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

[Nick James]

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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