[IAUC] CBET 1118: 20071103 : COMET 17P/HOLMES

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                                                  Electronic Telegram No. 1118
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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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