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</o:shapelayout></xml><![endif]--></head><body lang=EN-GB link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Peter<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> Comets-disc [mailto:comets-disc-bounces@lists.britastro.org] <b>On Behalf Of </b>Martin Smith<br><b>Sent:</b> 08 April 2020 21:53<br><b>To:</b> BAA Comets discussion list<br><b>Cc:</b> BAA Comets discussion list<br><b>Subject:</b> Re: [BAA Comets] Sudden changes on C/2019 Y4 (ATLAS)<o:p></o:p></span></p><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal>Single 180sec image of 2019Y4 tonight, now lacking the bright coma from a few days ago. 120mm f7.<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal><img width=383 height=472 id="_x0000_i1025" src="cid:image001.jpg@01D60DF7.01521C90" alt=Y4-20200408-120sec.jpg><o:p></o:p></p></div></div></div></div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>On Wed, Apr 8, 2020 at 8:03 AM Nick James <<a href="mailto:ndj@nickdjames.com">ndj@nickdjames.com</a>> wrote:<o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm'><p class=MsoNormal>Many thanks to everyone who has submitted images and astrometry of this <br>comet over the last few days.<br><br>The astrometry continues to show growing residuals aligned with the tail <br>direction. Last night these residuals were around 7 arcsec. I think that <br>this is mainly due to the fact that the bright inner part of the coma is <br>now much more diffuse and extended in the tailward direction and so the <br>astrometric centroiding algorithm biases the position away from the <br>original nucleus position.<br><br>A number of observers have reported fragments or condensations in the <br>tail. These are difficult to detect and I have not seen these in my <br>images over the last few nights. What I have seen is a gradual reduction <br>in the peak pixel ADU count and a flattening/broadening of the downtail <br>coma brightness profile. The attached plot shows a cut through the <br>photocentre aligned on the tail PA (positive offsets are tailward) for <br>five nights from March 25 to April 7. You can see that peak pixel ADU <br>falls from around 8000 to 1200 in that time (a fade of around 2 <br>magnitudes) and the profile is broader with a more gradual tailward <br>slope in the later images. It looks to me as if the nucleus has <br>completely fragmented and what we are seeing is a cloud of rubble <br>migrating down the tail. This explains the large astrometric residuals <br>in RA since the photometric centroid is no longer aligned with the <br>original nucleus.<br><br>If I am right the comet will continue to fade and become more diffuse <br>and we will end up seeing something like this:<br><br><a href="http://www.britastro.org/cometobs/2010x1/2010x1_20111022_ligustri.jpg" target="_blank">http://www.britastro.org/cometobs/2010x1/2010x1_20111022_ligustri.jpg</a><br><br>This is the dust trail left by C/2010 X1 (Elenin) when it fragmented in <br>2011. Only observations will tell. Please keep this comet under <br>observation and send your results to the section.<br><br>Nick.<br>_______________________________________________<br>Comets-disc mailing list<br><a href="mailto:Comets-disc@lists.britastro.org" target="_blank">Comets-disc@lists.britastro.org</a><br><a href="http://lists.britastro.org/mailman/listinfo/comets-disc" target="_blank">http://lists.britastro.org/mailman/listinfo/comets-disc</a><o:p></o:p></p></blockquote></div></div></body></html>