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IAUC 7467: C/1999 S4; P/2000 O2

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                                                  Circular No. 7467
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/iau/cbat.html  ISSN 0081-0304
Phone 617-495-7244/7440/7444 (for emergency use only)

     I. de Pater, University of California, Berkeley, with M. R.
Hogerheijde, M. C. H. Wright, R. Forster, W. Hoffman, L. E. Snyder,
A. Remijan, L. M. Woodney, M. F. A'Hearn, P. Palmer, Y.-J. Kuan,
H.-C. Huang, G. A. Blake, C. Qi, J. Kessler, and S.-Y. Liu, report
the detection of HCN from comet C/1999 S4 at the Berkeley-Illinois-
Maryland-Association (BIMA) Array in autocorrelation mode:  "The
peak antenna temperature in a 130" beam, averaged over July 21-24
(about 10 hr total on source), was 3.5 +/- 1 mK (a signal
suggesting an outgassing rate a few percent that of C/1995 O1).  No
signal was detected in cross-correlation mode with the combined
Owens Valley Radio Observatory and BIMA data (the virtual Combined
Array for Research in Millimeter-wave Astronomy).  This suggests a
source size slightly larger than expected for a Haser model."
     M. Kidger, Instituto de Astrofisica de Canarias, writes that
nightly observations made since July 23 in U, B, V, R, and Z
broadband filters with the 1-m Jacobus Kapteyn Telescope show what
appears to be the complete disruption of the comet's nucleus:  "The
central condensation was highly condensed and showed the typical
'teardrop' form on July 23.9 and 24.9 UT, although its brightness
decreased by a factor of about 3 between the two nights.  On July
25.9 the central condensation was seen to be strongly elongated
(length about 15") in p.a. 80 deg, with a very flat brightness
distribution.  The condensation's brightness faded further and its
length increased to about 30" and 45"-50" (p.a. 80 deg) on July
26.9 and 27.9, respectively.  On July 27.9, there was no evidence
of any local brightness peak that would indicate the presence of
subnuclei.  The expansion velocity of the condensation is about 40
m/s, indicating that it is particulate material and not gas.  The
gas tail, which virtually disappeared between July 23.9 and 24.9,
has reformed as an extension of the major axis of the central

     C. E. Delahodde, European Southern Observatory, reports the
recovery by O. R. Hainaut and herself of comet P/1994 A1 (= 1994a =
1993 XX) with the 3.6-m reflector.  The indicated correction to the
prediction by S. Nakano on MPC 31664 is Delta(T) = -0.10 day.
Further observations and improved orbital elements are given on
MPEC 2000-O32.

     2000 UT             R.A. (2000) Decl.
     July 25.33038   23 34 26.95   + 2 52 51.7
          27.24488   23 34 08.12   + 2 53 42.6

                      (C) Copyright 2000 CBAT
2000 July 27                   (7467)            Daniel W. E. Green

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