[BAA Comets] Sudden changes on C/2019 Y4 (ATLAS)

[Martin Smith]

In case there's any confusion the image I posted earlier was from two night
ago, not tonight. Sorry.

On Wed, 8 Apr 2020, 22:42 Peter Tickner, <peter.tickner1472 at btinternet.com>
wrote:

> 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.
>
>
>
> [image: 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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