Electronic Telegram No. 3608
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)
     Dennis Bodewits, Tony Farnham, and Michael F. A'Hearn, University of
Maryland, College Park, report results from observations of comet C/2012 S1
using the UltraViolet-Optical Telescope (UVOT) onboard the Swift space
telescope.  They obtained photometry using broadband V (central wavelength
546.8 nm, FWHM 75.0 nm) and UVW1 (central wavelength 260.0 nm, FWHM 70.0 nm)
filters.  The comet was observed on 2013 Jan. 30 at r = 4.95 AU, Mar. 11 at
r = 4.50 AU, Apr. 24 at r = 3.97 AU, and May 9 at r = 3.79 AU.  Within
apertures of 10" radius, they measured V magnitudes of 15.70, 15.76, 15.38,
and 15.27 (+/- 0.05) and [UVW1] magnitudes of 17.70, 17.76, 17.64, and 17.36
(+/- 0.1).  The V-band magnitudes correspond to values for Af(rho) of 1445,
1590, 1929, and 1897 cm (+/- 5 percent), respectively, normalized to a phase
angle of 0 degrees using the phase function by D. Schleicher (details given
at website URL http://asteroid.lowell.edu/comet/dustphase.html).  Dust colors
[UVW1] - V of around +2 were measured, which are close to the solar value
of +2.06.  The [UVW1] magnitudes, whose flux consists by approximation of
reflected continuum and OH emission lines, imply (by removing the continuum
to measure the OH flux; cf. Carter et al. 2012, A.Ap. 541, 70) an approximate
upper limit of Q(H2O) < 1 x 10^28 molecules per second.


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2013 CBAT
2013 July 30                     (CBET 3608)              Daniel W. E. Green