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                                                  Circular No. 6163
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     TWX 710-320-6842 ASTROGRAM CAM

     The Hubble Space Telescope Kuiper Belt Search Team (A. Cochran,
University of Texas; H. Levison and A. Stern, Southwest Research
Institute; and M. Duncan, Queens University) reports:  "We have
found several candidate Kuiper-belt objects using deep HST/WFPC2
images obtained in the ecliptic near morning quadrature.  These
objects are quite faint (V about 28), suggesting that they are
comparable in size to comet 1P/Halley.  The observations consist of
34 exposures of about 10 min each of a single 4-square-arcmin field
centered at R.A. = 3h41m20s, Decl. = +19 34'.7 (equinox 2000.0);
the exposures were obtained on a number of consecutive orbits
during 1994 Aug. 21.7-21.9 and 22.7-23.0 UT.  Each WF chip was
handled separately so that no assumptions were made about relative
orientation of the chips.  The first step in our data reduction was
to sum all of the images with no shifts to determine the positions
of stars and galaxies; this median sum was subtracted from each
image prior to further processing.  The stellar and galactic images
had a typical WFPC2 PSF, indicating that the telescope was pointing
at the same field on each of the 34 images.  After removal of stars
and galaxies, we stacked the resultant images with a variety of
shifts to simulate the drift rates of objects in the outer solar
system.  A typical Kuiper-belt orbit shifts by about 150 pixels in
the course of the 34 observations.  So far, we have searched for
objects in 44 viable Kuiper-belt orbits.  These orbits all are in
the 2:3 mean-motion resonance with Neptune, have eccentricities
between 0.1 and 0.3 and inclinations < 3.3 deg to the ecliptic.  In
addition, we also searched 44 unrealistic orbits as a control.
With our data-reduction procedure we have found a total of 244
candidate objects in our real orbits but only 185 in our control
orbits.  A chi-squared test indicates that there is less than a 1-
percent chance that these two samples are drawn from the same
parent population.  In addition, a comparison of the two flux
distributions shows that candidates in the real orbits tend to be
brighter than those in the control sample.  A visual inspection of
several of the brighter candidates confirms that they are most
likely real.  If our 59 excess candidates are indeed real members
of the Kuiper belt, there must be about 60 000 such objects per
square degree, or at least a total of 10**8 comets brighter than our
limiting magnitude in the restricted range of orbits similar to the
ones studied here."

1995 April 17                  (6163)            Daniel W. E. Green

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