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).Figure 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 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.Figure 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.Figure 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 designation||Internal SOHO designation||Telescopes||Maximum apparent magnitude||Discoverer|
|C/2020 P4-C||SOHO-4049||C2||~+5-6 (fainter than A and C)||M.Uchina|
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.
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. Figure 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.