MIDINFRARED IMAGES AND MODELS OF THE BETA-PICTORIS DUST DISK

New high resolution mid-infrared images of the inner parr of the beta Pictoris circumstellar dust disk are reported. These data are used to probe the inner disk geometry and to derive the 0-100 AU radial densit y profile, with a spatial resolution of 5 AU. Realistic grains (porous silicates with a core of olivine and pyroxene coated with a mantle '' organic refractories'') rather than solid Draine and Lee silicates, ha ve been incorporated in models describing the dust thermal emission. I n a first set of models, we assume a single composition for the grains . These models are not able to fully reproduce simultaneously our data and the IRAS fluxes. In a second set of models, we introduce another component, consisting of particles containing some ice beyond a limit around 90-100 AU. Such a model fulfils all the observational constrain ts (including the visible scattered flux) and resolves the discrepancy concerning the particles albedo which is predicted to be too small in the first set of models. It also predicts an ice feature at 50 mu m; a feature around 6 mu m characteristic of the organic refractory mater ial; and several crystalline olivine features at 24, 28 and 35 mu m Th ese features should be observable with the Short Wavelength Spectromet er (SWS) and Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory. 20 mu m images should also show an abrupt cut-off a round 90-100 AU due to the change of the grains composition. ISOCAM sh ould image essentially the scattered starlight, showing the transition between the thermal emission of the inner, non-icy, component, and th e scattered emission in the mid-infrared range from the outer componen t containing ice.