Astron. Astrophys. 364, L13-L18 (2000)

1. Introduction

One of the focal points of current astronomical research is the search for circumstellar disks and a study of their transformation into planetary systems. The aim is to understand how the solar system evolved, and to determine how many other planetary systems might exist in the Galaxy.

The huge brightness difference of typically 10⁶ to 1 between the central star and light scattered from the disk surface (Sonnhalter et al. 1995; Boss & Yorke 1996) makes it challenging to detect and resolve circumstellar disks in the optical and near-infrared. If the disk, however, is seen close to edge-on, it acts as a natural coronagraph and blocks out the light from the central star. The typical signature for such an alignment is a bipolar reflection nebula intersected by a central dust lane (Whitney & Hartmann 1992; Sonnhalter et al. 1995; Burrows et al. 1996). Edge-on circumstellar disk sources have been discovered in the Orion nebula (McCaughrean & O'Dell 1996) and in the Taurus T association (e.g., Burrows et al. 1996, Lucas & Roche 1997, Padgett et al. 1999, Koresko 1998, Stapelfeldt et al. 1998, Monin & Bouvier 2000).

It has already become apparent that different environments affect the evolution of circumstellar disks in various ways. In dense regions with many hot, luminous early type stars like, e.g., the Trapezium cluster in Orion or the starburst cluster NGC 3603, the harsh radiation environment leads to a rapid photoevaporation of circumstellar disks (O'Dell et al. 1993; McCullough et al. 1995; Störzer & Hollenbach 1999; Brandner et al. 2000). The low-density Taurus T association, on the other hand, is a more benign environment. Because of the high multiplicity among T Tauri stars in Taurus (Ghez et al. 1993, Leinert et al. 1993, Köhler & Leinert 1998), however, the majority of circumstellar disks might be affected by tidal truncation, like, e.g., HK Tau/c (Koresko 1998, Stapelfeldt et al. 1998) or HV Tau C (Monin & Bouvier 2000).

The Ophiuchi molecular cloud complex is the formation site of a proto-open cluster with a gaseous mass of around 550 (Wilking & Lada 1983) and at least 100 stellar members (Comerón et al. 1993; Barsony et al. 1997; Kenyon et al. 1998). Its stellar density is intermediate between that of the Taurus T association and the Trapezium cluster. The absence of hot, luminous early-type stars ensures that circumstellar disks are not subject to photoevaporation. The Oph region appears to be a more typical representative of the dominant star formation mode in the Galaxy than the low density environment of the wide-spread Taurus T association with its high percentage of binary and multiple systems.

In an effort to identify spatially resolved circumstellar disks in various environments, we carried out a VLT/ISAAC survey of southern starforming regions (Zinnecker et al. 1999). The survey also aimed at establishing a sample of circumstellar disk sources which are suitable for detailed follow-up studies with the VLT-Interferometer and the Atacama Large Millimeter Array.

© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
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