The object was spotted independently by Heiner Otterstedt (Germany) who posted his report only 1,5 minutes after John’s. Moments later, former SOHO comet hunter Sebastian Hönig suggested that John’s comet was the one reported by Michael in SWAN data. By combining the newly obtained SOHO observation with those from SWAN, he estimated (based on his own rough calculations) that the object would reach perihelion on December 21st, at only q= 0.23 AU from the Sun. These two parameters are comparable to those of the final solution published by the MPC. Figure 6 shows that comet entering the SOHO/LASCO C3 FOV. Notice the obvious tail dynamics, resulting from its interaction with the solar wind.

Unfortunately, despite its bright appearance in SOHO/LASCO, the comet soon rapidly faded and morphed into a diffuse appearance by the time it recovered by ground-based instruments (less than two weeks after exiting the C3 FOV) (Figure 7). It is hence clear that C/2004 V13 did not survive its solar encounter.

Figure 7: Comet C/2004 V13 (SWAN) as imaged by Michael Mattiazzo in early-January, 2005. Note the comet’s very diffuse appearance, indicating that it likely disintegrated. (c) Michael Mattiazzo.

Coincidentally, soon after the passage of comet SWAN in SOHO/LASCO, another obvious SOHO non-group comet was spotted! Designated C/2004 Y4 (SOHO), it was significantly fainter than C/2004 V13, but relatively bright as far as SOHO non-group comets go (around mag +6). It was discovered in Heiner Otterstedt in C3 images from Dec 24th, 2004, and was detectable in SOHO/LASCO until December 28th. As described by Heiner on his website: “It took a long clockwise arc around the edge of C3, becoming moderately bright and then faded out to the lower right of the Sun” (Figure 8). Overall, C/2004 Y4 displayed a condensed appearance, with a somewhat elongated morphology at its brightest. No obvious tail was observed. The comet was not recovered from Earth.

Figure 8: Comet C/2004 Y4 (SOHO) as seen around perihelion, at its brightest. Images are from December 25th and 26th, 2004. Notice the obvious coronal mass ejection that coincidentally took place at the passage of this comet. Image credit: ESA/NASA SOHO/LASCO C3.

2008 – Comet 210P/Christensen

Officially discovered by Eric Christensen of the Catalina Sky Survey (CSS) in May of 2003, this small comet (P/2003 K2) was unfortunately difficult to observe due to poor solar elongation. As a consequence, it was difficult to establish a highly reliable orbit at the time. Note that Michael Mattiazzo (see above) reported this object individually in SOHO/SWAN images taken the month prior, but its sighting could not be confirmed in these images alone. Fortunately, the comet was recovered by Alan Watson (Australia) in STEREO/SECCHI HI1-B images of December 2008 (Figure 9), as it was again heading towards perihelion. In those images the comet had already reached mag +10.

Figure 9: Comet 210P/Christensen as seen in STEREO/SECCHI HI1-B only a couple of days after its recovery by Alan Watson. Image credit: NASA/SSC STEREO/SECCHI HI1-B.

Interestingly, the comet passed almost perfectly between the Sun and the Earth during its 2008 return, making an easy target for SOHO. Indeed, the resulting forward scattering made it easily observable in SOHO/LASCO (between mag +5 and 6), as can be seen in the C3 animation and the C2 image extract below (Figures 10 and 11). A faint tail could also be seen. In addition, a few Kreutz-group sungrazers coincided with the appearance of this comet in SOHO/LASCO.

Figure 10: Comet 210P/Christensen during its transit through the SOHO/LASCO C3 FOV. The images were taken between December 21st and 23rd, 2008. Note also three small Kreutz-group comets were also visible. Image credit: ESA/NASA SOHO/LASCO C3.
Figure 11: Comet 210P/Christensen as seen in a SOHO/LASCO C2 image extract. Note is globular appearance and its faint tail. Image credit: ESA/NASA SOHO/LASCO C2.

2009 – Sungrazing Comet C/2009 Y4 (STEREO)

Also known as STEREO-23, this bright Kreutz-group sungrazer was discovered by amateur astronomer Alan Watson (Australia) via the Sungrazer citizen science project (see above). He first spotted the comet in near-real time STEREO/SECCHI HI1-A images of December 31st, 2009, where the comet was hardly detectable above the background noise level. However, the comet rapidly brightened over the next few days, reaching mag +1 on January 3rd. The days following its discovery the comet was recovered in SOHO/LASCO and STEREO-B SECCHI imagery. The animation below (Figure 12) shows STEREO-23 as seen in STEREO/SECCHI HI1-B images. Notice how the comet develops a long (a couple of degrees) tail, that remained visible even after it left the FOV. C/2009 Y4 (STEREO) is one of the brightest sungrazers discovered by STEREO.

Figure 12: Comet C/2009 Y4 (STERE0) as seen in STEREO/SECCHI HI1-B images. Notice the obvious tail dynamics, indicating strong interaction with the solar wind. Image credit: NASA/SSC STEREO/SECCHI HI1-B.

SOHO comet hunter Michal Kusiak (Poland) was the first to recover the comet in SOHO/LASCO. As it entered the SOHO/LASCO C3 FOV the comet was hardly detectable (around mag +10), but it quickly brightened to the point where it saturated the SOHO/LASCO C2 and C3 detectors on January 2nd and 3rd (Figure 13). Soon after reaching peak brightness in the early morning of January 3rd (UT), the comet started to rapidly disintigrate. It did not survive its solar encounter and was not recovered after it passed behind the solar occulter (Figure 14).

Figure 13: Comet C/2009 Y4 (STEREO) as seen near its peak brightness in SOHO/LASCO C3 (Image taken on January 3rd at 02:42 UT). Venus is the bright object seen to the right of the Sun. The horizontal bars associated with both these objects are due to saturation of the SOHO/LASCO detectors (“pixel bleeding”). Image credit: ESA/NASA SOHO/LASCO C3.
Figure 14: The final moments of comet STEREO-23 as seen in SOHO/LASCO C2. It did not re-emerge from behind the solar occulter. Image credit: ESA/NASA SOHO:LASCO C2.

Amateur astronomer Jiangao Ruan (China) discovered a small leading fragment in SOHO/LASCO C2 images (SOHO-1782), that was later also recovered in STEREO/SECCHI HI1-A . It was only visible in three or four SOHO/LASCO images (five are generally required in order to confirm a SOHO comet), hence had it not been for STEREO/SECCHI, this comet would have remained unconfirmed (probably an X/comet). It was too faint to appear in STEREO-B and SOHO/LASCO C3 images.

Figure 15: Comet SOHO-1782, a tiny leading fragment of STEREO-23, as seen in SOHO/LASCO C2. Unlike STEREO-23, this comet did not display any obvious tail, but appeared quite diffuse. Image credit: ESA/NASA SOHO/LASCO C2.

References: Other than my own observations/interpretations, the information present in this article are from the Sungrazer Project website,, the BAA comet section, and Seiichi Yoshida’s website. Other sources have been incorporated in the text. Images, if not otherwise specified, were acquired from NASA’s SOHO website and the STEREO Science Center.

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

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! 🙂 )