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IAUC 6345: C/1996 B2

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                                                  Circular No. 6345
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
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)
BMARSDEN@CFA.HARVARD.EDU or DGREEN@CFA.HARVARD.EDU (science)
Phone 617-495-7244/7440/7444 (for emergency use only)


COMET C/1996 B2 (HYAKUTAKE)
     M. Womack, Pennsylvania State University; M. C. Festou, Observatoire
Midi Pyrenees, Toulouse; and S. A. Stern, Southwest Research Institute,
report observations of comet C/1996 B2 with the National Radio
Astronomy Observatory's 12-m telescope at Kitt Peak: "The following
lines were detected during Mar. 16 and 17: H2CO 3(12)-2(11)
at 226 GHz, HCN 3-2 at 266 GHz, CO 2-1 at 230 GHz and CH3OH at
145 GHz.  Based on the detected brightness temperatures and an
assumed rotational temperature of 50 K we estimate production
rates of Q(H2CO) = 2*10**26/s, Q(CO) = 3*10**28/s, Q(CH3OH) = 5*10**26/s
and Q(HCN) = 3*10**26/s.  This Q(HCN) is close to that
found for CN by R. Millis and D. Schleicher on Mar. 19 (IAUC 6344),
suggesting a direct link between the two species.  However,
the CO and H2CO data indicate that H2CO is not the main parent of
the observed CO.  Interpolation between the present observations of
CO and those reported on IAUC 6335 (and on the assumption that the
fluorescence efficiencies of the millimeter lines have not significantly
changed) indicates a very rapid change in the production rates,
presumably near an r**-5 law, during the last two weeks.  If the
CO/H2O ratio is 20 percent, as suggested on IAUC 6335, the water
production is in excess of 10**29/s."

     Improved orbital elements and a detailed new ephemeris are
given on MPEC 1996-F03.  The following elements are from the same
solution but for a more current osculation epoch:

                    Epoch = 1996 Mar. 18.0 TT
     T = 1996 May   1.3966 TT         Peri. = 130.2016
     e = 0.999705                     Node  = 188.0432  2000.0
     q = 0.230085 AU                  Incl. = 124.9106

The earlier indication (IAUC 6329) that the comet is not "new"
is confirmed, the last perihelion passage having occurred some 9000
years ago.  Corrections to the IAUC 6330 ephemeris: Mar. 22,
Delta alpha = -0.01 timemin, Delta delta = +3.5 arcmin; 23, -0.02, +5.2;
24, -0.04, +7.2; 25, -0.09, +9.0; 26, -0.4, +9.0; 27, -7, +3.3;
28, -0.7, -5.8; 29, -0.19, -4.7; 30, -0.10, -3.6; 31, -0.06, -2.8;
Apr. 1, -0.04, -2.4; 2, -0.04, -2.0; then to the IAUC 6311 ephemeris:
Apr. 2, -0.08, -4.4; 7, -0.05, -2.3; 12, -0.04, -1.7; 17, -0.05, -1.4;
22, -0.06, -1.4; 27, -0.08, -1.7.

     Naked-eye magnitude estimates: Mar. 20.07 UT, 1.9 (B. H.
Granslo, Fjellhamar, Norway); 20.09, 2.3 (K. Cernis, Lavariskes,
Lithuania); 20.58, 2.5 (S. O'Meara, Volcano, HI); 20.98, 2.1 (P. Pravec,
Ondrejov, Czech Republic); 21.16, 2.2 (B. G. Marsden, Lexington, MA);
21.31, 1.6 (J. V. Scotti, Tucson, AZ); 21.48, 1.3 (C. S. Morris,
near Gorman, CA).

                      (C) Copyright 1996 CBAT
1996 March 21                  (6345)              Brian G. Marsden

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