Electronic Telegram No. 4254 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/2015 Y2 = P/2010 V1 (IKEYA-MURAKAMI) Further to CBET 4250, J.-F. Soulier, Maisconcelles, France, reports the discovery of a fifth fragment of comet P/2015 Y2, here denoted component E, some 2'.5 to the west-northwest of fragment A on images taken with a 0.3-m f/3.8 reflector on Feb. 4.0 UT at magnitude 20.1-20.5 (cf. MPEC 2016-C30, where it was incorrectly identified as component D). Fragment E was also observed by R. A. Kowalski via the Mount Lemmon Survey with a 1.5-m reflector on Feb. 3.2 at mag 19.9-20.9 (MPEC 2016-C30); by P.-J. Dekelver with the 2-m "Faulkes Telescope North" f/10 reflector on Feb. 1.4 and 4.4 at magnitudes 21.8-21.9 and 21.9-22.2, respectively; with the 1.8-m Pan-STARRS1 telescope on Haleakala on Feb. 4.4 at magnitude 21.5-21.9; and by others. F. Colas, L. Maquet, and F. Pierret, IMCCE, Observatoire de Paris, report the discovery of a sixth fragment of comet P/2015 Y2 at magnitude 22.0 on images made on Feb. 5.05-5.11 UT with the 1-m telescope at Pic du Midi observatory, here denoted component F; other magnitudes for the comet's fragments are as follows: A, 19.0; B, 21.2; C, 18.2, E ("D" on MPEC 2016-C30), 20.9. Colas notes that fragment F is 16" from component E (i.e., from "D" of MPEC 2016-C30). Fragments A, C, E, and F appear condensed and elongated. Fragment B is rather more complex, and it seems to be in complete disintegration, with now two fragments (each of mag about 23) on each side at 10" distance from the central diffuse fragment, producing a wide debris trail. The discovery of a seventh fragment and an eighth fragment is reported by F. Kugel, Dauban, France, on images taken with a 0.4-m reflector on Feb. 5.95 UT. These fragments, respectively designated G and H, both have brightness around magnitude 21.0, each appears in p.a. 289 deg with respect to fragment C at distances 15'04" (fragment G) and 20'44" (fragment H). Other distances (in p.a. 289 deg) and magnitudes of the various fragments from component C (which was then measured to be mag 17.8) on Feb. 5.95: B, 21", 21.0; A, 2'25", 18.2; D, 4'50", 20.8. Regarding Kugel's earlier images image taken on Jan. 3.2 in poor seeing, fragment A is slightly condensed and of red mag 19.3-19.5, while fragment B is diffuse and of similar brightness. On Jan. 5.23, fragment A is well-condensed and slightly brighter, while fragment B is elongated toward p.a. 290 deg; the image for that date is posted at http://lesia.obspm.fr/comets/lib/display-obs1.php?Num=15013. Kugel has also posted additional images and information at website URL http://www.astrosurf.com/obsdauban/pages/P10V1.html. Colas et al. discovered an apparent ninth fragment, of magnitude 22.6 and designated here as fragment J, located 70" to the west-northwest of fragment E on images from Pic du Midi on Feb. 6.1 UT, which also show more details in the B/D complex; an image has been posted at the following website URL: http://www.picdumidi.eu/T1M/P2010V1-2E.jpg. Fragments B and D now form a dust complex with new fragments. Fragment D is fading to the north of B, and there are two fragments on each side of B. All the dust tails of these fragments are merging to form a wide structure of debris. Colas gives the magnitudes of other fragments on Feb. 6.1 as follows: A, 19; B, 21; C, 18.2; E, 21; F, 22.6. Z. Sekanina, Jet Propulsion Laboratory, writes that the fifth fragment, referred to above properly as E, cannot possibly be identified with fragment D (cf. CBET 4250). The issue with component E is whether it split off from the primary nucleus C or from companion A. At present this remains uncertain, since the available observations from Feb. 3-4 are about equally well fitted by either hypothesis. Very tentatively, the separation appears to have occurred in late February 2013 if from component C, but in mid-August 2014 if from component A, with a formal (a crude lower limit to actual) uncertainty of +/- 1 month in either scenario. Interestingly, the fragmentation time in the latter case coincides within 4 sigma with the time of the 2014 observation of the comet from Castelmartini (code 160; MPC 90863); the component observed then needs to be identified. The time of separation of A from C has now been refined as 2012 December 1, with a mean error of +/- 31 days, whereas the differential nongravitational deceleration was found to equal 5.25 +/- 0.30 units of 10^{-5) the solar gravitational acceleration; the separation velocity hardly changed at all from the previously published value (CBET 4250). A solution forcing the fragmentation time to coincide with the 2010 outburst now leaves a discernible systematic trend in the residuals in declination. If the nucleus fragmented during the outburst, the products may not have survived until now. The assembly of sub-fragments referred to as components B and D, in the past days detected only intermittently with larger telescopes, appears to have consisted, at various times, of at least four different clusters, a conclusion consistent with the description of the Feb. 5 images by Colas et al. Attempts to fit their motions relative to component C suggest that they were released generally between mid-2015 and November 2015, all receding with the decelerations of more than 10 units; even the most massive among them were short-lived. Available information does not yet allow one to model the fragmentation history of components F, G, H, and J. Preliminary separations and position angles of component E from A and C, based on the respective fragmentation models (for 0h TT), are as follows: Feb. 2, 146", 286 deg (A), 287", 289 deg (C); Feb. 12, 150", 285 deg (A), 291", 287 deg (C); Feb. 22, 149", 285 deg (A), 286", 287 deg (C); Mar. 3, 145", 289 deg (A), 274", 288 deg (C). NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2016 CBAT 2016 February 8 (CBET 4254) Daniel W. E. Green