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IAUC 6360: C/1996 B2

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                                                  Circular No. 6360
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)

     W. M. Harris, F. Scherb, J. W. Percival and K. Nordsieck, University of
Wisconsin at Madison; and B. E. A. Mueller, D. Harmer and A. Macdonald,
National Optical Astronomy Observatories; report the detection of
a possible secondary condensation in the comet's central region on a
70-s exposure on Mar. 25.47 UT using the imaging camera on the 3.5-m WIYN
telescope at Kitt Peak: "The 7' x 7' field was obtained using a filter
centered at 392 +/- 10 nm under conditions of 0".6 seeing.  To enhance
detail the image was smoothed with a 4".0 (20 pixel) median filter and
divided into the original frame.  The divided result shows evidence for
a separation of the nuclear region into two features.  Both structures
are located on the orbital path of the comet with a central separation
of 0".66 or 51.2 km.  The sunward of the two is approximately 20-percent
brighter than the other and displaced by about 0".2 to the north.
Because the separation of the structures is comparable to the seeing limit,
they are not well enough resolved to determine if they are independent
nuclei, although their 51-km separation argues against a single object.
Both objects are 3-5 times brighter than the intensity of emission from the
tail or any other jet structure observed in the coma."

     With respect to the observations above and those reported by J.
Lecacheux et al. (IAUC 6354) and Tozzi et al. (IAUC 6357), as well as
related information in the World-Wide Web (notably by H. A. Weaver
regarding Hubble Space Telescope observations on Mar. 26.1 UT),
Z. Sekanina, Jet Propulsion Laboratory, e-mails: "A tentative solution
based on the available positions of what is apparently the same blob
indicates that a fragment separated from the comet's nucleus on
Mar. 21.0 +/- 0.3 UT at a relative nongravitational deceleration of
280 +/- 30 units of 10**-5 solar attraction, typical for minor cometary
companions whose lifetimes at 1 AU from the sun are usually 10-20
days.  If it continues to survive, the piece should be located at a
p.a. 50 deg from  the nucleus, separated by 34" from it on Mar. 30.0 and
by 45" on Apr. 5.0.  This prediction refers to the tip of the feature
nearest the nucleus.  As the feature becomes ever more elongated,
observers are likely to report average separations from the nucleus
that will be increasingly greater than this prediction, by up to
several arcseconds or more."

     Several readers, notably A. W. Harris, Jet Propulsion Laboratory,
have pointed out that, if the tail of a comet corresponds to the extended
radius vector (as is particularly likely for a gas tail), it is physically
impossible for its angular extent to exceed the phase angle.  They
therefore question some of the reported observations of tail length,
particularly on IAUC 6355.  The phase angle of comet C/1996 B2 reaches
a maximum of beta = 111 deg (IAUC 6359) around Apr. 6.

                      (C) Copyright 1996 CBAT
1996 March 28                  (6360)              Brian G. Marsden

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