Latest CBAT update on COMET C/2012 S1 (ISON)

The comet’s nucleus apparently disrupted near perihelion, with the
comet’s head fading from perhaps a peak brightness of visual mag -2 some hours
before perihelion to well below mag +1 before perihelion.  M. Knight, Lowell
Observatory, finds that the comet peaked around visual mag -2.0 around Nov.
28.1 UT, adding that the brightest feature in the coma of the comet faded
steadily after perihelion from about mag 3.1 in a 95″-radius aperture when the
comet first appeared from behind the SOHO coronagraph occulting disk on Nov.
28.92 to about mag 6.5 on Nov. 29.98.  K. Battams, Naval Research Laboratory,
writes that, based on the most recent LASCO C3 images (Nov. 30.912 UT), there
is no visible nucleus or central condensation; what remains is very diffuse,
largely transparent to background stars, and fading; it appears that basically
a cloud of dust remains from the nucleus.  S. Nakano, Sumoto, Japan, writes
that he measured the comet’s total magnitude in a 27′ photometric aperture
from the SOHO C3 camera images to be as follows:  Nov. 29.383, 0.5; 29.755,
1.4; 30.013, 2.0; 30.496, 3.0; 30.883, 5.4.

Z. Sekanina, Jet Propulsion Laboratory, reports that, from the position
of the northeastern boundary of the comet’s fan-shaped tail in three images
taken with the C3 coronagraph onboard the SOHO spacecraft between 0.7 and 1.9
days after perihelion (Nov. 29.46 to 30.66 UT), he finds that the comet’s
production of dust terminated about 3 hours before perihelion.  Although this
result is preliminary, it is unlikely to be significantly in error, because
the position angles of a perihelion emission are off in the three images by
14-22 deg, and those of post-perihelion emissions still more.  The peak
radiation-pressure accelerations derived from the tail boundary’s angular
lengths (estimated at 1.8-2.5 deg) are about 0.1-0.2 the solar gravitational
acceleration, implying the presence of micron-sized particles.  The estimated
time of terminated activity is consistent with the absence of any feature that
could be interpreted as a condensation around an active nucleus in the 20 or
so images taken with the C2 coronagraph on Nov. 28.8-29.0 UT (0.8 to 5.4 hr
after perihelion) and with the appearance of a very sharp tip (replacing a
rounded head) at the comet’s sunward end in the C2 images starting about 4 hr
before perihelion and continuing until its disappearance behind the occulting
disc around Nov. 28.74 UT (or some 50 minutes before perihelion).  The time
of terminated activity is here interpreted as the end of nuclear fragmentation,
a process that is likely to have begun shortly before a sudden surge of
brightness that peaked nearly 12 hr prior to perihelion.  Fine dust particles
released before perihelion moved in hyperbolic orbits with perihelion
distances greater than is the comet’s, thus helping some of them survive.
The post-perihelion tail’s southern, sunward-pointing boundary consists of
dust ejected during the pre-perihelion brightening.  However, the streamer of
massive grains ejected at extremely large heliocentric distances, so
prominently seen trailing the nucleus along the orbit before perihelion (cf.
CBET 3722), completely disappeared.  The dust located inside the fan, between
both boundaries, was released in intervening times, mostly during the last
two days before perihelion.  The strong forward-scattering effect (phase
angles near 120-130 deg) has tempered the rate of post-perihelion fading of
the comet, but the merciless inverse-square power law of increasing
heliocentric distance is necessarily the dominant factor in the comet’s
forthcoming gradual disappearance.

H. Boehnhardt, J. B. Vincent, C. Chifu, B. Inhester, N. Oklay, B.
Podlipnik, C. Snodgrass, and C. Tubiana, Max Planck Institute for Solar
System Research, Katlenburg-Lindau, reports that two diffuse tail structures
were analyzed in post-perihelion images obtained by the LASCO-C3 corongraph
onboard the SOHO spacecraft between Nov. 29.60 and 29.81 UT.  The southward
tail extended toward p.a. about 167 deg to about 0.4 deg distance from the
central brightness peak.  The eastward tail had an approximate position angle
of 68 deg and extended to at least 1.2 deg distance.  By Finson-Probstein
simulations, the eastward tail can best be interpretated as being caused by a
dust release about 1 hr around perihelion.  The maximum beta value in the
eastward tail reaches values up to 1.5, typical for graphite or metallic
grains of about 0.1 micron radius.  No indications are found for a
continuation of the release of similar dust after 2 hr post-perihelion.  The
shorter southward tail may be a relict of heavier grains released about 1-2
days before perihelion passage.  Diffuse cometary material is noticeable in
the p.a. range covered by the two dust tails.  The match of the synchrone
pattern for the eastward tail is not optimal, which may indicate secondary
effects to the dust grains involved.

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

(C) Copyright 2013 CBAT
2013 December 1                  (CBET 3731)              Daniel W. E. Green