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

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4. Discussion

We have obtained evidence for water ice in the coma of comet Hale-Bopp at [FORMULA] = 2.9 AU and inferred a mean icy grain size of 15 µm. (We note, however, that using a single grain size is not realistic for sublimating icy grains.) Davies et al. (1997) modelled their 1.5 and 2.0 µm observations at 7 AU in terms of ice/dust mixtures (intimate or mixtures) on a solid surface (although, again, most of the flux originates from the coma). For spatial mixtures, ice particle diameters of 5-10 µm were inferred, comparable to what we find. The water ice bands were not seen in subsequent observations at 4.6 AU (Davies et al. priv. comm. ). This is, however, not inconsistent with our detection at 2.9 AU, as the 3 µm band is much stronger than the 1.5 and 2 µm features.

Water outgassing at large [FORMULA] probably originates from icy grains in the coma. From the H2O production rate (Fig. 3; Crovisier et al. 1997a) and our measured H2O ice cross section and mass, we infer a grain lifetime of about 2.2 days. This is [FORMULA] 10 times longer than calculated for 15 µm grains at [FORMULA] K (Enzian 1997), indicating that this temperature is too high. The sublimation temperature can be directly estimated from [FORMULA], where Z is the ice sublimation rate and A, the total grain area, is equal to [FORMULA]. We find [FORMULA] mol cm- 2 s-1, i.e., [FORMULA] K at 2.9 AU. This is in very good agreement with calculations by Hanner (1981) for dirty (n" = 0.002 in the visible) 15 µm icy grains.

The dust mass production rates we infer, which pertain to large ([FORMULA]m) particles, are similar to those measured for micrometre-sized particles in the visible at the same [FORMULA]. Dust production rates have similarly been determined from millimetre/submillimetre measurements performed near perihelion (Senay et al. 1998; Wink et al. 1998). Although these determinations are uncertain because the appropriate dust velocity is poorly known, they also suggest that large particles importantly contribute to (or even dominate) the mass of the dust coma.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998