[IAUC] CBET 1018: 20070802 : COMET C/2006 P1 (McNAUGHT)

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                                                  Electronic Telegram No. 1018
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
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COMET C/2006 P1 (McNAUGHT)
     C. M. Lisse and N. Dello Russo, Applied Physics Laboratory, Johns
Hopkins University; Y. Fernandez, University of Central Florida; G. H.
Jones, Mullard Space Science Laboratory, University College, London;
and M. Sitko, Space Science Institute, report that the Spitzer Space
Telescope observed comet C/2006 P1 on May 4-5 using the IRS instrument
when the comet was at r = 2.4 AU and Delta = 2.2 AU.  A rounded,
central source of emission due to the coma was found in the peak-up
imager at 16 microns.  The 5- to 35-micron spectrum of the outflowing
dust showed only a mild excess (about 10 percent) due to silicate
emission at 8-13 microns.  The flux density at 10 microns was about
0.1 Jy, and at 20 microns was 0.6 Jy.  The effective temperature of
the dust was 190 +/- 10 K.  The local equilibrium temperature at 2.4 AU
was 182 K.  Lisse et al. estimate a production rate of dust to be about
6 x 10^3 kg/s.  The spectrum is remarkably featureless and dominated by
infrared emission from large particles, and is similar to that derived
from comet-surface mantles.  This is unexpected for a comet that, in
Dec. 2006-Jan. 2007, had demonstrated large outflows of material, a
highly structured dust tail due to the presence of 0.1- to 10-micron
dust particles, and was still emitting dust at the time of Spitzer
observations at a rate comparable to the strongly-mid-infrared-featured
comets C/1996 B2 (Hyakutake) and 1P/1982 U1 (Halley) when passing
closest to the earth.  Lisse et al. further surmise that either the
comet has a very thick surface mantle that was only temporarily breached
during the perihelion passage by a jet or the material being emitted in
May 2007 is from a surface mantle that has regrown since the comet's
perihelion passage four months earlier.


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                         (C) Copyright 2007 CBAT
2007 August 2                    (CBET 1018)              Daniel W. E. Green