[BAA Comets] Sudden changes on C/2019 Y4 (ATLAS)
Peter Tickner
peter.tickner1472 at btinternet.com
Wed Apr 8 22:42:38 BST 2020
I'm imaging it now and it is very hard to pick out an area to call the coma. It is visibly fainter again than two days ago.
Peter
From: Comets-disc [mailto:comets-disc-bounces at lists.britastro.org] On Behalf Of Martin Smith
Sent: 08 April 2020 21:53
To: BAA Comets discussion list
Cc: BAA Comets discussion list
Subject: Re: [BAA Comets] Sudden changes on C/2019 Y4 (ATLAS)
Single 180sec image of 2019Y4 tonight, now lacking the bright coma from a few days ago. 120mm f7.
Y4-20200408-120sec.jpg
On Wed, Apr 8, 2020 at 8:03 AM Nick James <ndj at nickdjames.com> wrote:
Many thanks to everyone who has submitted images and astrometry of this
comet over the last few days.
The astrometry continues to show growing residuals aligned with the tail
direction. Last night these residuals were around 7 arcsec. I think that
this is mainly due to the fact that the bright inner part of the coma is
now much more diffuse and extended in the tailward direction and so the
astrometric centroiding algorithm biases the position away from the
original nucleus position.
A number of observers have reported fragments or condensations in the
tail. These are difficult to detect and I have not seen these in my
images over the last few nights. What I have seen is a gradual reduction
in the peak pixel ADU count and a flattening/broadening of the downtail
coma brightness profile. The attached plot shows a cut through the
photocentre aligned on the tail PA (positive offsets are tailward) for
five nights from March 25 to April 7. You can see that peak pixel ADU
falls from around 8000 to 1200 in that time (a fade of around 2
magnitudes) and the profile is broader with a more gradual tailward
slope in the later images. It looks to me as if the nucleus has
completely fragmented and what we are seeing is a cloud of rubble
migrating down the tail. This explains the large astrometric residuals
in RA since the photometric centroid is no longer aligned with the
original nucleus.
If I am right the comet will continue to fade and become more diffuse
and we will end up seeing something like this:
http://www.britastro.org/cometobs/2010x1/2010x1_20111022_ligustri.jpg
This is the dust trail left by C/2010 X1 (Elenin) when it fragmented in
2011. Only observations will tell. Please keep this comet under
observation and send your results to the section.
Nick.
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