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Astron. Astrophys. 339, L9-L12 (1998)

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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 ([FORMULA]) 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 [FORMULA]: 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 [FORMULA] 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 [FORMULA] 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.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998