Electronic Telegram No. 4250 Central Bureau for Astronomical Telegrams Mailing address: Hoffman Lab 209; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A. e-mail: cbatiau@eps.harvard.edu (alternate cbat@iau.org) URL http://www.cbat.eps.harvard.edu/index.html Prepared using the Tamkin Foundation Computer Network COMET P/2010 V1 = P/2015 Y2 (IKEYA-MURAKAMI) Further to CBET 4235, P.-J. Dekelver, Maasmechelen, Belgium, reports that he and W. Cuppens have obtained a series of images of comet P/2015 Y2 with the 2-m f/10 "Faulkes Telescope North" at Haleakala over several weeks in January, showing additional components. On Jan. 4, components A and B were visible; component A appears bright and concentrated (which was measured to be magnitude 20.5-20.7), while component B appears more like a debris field. On Jan. 8 and 11, a third component (C) is visible at mag 21.6-22.4, more concentrated than component B (mag 22.3-22.7). On Jan. 18, another feature was identified, which Dekelver denoted component D (mag 22.2-22.9), but the "B" debris field was likely a cluster of sub-fragments of which features B and D were the brightest; components A and C were measured to have magnitudes 20.2-20.5 and 21.7-22.3, respectively. By Jan. 18, the fainter sub-fragments presumably disappeared due to their extremely short lifetimes, leaving a B/D debris field that will likely continue to disintegrate and evolve (see item by Sekanina, below). Dekelver addes that observations on Jan. 29.6 yield magnitudes 20.1-20.3 and 19.7-19.8 for components A and C, respectively. Their images have been posted at URL http://www.astronomie.be/pieterjan.dekelver/Mainpage.htm. Note that most other CCD astrometrists measure the brightness of components A and B to be 1-3 magnitudes brighter (cf. MPEC 2016-B77). Z. Sekanina, Jet Propulsion Laboratory, writes that recent developments (cf. MPEC 2016-B77) suggest that the 2010 outburst had more severe consequences for the comet's evolution than to accompany a single fragmentation event. It triggered a fragmentation process that continued over an extended period of time after the outburst (and apparently is still continuing) -- a fairly common phenomenon among split comets that is referred to as "cascading fragmentation" (e.g., Sekanina 2002, Ap.J. 566, 577). As already pointed out on CBET 4235, fragment A is not a primary component. Since component B was the only other fragment known by Jan. 4, it was the candidate for the primary due to its location, but not by its very diffuse appearance. The subsequent detection of fragment C (MPEC 2016-B77), about 12"-13" east of component B on Jan. 8 and 15"-16" on Jan. 11-19, resolved this ambiguity: nucleus C is the primary, the presumably most-massive piece -- though barely active until several days ago -- of the pre-outburst parent comet, unless yet another persistent fragment should still be detected to the east of component C. The computations suggest that companion A separated from the parent most probably in early November 2012 (with an uncertainty of +/- 2 months). at a rate of 0.36 +/- 0.04 m/s, and was subjected to a differential nongravitational deceleration of 5.2 +/- 0.6 units of 10^{-5} the solar gravitational acceleration. This solution fits the 12 most consistent offsets of A from C between Jan. 8 and 29 moderately better than a solution forcing the fragmentation time to coincide with the time of the 2010 outburst; it is suggested that component A should survive beyond the 2016 perihelion. The histories of companions B (probably a cluster of sub- fragments) and D cannot as yet be determined with confidence, except that these objects are likely to be more recent and less massive products of the fragmentation process than companion A. For companion B, a very tentative solution, based only on the observations Jan. 11-29, suggests that it may have split off in the second half of 2013 or the first half of 2014. The observations of B from Jan. 8 leave systematic residuals of 2" to 4" from this solution, and a more recent origin of B is plausible. It is possible that the Jan. 8 astrometric positions of B refer instead to D, in which case component D would have separated only in mid-October 2015, some 150 days before perihelion and, being subjected to a deceleration of about 25 units, would be a short-lived fragment. Further astrometric observations of nuclei C and (if they still exist) B and D are urgently needed. Predicted separations and position angles of component A from C (for 0h TT): Feb. 2, 142", 291 deg; Feb. 12, 143", 289 deg; Feb. 22, 140", 289 deg; Mar. 3, 134", 289 deg; Mar. 13, 125", 290 deg; Mar. 23, 116", 292 deg. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2016 CBAT 2016 February 2 (CBET 4250) Daniel W. E. Green