Central Bureau for Astronomical Telegrams

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IAUC 5947: 1993e

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                                                  Circular No. 5947
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Telephone 617-495-7244/7440/7444 (for emergency use only)
TWX 710-320-6842 ASTROGRAM CAM     EASYLINK 62794505

     For the Hubble Space Telescope observing team, H. A. Weaver,
Space Telescope Science Institute, reports the following
results from their analysis of HST images taken during Jan. 24-27:
"The HST data can only place upper limits on the sizes of the
cometary fragments, since there is no clear evidence of a point
source near any of the condensations.  The coma spatial brightness
distribution cannot be fitted by a simple analytical (or
numerical) model, so there is no reliable way of judging how much of
the brightness is due to an unresolved source.  If one is willing to
accept that the coma becomes completely flat at the position of a
fragment, then both the July 1993 (Weaver et al. 1994, Science 263,
787) and Jan. 1994 HST data indicate that the largest fragment could be
as large as 4 km in diameter.  However, since it seems unlikely for the
coma profile to flatten out like this, and since the new HST data show
that the coma between 0.1 and 0.6 arcsec is reasonably well
approximated by a power law with an index of -1.33, it is possible that
all of the light observed (to within the noise level) is due to the
coma.  In the new images, most of the condensations have a coma that is
circularly symmetric out to approximately 0.6 arcsec from the peak
pixel; then there is a strong divergence in the brightness distribution
on one side of the 'train' versus the other.  This behavior is also
present in the July 1993 HST data.  The spatial brightness profiles
have become significantly steeper in the new data compared to the old
(after taking into account the spherical aberration present in the old
data), indicating that there has been significant change in the dust
distribution between July 1993 and Jan. 1994.  There have also been
some changes in both the relative and absolute magnitudes of the
components.  Fragment 15 is now about 15 percent brighter than fragment
7a.  Fragments 5, 8a and 8b have markedly different spatial brightness
distributions from most of the others (but not all condensations have
been examined in detail).  There is a brightness spur on fragment 5
that extends into the 'dust-free' side of the train and that may
indicate that this nucleus is fragmenting further. Nine of the
fragments are significantly displaced from the train with all
displacements being into the dusty side.  The region around fragment 8a
looks more like a 'streak' than a strong condensation, possibly
indicating that there is only a swarm of material in this region,
rather than dust plus a dominant fragment.  The case of 8b seems to be
intermediate between that of 8a and the more well-condensed cases."

1994 March 10                  (5947)              Brian G. Marsden

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