Electronic Telegram No. 3723
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)
     [Editor's note:  this replaces the text on CBET 3722 (correction to
author's name)]
     Z. Sekanina, Jet Propulsion Laboratory, reports that the comet's images
taken with the SOHO's C3 coronagraph around Nov. 27.65 UT show two major
features:  (1) the main straight dust tail, deviating 14 +/- 1 deg from the
radius vector counterclockwise (thus pointing at p.a. 272 +/- 1 deg); and (2)
a very narrow dust synchronic streamer, deviating in the same images 19 +/- 1
deg from the radius vector also counterclockwise (thus pointing at p.a. 277
+/- 1 deg).  No ion tail is apparent, but a saturation effect is.  The
orientation of the main tail is consistent with the expected loci of
microscopic ejecta from the outburst on Nov. 14.  The tail extends all the
way to the edge of the field and is therefore very probably longer than about
3 deg, which is equivalent to a lower limit of the peak radiation-pressure
acceleration of 0.4 the solar gravitational acceleration, characteristic of
micron- and submicron-sized dust particles.  The tail's width, estimated at
6'-7', suggests ejection velocities of up to about 100 m/s.  In contrast, the
narrow synchronic feature north of the tail implies the presence of massive
grains ejected with near zero velocities from the comet at extremely large
heliocentric distances, possibly as far as 20 AU from the sun.  The streamer's
length, estimated at 2.5 deg at the time the images were taken, suggests a
peak radiation-pressure acceleration on this debris on the order of 0.1
percent of the solar gravitational acceleration, thus referring to millimeter-
sized and larger debris.  Having survived until now, this material cannot be
composed of icy grains.  All these results are preliminary.

     J. N. Marcus, St. Louis, MO, U.S.A., writes:  "Considerable sublimation
of submicron-size silicate dust grains can be expected to occur in the coma
and tail of comet C/2012 S1 near perihelion, if its behavior is similar to
that of sungrazing comet C/2011 W3 (Lovejoy), whose nucleus survived for a
few days beyond perihelion (see Sekanina and Chodas 2012, Ap.J. 757, 127).  If
C/2012 S1 can survive until its perihelion passage, then its brightness, based
upon my preliminary photometry of C/2011 W3 in the SOHO LASCO C3 and C2
coronagraph fields, should start to fall below any power-law formula at about
r = 0.1 AU (Nov. 27.8 UT), peak at magnitude -1 to -3 at r = 0.03-0.04 AU
(Nov. 28.5-28.6), and then decline by 2 or more magnitudes by the time of
perihelion passage on Nov. 28.8 (details of my study of photometric behavior
of near-sun comets will be submitted to the ICQ soon).  Thus, a strong falloff
in brightness at these times should not necessarily be construed as
disintegration of the nucleus.  After perihelion, the comet's brightness would
temporarily increase as submicron grains re-populate the coma and dust tail."

     K. Battams, Naval Research Laboratory, notes that the comet brightened in
SOHO LASCO C3 satellite images to around total visual magnitude 0.5 on Nov.
27.64 UT and to around mag -0.5 to -1.0 on Nov. 27.83, adding that two tails
are now present extending 2-3 degrees to the edge of the field (see website
URL http://sohowww.nascom.nasa.gov/data/realtime-images.html).  Battams
observes that the tail is now more impressive than that of C/2011 W3 was at
about the same distance pre-perihelion in C3 images.


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

                         (C) Copyright 2013 CBAT
2013 November 27                 (CBET 3723)              Daniel W. E. Green