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IAUC 7471: C/1999 S4; 2000cx

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                                                  Circular No. 7471
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)

     Z. Sekanina, Jet Propulsion Laboratory, reports: "The unusually large
nongravitational forces found by B. G. Marsden (MPEC 2000-O07) suggest that
comet C/1999 S4 was a trailing fragment of a more massive comet that has
been moving in the same orbit, arrived at perihelion long (centuries?) ago
but (not surprisingly) was missed.  Trailing fragments of known comet pairs
have a tendency to sudden disintegration (e.g., Sekanina 1997, A.Ap. 318, L5).
If much of the comet's mass did indeed dissipate into a cloud of dust in the
recent event, as suggested by M. R. Kidger (IAUC 7467) and others, the total
mass involved could be estimated by further monitoring the tail.  Experience
with the past initially bright comets that later became headless and
disappeared shows that a narrow, bandlike tail--a developing synchronic
formation--should survive the head by several weeks or even longer
(Sekanina 1984, Icarus 58, 81).  A very preliminary analysis suggests that
the event may have begun as early as July 23.6 UT and involved
submillimeter-sized and larger dust (repulsive accelerations up to 0.024 of
the solar attraction).  The position angle and approximate length of
this tail feature are then predicted to reach: July 30.0 UT, 90 deg, 2';
Aug. 4.0, 98 deg, 4'; 9.0, 102 deg, 7'; 14.0, 104 deg, 10'; 19.0, 105 deg, 12';
24.0, 106 deg, 15'; 29.0, 106 deg, 17'.  Especially toward the end of this
period, the predicted length probably is a crude upper bound.  If no such tail
persists, the comet's upper mass limit should be tightly constrained, or the
amount of dust lost in the event did not represent a substantial fraction
of the total mass."
     Total visual magnitudes: July 25.90 UT, 6.6 (M. Lehky, Hradec
Kralove, Czech Republic, 10 x 80 binoculars); 26.87, 6.5 (J. Kysely, Vlasim,
Czech Republic, 7 x 50 binoculars); 27.86, 6.6 (M. Reszelski, Szamotuly,
Poland, 20 x 60 binoculars); 28.86, 7.0 (M. V. Zanotta, Alpe di Ponna, Italy,
7 x 42 binoculars); 29.48, 7.4 (Y. Nagai, Yamanashi, Japan, 20 x 100
binoculars); 30.47, 7.9 (S. Yoshida, Ibaraki, Japan, 0.25-m reflector).

     D. C. Leonard, A. V. Filippenko, R. Chornock and W. D. Li, University of
California at Berkeley, report: "Spectropolarimetry (range 430-690 nm)
obtained on July 28 with the Shane 3-m reflector, Lick Observatory,
indicates continuum polarization of about 0.5 percent.  A strong
polarization modulation (about 0.3 percent) across the Si II (635.5 nm)
P-Cyg feature implies significant polarization intrinsic to the supernova
itself, possibly resulting from an aspherical scattering atmosphere (e.g.,
Hoeflich 1991, A.Ap. 246, 481).  According to photometry by KAIT (see IAUC
6627, 7126), SN 2000cx reached maximum B brightness (preliminary B = 13.3)
on July 27.

                      (C) Copyright 2000 CBAT
2000 July 30                   (7471)              Brian G. Marsden

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