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IAUC 8701: 174P; 73P

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IAUC number

                                                  Circular No. 8701
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
URL http://cfa-www.harvard.edu/iau/cbat.html  ISSN 0081-0304
Phone 617-495-7440/7244/7444 (for emergency use only)

     S. Tegler, Northern Arizona University; G. Consolmagno,
Vatican Observatory; and W. Romanishin, University of Oklahoma,
obtained R-band CCD images on Apr. 2.3 UT with the Vatican Advanced
Technology 1.8-m telescope at Mt. Graham, which show that the coma
previously reported around this object (IAUCs 8656, 8660, 8677) has
developed a complex structure.  A low-surface-brightness coma of
diameter 2' is centered 1' east of the nuclear condensation.  A
higher-surface-brightness condensation about 12" in diameter is
centered about 7" west of the nuclear condensation, which itself is
of mag R about 20.1 and is clearly offset from the center of the
higher-surface-brightness structure, suggesting a detached coma
produced by a transient event.  A rough measurement of the entire
2'-diameter object gives a total R magnitude of 16, while the
higher-surface-brightness region has R = 17.9 (as measured in a
12"-diameter circle centered 7" west of the nuclear condensation).

     This comet continues to fragment (cf. IAUC 8693, CBET 453),
including a splitting of component 'G' (evidently around Mar. 6;
cf. CBET 464), which is now near total visual mag 12.  Component
'B', now in outburst about as bright as component 'C' (near total
visual mag 9.0-9.5), appears also to have fragmented (cf. CBET 473).
     M. L. Sitko, B. A. Whitney, and M. J. Wolff, Space Science
Institute; C. M. Lisse, Johns Hopkins University; E. F. Polomski,
University of Minnesota; R. W. Russell and D. K. Lynch, The
Aerospace Corporation; and D. E. Harker, University of California,
San Diego, report on observations made of comet 73P's component 'C'
using the Infrared Spectrograph of the Spitzer Space Telescope on
Mar. 17.05 UT.  A smooth continuum was observed between 5 and 8.5
microns, followed by a structured silicate emission feature
extending to 12.2 microns, and then a continuum extending to 13.5
microns.  An underlying blackbody with a temperature of 235 +/- 5 K
was fitted to the continuum fluxes at 7.8 and 12.5 microns.  The
grain temperature was about 2-3 percent higher than that of an
equilibrium blackbody at the comet's heliocentric distance of 1.47
AU.  The strength of the silicate feature was 33 percent above the
underlying continuum.  Within the silicate band, emission peaks at
9.4, 9.9, 10.4, and 11.2 microns were observed -- consistent with a
mixture of olivine and pyroxene dust.  The measured flux at
wavelength 10.5 microns (slit size 3".7 x 57") was 0.75 Jy, or
magnitude [10.5 microns] = 4.3.

                      (C) Copyright 2006 CBAT
2006 April 11                  (8701)            Daniel W. E. Green

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