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THE ASTRONOMER Electronic Circular No 1747      2002 Feb 25 16.50UT
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             GMH at AST.STAR.RL.AC.UK
WORLD WIDE WEB                         http://www.theastronomer.org
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COMET C/2002 C1 (IKEYA-ZHANG)
Mark Kidger continues (cf TA E-Circular 1746): A second point to
watch for is the light curve of Ikeya-Zhang. Jonathan Shanklin
suggests in his note that this comet is brightening very rapidly
(15 log r). This is a very fast brightening rate consistent with a
gassy comet - gassy objects rarely brighten rapidly close in to the
Sun. This is from the data archive from the Spanish Cometas_obs
mailing list. Note how there was an initial steep increase in
brightness in m1, but this has since stagnated. Interestingly the
brightness in the standard 10 arcsecond CCD aperture is rising
steadily, suggesting that the comet is getting more condensed. If
we take the archetypal active evolved object, 1P/Halley, the
brightening rate at r < 1.7AU was found by Daniel Fischer to be 9.1
log r. C/1995 O1 (Hale-Bopp), also an active multiple-return
object, brightened at 7.5 log r for most of its inbound passage.
Both suggest that to expect 15 log r to continue through to
perihelion may be overoptimistic. Comets are full of surprises.
C/2000 WM1 (LINEAR) did reach the upper bound of predictions (a
little below magnitude 2), but took an unusual route to get there!
It may be too early to call Ikeya-Zhang yet.

Martin Mobberley comments on observing prospects:
The comet stays below 15 degrees altitude in the western evening
sky at astronomical twilight, ie a difficult, but not impossible,
object through Feb and March. If it reaches magnitude 4, it is
going to be well worth hunting for in evening March twilight.
Then, from early April, it climbs rapidly into the north-eastern
dawn sky, exceeding 20 degrees altitude at astronomical twilight by
April 8 and heading further north, peaks at +62 degrees on April
25. For people with an exceptional western country horizon it
should be a nice binocular target throughout March, increasing from
magnitude 5 to 4. For those without a perfect west horizon, it will
be nice and high in the morning north-eastern sky from mid-April
and still magnitude 5 (if the MPEC mag law is correct). Any comet
with a q of 0.5 that should reach mag 4 holds a good prospect of a
nice tail, visible in binoculars.
Guy M Hurst