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Astron. Astrophys. 327, 1123-1136 (1997)

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1. Introduction

At a distance of 16.5 pc from the Earth lies the star [FORMULA] Pictoris for which the infrared excess detected by IRAS (Aumann 1984) was attributed to a dusty disk in an intermediate stage between the solar protoplanetary nebulae and an actual solar system (e.g. review by Artymowicz, 1994 and references therein). This system was studied from the UV (Deleuil et al., 1993 and references therein) to the millimeter range (Chini et al., 1991). Circumstellar dust grains producing the infrared excess are present with sizes ranging from the µm domain to the mm domain (e.g. see the review by Artymowicz (1994) and references therein) and are arranged geometrically in a disk seen almost edge-on (Smith and Terrile, 1984). A gaseous component is also present around the star (Sletteback and Carpenter, 1983; review by Vidal-Madjar and Ferlet, 1994 and references therein). Sporadic gas pockets, revealed by UV and visible spectroscopic observations of redshifted metallic lines seen in absorption (Kondo and Bruhweiler, 1985; Ferlet, Hobbs and Vidal-Majar, 1987; Lagrange, Ferlet and Vidal-Majar, 1987), are attributed to the evaporation of kilometer sized bodies, perhaps the [FORMULA] Pic analogue of comets, as they fall onto the star (Beust, 1994 and references therein). Slow evaporation of a belt of bodies orbiting at 15-30 AU from the star (somewhat analogous to the Kuiper belt) is also the main ingredient for a model recently proposed to explain the spatial profile of the outer part of the dusty disk (Lecavelier des Etangs et al., 1995).

[FIGURE] Fig. 1. TIMMI images of the [FORMULA] Pictori s dust disk at 11.9µm. The raw image is on the left and the image deconvolved using Multi-Scale Maximum Entropy Method is on the right. The pixel field of view is 0.33". The resolution achieved in the deconvolved image is very close to 0.33 arcsec.

The study of the inner part of the dusty disk (r [FORMULA] 100 AU) is very exciting since it is where planets could exist. In visible, ground-based observations, the contrast between the star and the disk is so high that blooming (charge leakage) and seeing effects have made it impossible to observe the disk within 30 AU from the central star (Golimovski et al., 1993; Lecavelier des Etangs et al., 1993; Kalas and Jewitt, 1995). Even the recent HST observations did not give access to the region within 20 AU (Burrows et al., 1995). A similar limit is reached with ground-based near-IR observations (K band) using adaptative optics (Mouillet and Lagrange, 1996). As shown by Lagage and Pantin (1994, hereafter LP94), the inner region is accessible to mid-IR observations. The first images have shown an inner region deficient in matter, as expected by the models developed to fit the IRAS spectral energy distribution (Gillett, 1986; Diner and Appleby, 1986; Telesco, Becklin and Wolstencroft, 1986; Artymowicz, Burrows and Paresce, 1989; Backmann, Gillett and Witteborn, 1992), and, surprisingly, a large asymmetry between the NE and SW components of the disk.

In order to confirm and improve these preliminary results, we re-observed [FORMULA] Pictoris, again with the mid-infrared camera TIMMI (Thermal Infrared Multi Mode Instrument) mounted on the ESO 3.6m telescope at La Silla, Chile. The new set of images and the associated data reduction are discussed in Sect.2. Sect.3 presents the radial density profile from the data in the range 0-100 AU from the star. We discuss the model used to calculate the thermal emission of the grains, which is essential for inferring the density from the observed flux. The main ingredients of the model are discussed, as well as the simplifying assumptions. We determine dust composition and size distribution by using both constraints provided by the silicate feature observed in the mid-IR (Knacke et al., 1993, Aitken et al., 1993) and physical considerations, such as the dynamic behaviour of the smallest grains that experience radiation pressure forces. In Sect.4, we have extrapolated our inner density to outer regions on the basis of visible data, keeping the same composition of the grains (one component models) in order to be able to try to fit the other observables of the disk (IRAS fluxes, scattered flux, other infrared fluxes). We find that none of these one component models are able to fully reproduce the IRAS measurements. In Sect.5, we relax the assumption of uniform grain composition throughout the disk, by introducing porous silicate particles containing some ice beyond 90 AU. Sect. 6 is devoted to the discussion of some of the consequences of our model, such as the presence of an ice feature at 50 microns, which should be observable by ISO.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998