Central Bureau for Astronomical Telegrams
IAUC 4624: 1987g; GU Sgr
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Read IAUC 4625
Circular No. 4624
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
TWX 710-320-6842 ASTROGRAM CAM Telephone 617-495-7244/7440/7444
MARSDEN or GREEN@CFA.BITNET MARSDEN or GREEN@CFAPS2.SPAN
PERIODIC COMET TEMPEL 2 (1987g)
Z. Sekanina, Jet Propulsion Laboratory, California Institute
of Technology, reports: "Recent near-nuclear photometry of P/Tempel
2 (IAUC 4582, 4603, and 4614) has confirmed that the earth has been
near the comet's equatorial plane, as predicted (IAUC 4553). Based
on P/Tempel 2's spin-vector model with the emission source at
latitude +60 deg (Sekanina 1987, ESA SP-278, p. 323), comparison of the
1988 results with Jewitt & Luu's (1988, Ap.J. 328, 974) data from
early 1987 suggests that the nuclear dimensions are about 18 x 11 x 7
km (making P/Tempel 2 larger than P/Halley), if the albedo by
A'Hearn et al. (IAUC 4614) is accepted. On the assumption that
A'Hearn et al.'s (IAUC 4622) values of the water production refer
to the discrete source, its active surface area at the time was
between 1.2 and 2.8 km2, or 1/2 percent of the nuclear surface on
the average. With the sun below the vent's horizon for about one-third
of the rotation, major variations in the water production on
timescales as short as a small fraction of a day could be expected,
but these are likely to be masked in the observations by the much
longer photodissociation time. The derived nuclear shape and low
activity imply the possibility of a fairly stable rotation. During the
current apparition, the spin-vector model can extensively be tested
by studying the comet's fanlike appearance, which in the past was
observed to persist from 55 days pre-perihelion to almost 200 days
post-perihelion. Observers are encouraged to image the near-nuclear
region at high resolution in the continuum. The position angle
of the fan's axial direction is predicted to be as follows: July 18,
325 deg; Aug. 7, 324 deg; 27, 323 deg; Sept. 16, 320 deg; Oct.
6, 321 deg; 26, 329 deg; Nov. 15, 348 deg; Dec. 5, 7 deg; 25, 18 deg;
1989 Jan. 14, 24 deg; Feb. 3, 27 deg; 23, 30 deg. The fan's cone
(opening) angle should be near 60-70 deg at the beginning and end of
this period, but in excess of 90 deg in late 1988, when the fanshaped
appearance may temporarily be inhibited or lost. Also, the
fan's light distribution is unlikely to be axially symmetrical and
especially in 1989 most ejecta (from the vent's afternoon activity)
should appear to the north of the fan's projected axis, at position
angles near 0 deg. The fanlike shape is expected to be less
pronounced in the various molecular passbands."
GU SAGITTARII
J. E. Bortle, Stormville, NY, reports that this R CrB-type star
is fading, at mv = 14.3 on July 3.1 UT. Normal brightness is near
B = 11.3.
1988 July 8 (4624) Daniel W. E. Green
Read IAUC 4625
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