IAUC 8892 (2007 Nov. 7) COMET 17P/HOLMES
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B. Yang and D. Jewitt, University of Hawaii, report near-
infrared (0.8- to 4.2-micron) spectra of comet 17P taken with the
3.0-m NASA Infrared Telescope Facility by S. J. Bus on Oct. 27 and
28 UT. Two broad absorption bands were found centered at 2.02
microns (the depth being 10 percent of the continuum) and 3.03
microns (30-percent deep), respectively, in the reflectance spectra
of 17P. These features, which appeared on both nights, are
consistent with abundant water ice grains in the central coma. The
overall slope of the infrared reflectance spectrum of this comet is
blue, with a normalized reflectance gradient near -2.3 percent per
100 nm.
CBET 1123 : 20071107 : COMET 17P/HOLMES COMET 17P/HOLMES
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Further to CBETs 1111 and 1118, B. Gaillard, J. Lecacheux, and F. Colas,
Paris Observatory, elaborate on observations of comet 17P obtained from Oct.
24 to Nov. 4 with the 1-m telescope at Pic du Midi Observatory. During this
10-day interval beginning just at the outburst, they have followed the
evolution of dust streams and condensations inside the expanding "coma blob".
The dust streams appear to come from cometary fragments that quickly recede
from the nucleus, though such fragments remain below their detection
threshold. Nevertheless, they have measured (with a 2000-km precision) the
increasing distance from the nucleus of the well-defined head of these
streams. Apparent velocities ranging from 50 to 100 m/s (projected on the
sky) have been found for four of these stream heads, showing no trace of
acceleration. The separation of the parent fragments from the nucleus
is calculated to have occurred at different times between Oct. 23.7 and
24.8 UT, so the most probable scenario is that the comet outburst began a
few hours before Oct. 24.0. Different discrete releases of dusty material
took place over the two subsequent days, with a dominant event around Oct.
24.40 (cf. CBET 1111) being perhaps responsible for the final brightness
rise of the comet, and undoubtedly for the obvious circular shell that
continues expanding at 570 m/s. The measured dust streams are no longer
fed by new material and will vanish via increasing dilution in the space.
On the other hand, the pseudo-nucleus, which seemed inactive until Oct.
29, now is active again: its brightness, which was declining since Oct.
24, has stabilized, and a thin permanent dust trail is observed, perhaps
announcing another outburst.