Electronic Telegram No. 3496
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)
     Jian-Yang Li, Planetary Science Institute; H. A. Weaver, Applied Physics
Laboratory, Johns Hopkins University; M. S. Kelley, T. L. Farnham, and M. F.
A'Hearn, University of Maryland; M. M. Knight, Lowell Observatory; M. J.
Mutchler, Space Telescope Science Institute; P. Lamy, Laboratoire
d'Astrophysique de Marseille; and I. Toth, Hungarian Academy of Sciences,
obtained broadband images of comet C/2012 S1 (cf. CBET 3238) on Apr. 10 (when
at r = 4.15 AU and D = 4.24 AU) using the Hubble Space Telescope (HST)
WFC3/UVIS cameras with the F606W and F438W filters.  The observations were
performed in three HST orbits, with the second orbit about 2.5 hr after the
first, and the last about 13.5 hr from the second.  Preliminary analysis
using a coma-nucleus separation technique (cf. Lamy et al. 2009, A.Ap. 508,
1045) suggests a nuclear radius less than 2 km.  The total brightness of the
comet within a 0".24-diameter aperture remained unchanged within 0.03 mag
over the 19-hr observing window.  Broadband photometry yielded Af(rho) = 1376
cm at 589 nm, and 1281 cm at 433 nm, consistent with a red slope of 5 percent
per 0.1 micron for the coma dust.  A well-defined jet is visible after
removing the 1/r brightness distribution.  The jet is centered on position
angle 290 deg, with a cone angle of 45 deg, a projected length of 1".6, and a
slight curvature towards the north near the end.  No temporal change in the
morphology is observed over the three epochs, suggesting that the jet is
circumpolar.  Under this assumption, the jet's apparent position constrains
the rotational pole to lie within 30 deg of R.A. = 330 deg, Decl. = 0 deg
(equinox 2000.0).  The images are posted at the following website URL:
http://hubblesite.org/newscenter/archive/releases/2013/14/.

     Total-magnitude estimates (visual unless otherwise noted):  2012 Oct.
14.77 UT, 17.3 (K. Yoshimoto, Yamaguchi, Japan, 0.16-m reflector + CCD);
Dec. 19.82, 16.5 (Yoshimoto); 2013 Jan. 13.90, 14.8 (J. J. Gonzalez, Leon,
Spain, 0.20-m reflector; visual); 21.85, 15.8 (S. Shurpakov, Baran', Belarus,
20-cm reflector + CCD); Feb. 2.90, 15.0 (U. Pilz, Leipzig, Germany, 32-cm
reflector; visual); 11.57, 15.8 (Yoshimoto); 23.74, 15.8 (Shurpakov); Mar.
6.72, 15.5 (Shurpakov); 10.76, 16.1 (T. Scarmato, Calabria, Italy, 25-cm
reflector + CCD + R_c filter); 23.79, 15.8 (Scarmato); Apr. 6.82, 16.2
(Scarmato); 9.48, 15.8 (Yoshimoto); 19.81, 16.2 (Scarmato).


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

                         (C) Copyright 2013 CBAT
2013 April 25                    (CBET 3496)              Daniel W. E. Green