Electronic Telegram No. 3590
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
CBAT Director:  Daniel W. E. Green; Hoffman Lab 209; Harvard University;
 20 Oxford St.; Cambridge, MA  02138; U.S.A.
e-mail:  cbatiau@eps.harvard.edu (alternate cbat@iau.org)
URL http://www.cbat.eps.harvard.edu/index.html
Prepared using the Tamkin Foundation Computer Network


COMET C/2012 S1 (ISON)
     C. M. Lisse, R. J. Vervack, and H. A. Weaver, Applied Physics Laboratory,
Johns Hopkins University; J. M. Bauer, Jet Propulsion Laboratory, California
Institute of Technology; Y. R. Fernandez, University of Central Florida; M.
S. P. Kelley, University of Maryland; M. M. Knight, Lowell Observatory; D.
Hines, Space Telescope Science Institute; J.-Y. Li, Planetary Science
Institute; W. Reach, SOFIA, Universities Space Research Association; M. L.
Sitko, University of Cincinnati; P. A. Yanamandra-Fisher, Space Science
Institute; and K. J. Meech and J. Rayner, University of Hawaii, report the
detection of comet C/2012 S1 using the Spitzer Space Telescope (SST) on
June 13.00-13.96 UT at 3.6 and 4.5 microns, when the comet was 3.35 to 3.34
AU from the sun, 3.29 AU from SST, and 4.25 AU from the earth.  Imaging and
photometry were obtained at 3.6 and 4.5 microns, with flux densities of 0.66
+/- 0.04 mJy and 0.89 +/- 0.02 mJy, respectively, found in an 18200-km-
radius aperture at phase angle 17.4 deg.  No photometric variability was
seen at the 3-percent level over the course of the observations.  The comet
showed a linear anti-solar dust tail > 3 x 10**5 km in projected length in
both bands, and a 1/rho profile gas coma extending > 10**5 km from the nucleus
(where rho denotes the projected distance on the sky from the comet's nucleus)
in the 4.5-micron band.  Supporting Cousins VRI-band photometry in 18200-km-
radius apertures with the 1.1-m Hall telescope at Lowell Observatory on June
11.16 at a phase angle of 7.3 deg yielded magnitudes V = 15.9 +/- 0.1, R =
15.5 +/- 0.1, and I = 15.2 +/- 0.1, and supporting R-band photometry with the
3.5-m Astrophysical Research Consortium telescope at Apache Point Observatory
on June 14.13 at phase 6.9 deg yielded R = 15.7 +/- 0.1, also in the same
18200-km-radius aperture.  Af(rho) values of 840, 890, 840, and 650 cm (all
+/- 80 cm) were found at band V, R, and I and at 3.6 microns, respectively.
Together, allowing for phase-angle differences in the observations, the
ground-based and Spitzer photometry imply near-neutral dust scattering from
the visual through the infrared.  An excess at 4.5 microns due to emission
from a neutral-gas coma is clearly found both morphologically and
photometrically.  The gas coma's total flux and spatial profile and the
comet's discovery distance imply a coma dominated by strong CO_2 emission at
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ruled out.  Assuming that the gas coma consists of CO_2 from a nucleus source
with outflow at 0.35 km/s (Biver et al. 2013, private communication), the
authors find a gas-production rate of Q(CO_2) = 1.9 x 10**26 molecules/s and a
minimum effective radius for the nucleus of 0.08 km (based on the methodology
of Cowan and A'Hearn 1979, Moon and the Planets 21, 155).  Assuming that the
gas coma consists of CO from a nucleus source with outflow at 0.35 km/s, they
report Q(CO) = 2.1 x 10**27 molecules/s and a minimum effective radius of 0.14
km for the nucleus.


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
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                         (C) Copyright 2013 CBAT
2013 July 23                     (CBET 3590)              Daniel W. E. Green