Central Bureau for Astronomical Telegrams
IAUC 6248: C/1995 O1
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Circular No. 6248
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
BMARSDEN@CFA.HARVARD.EDU or DGREEN@CFA.HARVARD.EDU (science)
Phone 617-495-7244/7440/7444 (for emergency use only)
COMET C/1995 O1 (HALE-BOPP)
Z. Sekanina, Jet Propulsion Laboratory, reports: "The
evolution of the bright jet on images published on the World Wide
Web since late September suggests that the general scenario that I
proposed on IAUC 6223 and 6240 remains viable, with two additional
refinements. The three spiral jets in late August, late September,
and mid-October were almost certainly made up of dust ejecta from
the same source, located in the comet's equatorial plane (defined
relative to the apparent pole of rotation at the time of emission;
cf. below). It appears that, shortly after the outburst's onset
near local noon, the production of dust from this source peaks
sharply, then gradually subsides, and eventually terminates at
local sunset. The production peak explains the presence of the
rectilinear jet in the fourth quadrant (as well as a brightness
excess in the spiral feature's westernmost portion), eliminating
the need for a second source (cf. IAUC 6240). The systematic
variations in the straight jet's position angle from about 290 deg
in the August emission episode, to 320-330 deg in the September
event, and to about 350 deg in the October episode (as well as
similar variations in the p.a. of the easternmost tip of the spiral
feature) suggest that the comet is not in the state of pure spin,
unless one considers the unlikely case of an active area's
migration over the nucleus. To match the feature's evolution
during the three outbursts requires that the apparent rotation pole
have moved by about 35 deg between late August and late September,
from 40 deg to 70 deg in R.A. and from 0 deg to -20 deg in Decl.
(equivalent to a change from 150 deg to 120 deg in the apparent
obliquity), and by an additional about 20 deg or more between late
September and mid-October. The temporal separations of about 5
weeks between the first two emission episodes, and one half this
duration between the last two events, are signatures of the comet's
complex rotation state. This evidence implies a recurrence period
of about 18 days and suggests that the source failed to activate
(in the sense to generate a major dust feature) in the last days of
July and again in early September, thereby twice 'skipping' a
cycle. Such 'duds' in cometary activity are not altogether
uncommon (especially not at large r). A recurrence period of one
half the proposed duration or shorter would imply too many non-
events and is less likely. The apparent spin rate during the three
outbursts is still unknown. The (unforeshortened) expansion
velocities of the particles that made up the spiral feature in the
last two episodes are found to have ranged from 30 to 50 m/s, being
comparable with those in the August event (cf. IAUC 6240).
Observations of additional dust features should further help refine
the proposed emission scenario."
1995 October 20 (6248) Daniel W. E. Green
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