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IAUC 6354: C/1996 B2

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                                                  Circular No. 6354
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; L. Jorda, Meudon Observatory
and Laboratoire d'Astronomie Spatiale, Marseille; A. Enzian,
Laboratoire de Glaciologie, Universite de Grenoble; J. Klinger,
Laboratoire de Glaciologie, Grenoble; F. Colas, Bureau des
Longitudes, Paris; E. Frappa, Planetarium of Saint-Etienne; and P.
Laques, Pic du Midi Observatory, write:  "Circumnuclear dust jets
of the comet have become an easy target for telescopes equipped
with accurate guiding systems.  Filters selecting the dust
continuum (such as those covering 484.5-486.5 nm and 684-689 nm in
the IAU standard set) provide better contrast, but broadband
filters permit shorter exposures.  The synodic period of rotation
of the nucleus prior to Mar. 23 was found to be a submultiple of 24
hr, as main jets crossed the nucleus-sun line each day at the same
hour.  At present our preferred period is 6.0 hr (revising our
estimate on IAUC 6344), but we do not exclude the value of 8.0 hr.
Cooperative coverage by observatories at various longitudes is
needed.  For several days following Mar. 23, the viewing geometry
of the nucleus for earth-based observers was rapidly changing.  The
inner part of the tail of C/1996 B2 (about 5000 km long) has a
conspicuous yellow color to telescopic visual inspection.  From our
multicolor CCD imaging with the Pic-du-Midi 1.05-m reflector, it
clearly has the photometric properties of dust reflecting sunlight.
We have used, since Mar. 10, a set of 11 narrow-band filters (in
the range 426-925 nm), including CO+ and H2O+ emissions, C2
emission and adjacent continuum, plus various broadband red filters
for 830 nm and longer wavelengths.  The dust particles have been
collimated in a narrow region of apparent width 300 km in the exact
anti-solar direction during Mar. 10-23.  During Mar. 23.9-24.2 UT,
we detected luminous knots slowly receding from the nucleus.  The
faintest, of mag 13, was at the projected distance of 21".5 on Mar.
24.0 and moved outwards at the projected velocity of 0".8/hr;
assuming that this object followed the extended solar radius
vector, the geometric conditions at this time imply a real distance
from the nucleus of 2340 km and an average real recession velocity
of 12 m/s.  A second, more luminous knot was closer to the nucleus
and receded similarly.  The luminosity of the inner tail,
especially its 2000-km inner part, increased conspicuously between
Mar. 21.2 and 21.9."

                      (C) Copyright 1996 CBAT
1996 March 26                  (6354)            Daniel W. E. Green

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