-------------------------------------------------------------------
THE ASTRONOMER Electronic Circular No 2396      2007 Dec 04 17.29UT
Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise,  Basingstoke,
Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074
INTERNET: GUY@TAHQ.DEMON.CO.UK      Backup: gmh@wdcc1.bnsc.rl.ac.uk
WORLD WIDE WEB                         http://www.theastronomer.org
-------------------------------------------------------------------
COMET 17P/HOLMES
Z. Sekanina, Jet Propulsion Laboratory, reports on CBET 1118 (abridged 
version follows) that information on the appearance, morphology, and 
light curve of the comet allows one to make tentative conclusions on the 
nature of its megaburst from Oct. 23-24.  The brightness data suggest 
that, before the event, the comet had been active but fading with 
heliocentric distance (r) -- from late May following an r^(-n) law with 
n approximately equal 16.  At the event's onset, most probably on Oct. 
23.7 +/- 0.2 UT, the comet's total visual magnitude normalized to 1 AU 
from the earth was 15.3.  This is about 30 times brighter than expected 
for the nucleus based on its diameter of 3.3 km, geometric albedo of 4 
percent, and a phase law of 0.035 mag/deg. The extremely steeply 
increasing brightness at the beginning of the megaburst suggests that 
the rate of dust injection into the coma accelerated with time, or the 
average particle size was then rapidly decreasing with time (perhaps as 
a result of runaway particle fragmentation), or both.

The peak-brightness plateau was reached some 24 hours after the event's 
onset, with a normalized total magnitude of 1.4 +/- 0.2. The amplitude 
was thus very close to 14 magnitudes, or a factor of around 400000. The 
plateau brightness implies the presence in the coma of dust particles 
whose integrated cross-sectional area is 57 +/- 10 million km^2.  For a 
particle- size distribution with an average diameter of 2 microns, the 
estimated mass of this dust cloud is 10^(14) g at an assumed bulk 
density of 1.5 g/cm^3. This is almost exactly the mass that Sekanina 
found for a typical
major pancake-shaped companion nucleus of the split comets.

In summary, the enormous scale of the megaburst is a result of the
fact that the jettisoned layer was extremely poorly cemented and 
disintegrated in a cataclysmic manner.  Thus, one may expect a potential 
inverse relationship between the prominence of an outburst and the 
appearance of persistent companion nuclei.  Except for the amplitude, 
the 17P event was similar to the outbursts of 29P/Schwassmann-Wachmann 
which, likewise, has never been observed to split.  On the other hand, 
the brightness in the outbursts experienced by comet 
41P/Tuttle-Giacobini-Kresak in 1973 subsided very rapidly, suggesting a 
different mechanism (probably involving only gas).  It will be 
interesting to see whether 17P is subjected to a second outburst in 
early January, as in 1892.

SUPERNOVA 2007oc IN NGC 7418A
Discovery by Chilean Automatic Supernova Search program of a possible 
supernova (CBET 1114):
SN      2007 UT      R.A.  (2000.0)  Decl.    Mag.     Offset
2007oc  Nov  3.06   22 56 41.77  -36 46 22.3  13.6   6.1"E, 1.3"S
NGC 7418A was also the host galaxy of SN 1983M.

SUPERNOVA 2007od IN UGC 12846
Discovery by PIKA of a possible supernova (CBET 1116):
SN      2007 UT      R.A.  (2000.0)  Decl.    Mag.     Offset
2007od  Nov  2.85   23 55 48.68  +18 24 54.8  14.4  38"E   31"S
The report was prepared by Herman Mikuz and the discovery made by S. 
Maticic.

SUPERNOVAE 2007oe-2007om
Sloan Digital Sky Survey II collaboration report of nine supernova 
discoveries, all fainter than magnitude 19 according to CBET 1117.
Guy M Hurst