[IAUC] CBET 1111: 20071029 : COMET 17P/HOLMES

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                                                  Electronic Telegram No. 1111
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
     C. Snodgrass, European Southern Observatory; A. Fitzsimmons, Queen's
University, Belfast; H. Boehnhardt, Max-Planck-Institut fuer
Sonnensystemforschung; T. Lister, Las Cumbres Observatory; and T. Naylor
and C. Bell, University of Exeter, report on imaging of an expanding coma
associated with comet 17P in images taken with the 2.5-m Isaac Newton
Telescope (La Palma) and the 2.0-m Faulkes Telescope North (Haleakala)
between Oct. 24.9 and 26.5 UT.  Images were taken in broadband BVRI and
narrowband filters centered at 383, 515, and 816 nm.  The comet shows
the same appearance in all filters and is clearly dust-dominated.  The
unprocessed data clearly show a circular coma, with a fan of material
extending from the central condensation at p.a. 225 deg.  Processing using
adaptive Laplace filtering with a pyramid-box size of 4" to 10" produces a
ring marking the outer edge of the dust associated with this event.  The
central brightness peak in the coma is offset from the ring center by
around 1".3 at Oct. 24.9 and by 9".5 by Oct. 26.5.  The brightness
distribution in the ring varies with position angle, being fainter in the
sunward-coma hemisphere.  From the diameter of the edge of the expanding
structure, an approximate expansion speed of 400-600 m/s is estimated.
Analysis using a Larson-Sekanina algorithm reveals four or five arc-like
structures present in the near-nuclear region.  These could be due to
a fragmentation of the nucleus, but further observations of the central
coma region will be required to confirm this.  The brightness of the
central condensation within a 2"-radius aperture was R = 9.0 on Oct. 25.52
and R = 10.5 on Oct. 26.5.

     F. Colas and J. Lecacheux, Paris Observatory, report on observations of
comet 17P from Oct. 24 to 27 with the 1-m telescope at Pic du Midi
Observatory.  From a set of selected CCD images, the onset of the outer
dust shell, which is centered on the pseudo-nucleus, is estimated to have
occurred on Oct. 24.40 UT (i.e., the day after the main outburst; cf. IAUC
8886).  The outer shell's mean diameter, measured at the sharp edge, was
135$''$, or 160000 km, on Oct. 26.0.  The radius is growing a constant rate
of 0.575 km/s.  A detached coma blob, also expanding, is moving away from
the pseudo-nucleus:  it was apparently released on Oct. 24.8, or slightly
earlier; the motion vector of its photo-center is 77 m/s in p.a. 216 deg.

     G. Sostero and E. Guido, Remanzacco, Italy, report on their CCD
photometry of comet 17P with a 0.25-m f/12 reflector (image scale 0".6/pixel;
seeing around 2".7 FWHM) on Oct. 25.0 UT.  The total magnitudes as measured
through narrowband filters centered on dust continua were 2.2 in a red
filter (centered at 647 nm; FWHM = 10 nm) and about 3.5 in a blue filter
(centered at 450 nm; FWHM = 10 nm).  The photometric profile of the coma
showed an asymmetric distribution, with a sharp central condensation nearly
8" in diameter, offset almost 10" toward the southwest from the outer faint
coma whose total diameter was about 50".  The "color index" of the dust, in
the sense "blue continuum minus red continuum" was about +1.4.
Stacking of ninety red-filtered and fifth blue-filtered exposures, 10 s
each, consistently show the presence of a plume-like feature extending
almost 15" toward the southwest.

     J. Young writes concerncing his CCD images obtained on Oct. 26.4 UT
with the Table Mountain 0.61-m f/16 Cassegrain reflector:  Careful
impressions with extremely short exposures (0.1 s) up to long exposures (240
s) show an outer round coma shell of 3'.5.  This shell is extremely bright
with a consistent brightness throughout, and it is centered exactly on the
area of the nuclear condensation.  The coma/head area is 6" in diameter
and round without any kind of additional or secondary 'hot' spots.  However,
there is an extension from the head to the southwest at p.a. 220 degrees for
20"; this extension (about 8" in width) has no mottling along its length, is
very subtle, and is ever-so-slightly curved.  The end of the extension
terminates in a rounded 'bubble' appearance.  Very close inspection of this
extension shows a thin line for the first half of its length, also with the
same curvature.  A subtle, secondary, round shell of 40" is centered around
the end of this extension and is very noticeable in even the shortest
exposures.  No hint of any tail is seen extending from the outer shell, but
with the full moon and some lingering smoke (local fires), any such tail
might not be visible.

     J. McGaha (Tucson, AZ, U.S.A., 0.62-m reflector + R_c filter)
reports the following magnitudes from CCD photometry obtained of comet
17P on Oct. 25.35 UT, when he measured an outer coma diameter of 79"
and an inner coma diameter of 14":  60" aperture, mag 2.2; 30"
aperture, mag 2.4; 8" aperture, mag 4.0.


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2007 CBAT
2007 October 29                  (CBET 1111)              Daniel W. E. Green