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Astron. Astrophys. 361, L9-L12 (2000)

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1. Introduction

The discovery of infrared excesses around an important fraction of main-sequence stars (Aumann et al. 1984; Plets & Vynckier 1999) and attributed to the presence of cool dust grains orbiting the star and geometrically arranged in a disk, has triggered a lot of studies because these disks may be linked to planetary formation. For a long time the disk around the star [FORMULA] Pictoris was the only example to be resolved both in the visible (Smith & Terrile 1984) and in the mid-infrared range (Lagage & Pantin 1994). Recent discoveries of young "debris" circumstellar disks around relatively "old" and isolated (i.e. not associated to any star forming region) stars have shown that this phenomenon extends towards Pre-Main-Sequence stars (cf the photometric survey by Malfait et al. 1998a). The detection and resolution of disk around stars such as HD 141569 (Weinberger et al. 1999; Augereau et al. 1999) or HR 4796A thanks to high-resolution observations in the visible/near-infrared range (Schneider et al. 1999; Augereau et al. 1999) or mid-infrared images (Koerner et al. 1998; Jayawardhana et al. 1998) have shown the possibility to observe and study precursors to main-sequence dust disks. These so-called "baby-[FORMULA] Pic" dust disks are the denser precursors to main-sequence debris disks. IRAS and ISO/SWS observations have shown that they usually produce a huge infrared excess (typically 250 times the infrared excess produced by the [FORMULA] Pic disk); some of them, as HD 100546 for instance, showing prominent signatures of crystalline water ice (Waelkens et al. 1996, Malfait et al. 1998b), are particularly interesting targets in the visible/near-infrared because of high particle albedo. Observing this class of disks at various stages of evolution will help in finding a comprehensive scenario for the origin, the evolution, and the lifetime of the Vega phenomenon, i.e. how these disks form, how long they last, and how they disappear.

We report in this paper the first images of the disk around HD 100546, an isolated young main-sequence star. Using a model of scattering by dust grains, we derive the morphology of the disk outwards 10 AU and show evidence for a density maximum around 40 AU. The structure of the disk is compared to the best known example of the Vega Phenomenon, the disk around [FORMULA] Pic.

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000