Mid-infared silicate dust features in -Pictoris as seen by ISOCAM

Pierre-Olivier Lagage ¹, O. Boulade ¹, C. Cesarsky ¹, Th. Douvion ¹, V. Mannings ², E. Pantin ¹, & A. Sargent ²,

¹ DSM/DAPNIA/SAp, CEA-Saclay, 91191 Gif sur Yvette Cedex FRANCE

² Caltech, Department of Astronomy, Pasadena

One of the major discoveries made with the IRAS satellite has been the finding of an infrared excess around many main sequence stars, such as Vega or beta-Pictoris (e.g. Aumann 1985, PASP 97, 885; review by Backman, Paresce 1993, Protostars and Planets III, p.208). This excess was interpreted as due to the thermal radiation from a disk of micron size dust grains orbiting around the star. The interpretation was rapidly confirmed by imaging the outer part of the dust disk (Smith and Terrile 1984, Science 226, 1421). The small dust grains present at the time the star was born, have disappeared, either forming planets or being expulsed from the system (see review by Artymowicz 1994 ``Circumstellar Dust Disks and Planet formation'', edition Frontieres, p. 47). Thus the dust seen by IRAS is of secondary origin. Collisions between planetesimals, or sublimation of comets, are the best ways to replenish the dust (Weissman 1984, Science 224; Lissauer 1993, ARAA 31).

Ground-based spectroscopic observations of the 10 microns silicate dust feature have shown that the dust in beta-Pictoris and in some comets were similar (Knacke et al. 1993, ApJ 418, 440; Aitken et al. 1993, MNRAS 265, L41). ISO observations of comet Hale-Bopp have shown that a significant part of the dust was in the form of crystalline olivine (Crovisier et al. 1997, Science 275, 1904). Such crystalline silicates have also been found around young stars such as HD100546 (Malfait et al. 1998, AA 332, L25) or evolved objects such as the Red Rectangle (Waters et al. 1998, Nature 391, 868).

In this paper, we discuss the results from spectro-imaging observations of the beta-Pictoris dust disk, performed with the Circular Variable Filter of ISOCAM (between 6 and 15 microns); the smallest ISOCAM pixel field of view was used (1.5 arcsec). The disk is spatially resolved. A prominent 11.3 micron feature due to crystalline olivine feature does not show up in the spectrum. On the other hand, features caracteristic of crystalline pyroxene seems to be present (to be confirmed by additional data processing), so that pyroxene could be the dominant silicates in beta-Pictoris dust grains, as it is the case in Interplanetary Dust Particles.