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IAUC 6583: C/1995 O1

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                                                 Circular No. 6583
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
URL http://cfa-www.harvard.edu/cfa/ps/cbat.html
Phone 617-495-7244/7440/7444 (for emergency use only)

     K. Birkle, Max-Planck-Institut fur Astronomie, Heidelberg; and
H. Bohnhardt, Universitats Sternwarte, Munich, write:  "We have
detected ring- and spiral-like coma structures on CN-filtered
(388.5 +/- 2.3 nm) exposures of C/1995 O1 obtained with the Calar
Alto 1.2-m telescope.  After adaptive Laplace filtering of the
images (via a MIDAS procedure developed by G. Richter, Astronomical
Institute, Potsdam), two almost concentric and slightly elliptical
rings (axis ratio about 1.1) of diameter about 57" and 125" along
the major axis were found on Feb. 7.245 UT.  On Feb. 25.239,
another pattern of double rings was observed with diameters 85" and
175".  On both nights, the brightness peak in the coma was
displaced to the southwest of the common ring center, and the CN
emission appeared weakest in the northern part of the rings.  Two
strong, straight jets were observed to the south of the central
coma peak:  one pointed constantly towards p.a. 170 deg, while the
other one changed from p.a. 215 deg on the first night to p.a. 245
deg on the second night.  On Feb. 25, a shell-like spiral-arm
segment extended from the southeast to the northeast coma quadrant
at about 130" to 250" from the central coma peak.  The variable
jet in the southwest coma quadrant continued in a short spiral arm
curved toward the south at about 115" from the center.  A spiral
arclet of similar shape was found in the southwest quadrant at
about double the projected distance from the coma center.  It is
very likely that the ring and spiral arm patterns arise from two
different active regions on the nucleus (one for the jet and one
for the two rings).  Assuming the dust-shell rotation period given
by Lecacheux et al. (IAUC 6560), we conclude from the angular
separation of the two rings a CN gas-expansion velocity of about
1.2 km/s on Feb. 7 and 1.4 km/s on Feb. 25."
     L. Jorda, Max-Planck Institut fur Aeronomie; J. Lecacheux,
Observatoire de Paris, Meudon; and F. Colas, Bureau des Longitudes,
report:  "Further CCD observations of the bright southwestern jet
of comet C/1995 O1 at the Pic du Midi 1.05-m telescope confirm the
spin period of 11.47 +/- 0.05 hr (IAUC 6560) but show a periodic
oscillation of this period between 11.20 +/- 0.10 hr and 11.65 +/-
0.10 hr with a superperiod of 22 +/- 2 days.  The maximum spin
period was reached on about Feb. 14 and Mar. 8, and the minimum
near Feb. 27.  A complex rotational state of the nucleus is likely
to explain this variation, although we are not able to detect the
periodic tilt of the spin axis that should be associated with it.
A sequence of images covering a whole rotation period is available
at URL http://www.bdl.fr/s2p/comete/halebopp/hbrot.html."

                      (C) Copyright 1997 CBAT
1997 March 11                  (6583)            Daniel W. E. Green

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