The results shown above suggest the presence in coma of comet Hale-Bopp of discrete regions where grains with different size and/or composition are present. Spatial gradients in the continuum emission of a comet coma have been firstly reported by Jewitt & Meech (1986a). They showed that the grain continuum on the sunward side of the nucleus of comet P/Halley was redder than on the antisunward side. The same effect is shown in Fig. 4 and 5. The authors attributed these variations to particle size sorting by solar radiation pressure. Although a comprehensive model of the observations would require knowledge of size distribution, complex index of refraction, structure and shape of the particles (data not fully determined), some qualitative constrains on the nature of the grains can be placed, in the light of the already available data. The wavelength dependence of the intensity of solar light scattered by the grains, , is proportional (neglecting phase dependence effects and considering an optically thin coma) to the efficiency factor for scattering of the particles, . When the particles are very much larger than the wavelength (Mie size parameter 1 where a is the particle radius), becomes independent of wavelength. If grains in regions A, B have the same composition but different sizes, spectra plotted in Fig. 5 indicate that . This implies the presence of particles having different size, likely in the m interval. On the contrary, if wavelength dependence is the result of the compositional, rather than size effects, region A spectra could be fitted by particles scattering more efficiently than silicate grains (region B) at shorter wavelengths, such as icy or carbonaceous particles. Several pieces of evidence can be interpreted in terms of the size and composition of dust grains. The intensity of thermal emission in the 8 - 13 µm range indicates the presence of small ( 1 µm) grains in the inner coma (Hayward & Hanner, 1997). The authors suggest also that much of this fine material originates from localised active areas on the nucleus. The strong silicate emission at 11.2 µm (Crovisier et al. 1996, 1997) has been found consistent with Mg-rich crystalline olivine (forsterite), maybe in the form of sub-micrometric grains (Colangeli et al 1995). Larger grains ( 5 µm) are also suggested (Lisse et al. 1997) to explain infrared observation of the dust. H2 O sublimation from icy grains has been seen when the comet was at large heliocentric distances (Davies et al. 1997; Biver et al. 1997). Additional organic components may also be present (Davies et al. 1997; Sarmecanic et al. 1997). A preliminary analysis of the C2 ( = 0) emission feature, indicated in Fig. 5, shows a spatial correlation with the distribution of dust. This fact strengthens the organic nature of dust grains.
© European Southern Observatory (ESO) 1998
Online publication: April 15, 1998