Electronic Telegram No. 1118 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions) CBAT@CFA.HARVARD.EDU (science) URL http://www.cfa.harvard.edu/iau/cbat.html COMET 17P/HOLMES J. M. Trigo-Rodriguez, Institute of Space Sciences, Consejo Superior de Investigaciones Cientificas and Institut Estudis Espacials de Catalunya, has analyzed infrared images of comet 17P taken with the 1.5-m Carlos Sanchez Infrared Telescope (IAC, Tenerife) by P. Abraham and A. Konkoly (Konkoly Observatory) between Oct. 26.1 and 26.2 UT. An asymmetric distribution of the coma is visible, together with a bright fan of material extending from the nucleus in p.a. 220 deg. Near the false nucleus, the fan is formed by two bright blobs separated by a clear gap. Both features are quickly expanding at an approximate velocity of about 600 m/s. This observation confirms the report by Colas and Lecacheux on CBET 1111 regarding the release of bright expanding blobs. These bright features expand rapidly from the false nucleus, suggesting a massive release of material from a very active region of the cometary surface. The expanding blobs are not seen in filtered B, V, R, and I images taken with the 0.8-m IAC80 Telescope (Tenerife) between Oct. 27.1 and 27.2 by R. Barrena, P. Montanes-Rodriguez, M. F. Nunez, and P. Abraham. A fan extends from the false nucleus in p.a. 220 deg, and the outer coma diameter on those images was 4'.1, or 285000 km. The outer shell of the coma continues expanding on Oct. 30.1 images taken by P. Montanes-Rodriguez; the coma keeps its general asymmetric shape and quite uniform brightness, but its diameter has increased to 7'.1 or 505000 km. A Larson-Sekanina algorithm applied to IAC images reveals several arc-like structures probably produced from active jets, but no clear evidence of fragmentation of the nucleus. J. Lecacheux and F. Colas, Paris Observatory; and B. Gaillard report on images taken on Oct. 28.85 and 31.23 UT at the f/17 focus of the Pic du Midi 1-m telescope under sub-arcsecond conditions: the elongated semi- parallel luminous streaks reported by many observers across the expanding "coma blob" (cf. our previous report in CBET 1111). These streaks are curved dust tails that appear to emanate from about fifteen sites spread around the nuclear condensation, though they are more numerous and brighter in the south- southwest region from the bright condensation; the emanation sites are about 25000 +/- 10 000 km from the condensation (projected on the sky). The set of tails seems to be fan-shaped (divergent) but in a fountain-like pattern. There are three main tails responsible at low resolution for the diffuse light maximum that appears like a "blob". The main nuclear condensation possesses its own dust tail, though it is smaller than the other tails. See http://www.picdumidi.eu/ for the processed images. A. Arai, M. Uemura, M. Sasada, K. S. Kawabata, T. Yamashita, T. Yasuda, R. Matsui, H. Tanaka, O. Nagae, M. Isogai, and T. Ohsugi, Hiroshima University; R. Furusho, Waseda University; J. Watanabe, National Astronomical Observatory of Japan; and M. Kino and S. Sato, Nagoya University, report on behalf of the Kanata observation team that they obtained CCD images of comet 17P/Holmes on Oct. 25.62 (R_c band), 27.66 (I_c), 29.70 (V), and 30.70 UT (I_c) with the KANATA 1.5-m telescope (+ TRISPEC) at Higashi-Hiroshima Observatory. An elongated structure has developed near the nucleus, which consists of a collimated flow and a diffuse expanding cloud embedded in the coma. Preliminary analysis of the last image shows that the position angle of the flow is about 190 deg, being significantly misaligned with that of the cloud (p.a. 210-220 deg). The brightness center of the cloud moved about 33" between Oct. 27 and 30, suggesting a mean velocity of 11"/day (150 m/s at 1.5 AU) on the projected sky. A sample of images can be seen at website URL http://kanatatmp.blogspot.com/search/label/Holmes. Low-resolution spectropolarimetry of the nuclear region on Oct. 25 and 27 showed the comet to be at negative polarization, with preliminary analysis showing the degree of polarization (0.5-0.7 percent) to be smaller than that of a typical comet at similar phase angle. M. Montalto, A. Riffeser, S. Wilke, and U. Hopp, University Observatory, Munich, report that B, V, R, and I imaging with a 0.8-m telescope at Wendelstein Observatory, obtained during Oct. 26-31 show an almost-stellar bright core (with a few arcsec FWHM) and a large cloud that is expanding and fading. The bright central condensation and the centroid of the cloud were separated by 15" on Oct. 27.1 UT, and by 61" on Oct. 31.9; analysis of 103 images shows that the two centers are linearly increasing in distance with a projected velocity of 9".61 +/- 0".4 per day (suggesting that they were coincident on Oct. 25.568 +/- 0.2). Z. Sekanina, Jet Propulsion Laboratory, reports that information on the appearance, morphology, and light curve of the comet allows one to make tentative conclusions on the nature of its megaburst from Oct. 23-24. The brightness data (calibrated by linking K. Kadota's pre- and post-outburst CCD magnitudes with the naked-eye estimates) suggest that, before the event, the comet had been active but fading with heliocentric distance (r) -- from late May following an r^(-n) law with n approximately equal 16. At the event's onset, most probably on Oct. 23.7 +/- 0.2 UT, the comet's total visual magnitude normalized to 1 AU from the earth was 15.3. This is about 30 times brighter than expected for the nucleus based on its diameter of 3.3 km, geometric albedo of 4 percent, and a phase law of 0.035 mag/deg (Lamy et al. 2000, BAAS 32, 1061; Snodgrass et al. 2006, MNRAS 373, 1590). The extremely steeply increasing brightness at the beginning of the megaburst suggests that the rate of dust injection into the coma accelerated with time, or the average particle size was then rapidly decreasing with time (perhaps as a result of runaway particle fragmentation), or both. Indeed, the reported constant rate in mag/hr (IAUC 8886) implies an exponential increase in the cross-sectional area of dust, neglecting the minor contribution from the molecular and atomic species (IAUC 8887). The peak-brightness plateau was reached some 24 hours after the event's onset, with a normalized total magnitude of 1.4 +/- 0.2. The amplitude was thus very close to 14 magnitudes, or a factor of around 400000. The plateau brightness implies the presence in the coma of dust particles whose integrated cross-sectional area is 57 +/- 10 million km^2. For a particle- size distribution with an average diameter of 2 microns, the estimated mass of this dust cloud is 10^(14) g at an assumed bulk density of 1.5 g/cm^3. This is almost exactly the mass that Sekanina (1982, in *Comets*, ed. by L. L. Wilkening, University of Arizona Press, p. 251) found for a typical major pancake-shaped companion nucleus of the split comets. Recently, he pointed out (Sekanina 2007, Proc. SPIE 6694, p. 0I) that such companions appear to be jettisoned thick "talps" (layers), which were lately proposed to make up the nuclei of the short-period comets and for which a similar mass estimate was suggested (Thomas et al. 2007, Icarus 187, 4; Belton et al. 2007, Icarus 187, 332). The scenario of a complete disintegration of such an object in the coma of 17P/Holmes is also supported by the nearly symmetrical outer dust halo, expanding at an average (but possibly decreasing) rate of 0.5 km/s (based on fitting the halo measurements reported on a "Yahoo comet group" e-mail-list website by P. C. Sherrod) and implying the event's onset around Oct. 23.8. The halo, containing much of the ejecta's mass, is a projected cloud of microscopic dust grains originating from the cataclysmic breakup of the jettisoned layer upon its release. Only several sizable (tens of meters?) fragments of the layer survived the early phase and, as companions, began to recede from the primary nucleus at velocities of at least a few m/s, while continuing to fragment. The expanding parallel streaks of light observed at position angles of 210-220 deg (essentially along the extended radius vector) are the tails consisting of microscopic dust particles released during and after the fragments' separation, after most dust was injected into the outer halo. Only one of these tails starts from the nucleus condensation, the other three (or more) appear to emanate from "nothing" as their parent bodies, the above-mentioned fragments, are too faint to observe. The parents' lateral momentum is carried by the dust in these "side" tails, whose southwest ends contain the smallest, submicron-sized particles with the highest radiation pressure accelerations (somewhat exceeding solar attraction). The disintegrating parent fragments may not be located right at the northeast tips of these "side" tails because of possible gaps due to the dearth of larger (low-acceleration) particles. If there are nonparallel streaks in this general region that are not processing artifacts, they must be of different nature. In any case, the inner dust halo appears to be associated with this later phase of the event and it thus differs dynamically from the outer halo. Geometry, including the fact that the comet is near its opposition with the sun, also affects the projection of dust motions onto the plane of the sky, making the north and east (i.e., sunward) boundaries of the outer halo sharper. In summary, the enormous scale of the megaburst is a result of the fact that the jettisoned layer was extremely poorly cemented and disintegrated in a cataclysmic manner. Thus, one may expect a potential inverse relationship between the prominence of an outburst and the appearance of persistent companion nuclei. Except for the amplitude, the 17P event was similar to the outbursts of 29P/Schwassmann-Wachmann (e.g., in the shape of the light curve; cf., e.g., Beyer 1962, Astron. Nachr. 286, 219), which, likewise, has never been observed to split. On the other hand, the brightness in the outbursts experienced by comet 41P/Tuttle-Giacobini-Kresak in 1973 (e.g., Sekanina 1984, Icarus 58, 81) subsided very rapidly, suggesting a different mechanism (probably involving only gas). It will be interesting to see whether 17P is subjected to a second outburst in early January, as in 1892. A. Fitzsimmons reports the following corrected R magnitudes for the nuclear condensation of 17P/Holmes in a 2" aperture (cf. the report by Snodgrass et al. on CBET 1111): Oct. 25.52 UT, 6.6; 25.55, 6.7; 26.40, 8.0; 26.45, 8.1; 28.15, 9.4; 28.43, 9.5. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2007 CBAT 2007 November 3 (CBET 1118) Daniel W. E. Green