Astron. Astrophys. 339, L9-L12 (1998)
1. Introduction
The possible existence of an icy grain halo around cometary
nuclei has been debated for a long time. Indeed, icy particles are
likely to be lifted off the nucleus along with refractory particles
during cometary activity. At small heliocentric distances
( ) such icy grains are expected to have short
lifetimes and thus to be difficult to detect from ground-based
observations (Hanner 1981). On the other hand, indirect evidences
exist for the presence of icy grains at large :
for example, the production of water from some distant comets (A'Hearn
et al. 1984), including C/1995 O1 (Hale-Bopp) (Biver et al. 1997),
cannot be explained by free sublimation at the nucleus surface but
rather from the sublimation of icy grains (Enzian et al. 1998;
Prialnik 1998).
Although previous searches for the water ice feature around
3.1 µm were inconclusive (e.g., in C/1983 O1
ernis; Hanner 1984a), water
ice was detected in comet C/1995 O1 (Hale Bopp) at
AU from 1.50 and 2.04 µm
absorption features (Davies et al. 1997). Other water ice features
exist in the far-infrared at 44 and 65 µm. They have been
observed in circumstellar shells ("Frosty Leo"; Omont et al. 1990),
disks around young stars (HD 100546; Malfait et al. 1998), and
protostars (e.g., RAFGL 7009S; Dartois et al. 1998).
We report here on observations of the 3.1, 44 and
65 µm water ice features in comet Hale-Bopp at
AU from the Sun using the Infrared Space
Observatory (ISO). We also investigate the continuum spectrum at
43-195 µm to constrain the dust size and production rate
of large particles.
© European Southern Observatory (ESO) 1998
Online publication: September 30, 1998
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