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IAUC 6367: C/1996 B2

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                                                  Circular No. 6367
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
Phone 617-495-7244/7440/7444 (for emergency use only)

     J. Lecacheux, Meudon Observatory; E. Frappa, Planetarium of
Saint-Etienne; P. Laques, Pic du Midi Observatory; F. Colas, Bureau
des Longitudes, Paris; and L. Jorda, Observatoire de Meudon and
Laboratoire d'Astronomie Spatiale, Marseille, report:  "Monitoring
has continued at the Pic-du-Midi 1.05-m telescope of fragments
moving away from the comet's nucleus.  Small fragments are
difficult to identify from night to night, but measurement of their
velocity is possible in a few hours.  Within 10~000 km of the
nucleus, their velocity dr/dt along the extended radius vector is
typically on the order of 5R m/s, where R is expressed in units of
1000 km.  The major fragment observed by Tozzi et al. on Mar. 25.93
UT (IAUC 6357), and later by the Hubble Space Telescope on Mar.
26.06 (IAUC 6363), had a velocity of 11.5 m/s at that time,
according to our measurements; we located it subsequently on Mar.
28.9 at R = 4800 km (diffuse appearance), then at R = 7450 km on
Mar. 30.9 (stellar appearance).  Its magnitude brightened from 16
to 14 between Mar. 30.79 and 30.86.  It seemed diffuse again and
was of mag 13 at R = 9150 km on Mar. 31.81."
     W. M. Harris, F. Scherb, J. W. Percival, and K. Nordsieck,
University of Wisconsin; K. Honeycutt, Indiana University; and
B. E. A. Mueller, D. Harmer, and A. Macdonald, National Optical
Astronomy Observatories, report on multiple observations of a
symmetric conical structure extending behind comet C/1996 B2 in
images taken with the 3.5-m WIYN telescope at Kitt Peak:  "The
conical structure has been consistently observable in images taken
since Mar. 24 using two filters that isolate bright, resonant CN
and OH neutral-gas emissions.  It has not been detected in WIYN
images targeting CO+ and H2O+ ion emission lines, at narrowband
continuum points at 683.2 and 487.5 nm, or in the broadband R.  The
shape of the feature is most reminiscent of a 3-dimensional analog
to a ship's wake.  It emanates from the near-nuclear vicinity with
an 80o-120o opening angle and is visible as diffuse, low-contrast
emission out to a distance of roughly 2000 km.  At this point, a
sharp increase in brightness is observed as a partial arc
surrounding the primary dust lane behind the nucleus.  This may
represent a line-of-sight enhancement of the optical depth of gas
in the feature, or possibly a localized gas-density increase.
Emission is also detectable from beyond the arc, but the contrast
is again very low.  An image showing the conical structure is
available at URL http://www.noao.edu/noao/comets/hy/wiynhy.html."

                      (C) Copyright 1996 CBAT
1996 April 1                   (6367)            Daniel W. E. Green

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