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