Astron. Astrophys. 323, 513-523 (1997)

Astron. Astrophys. 323, 513-523 (1997)

1. Introduction

The detection of circumstellar dust around many main-sequence stars, i.e. the Vega phenomenon, has been one of the highlights of the IRAS mission (e.g. Aumann et al. 1984). It is probable that these dust excesses are in some way related to the existence of large bodies, e.g. comets or planetesimals, around these stars, at typical distances of ten astronomical units and more from the star (e.g. Telesco and Knacke 1991). In fact, collisions with such larger bodies could be the source of the replenishment of the dust debris disk, which is required in view of the appreciable ages of the stars, that appear to be much older than the timescales of the various effects which cause the disappearance of the circumstellar disk that initially surrounded the star (e.g. Backman and Paresce 1993).

The similarity of the debris disk in Vega-like stars with the Kuiper belt in our solar system has been pointed out by several authors (Backman and Paresce 1993; Weissman 1995; Backman et al. 1995). The Kuiper belt, whose existence was anticipated by Edgeworth (1949) and Kuiper (1951), is a reservoir for short-period comets (Fernandez 1980), that extends beyond the orbit of Neptune to some 60 AU from the Sun (for a review, see Weissman 1995).

Jura (1990) first addressed the question what happens to Vega-like disks when the central star ends its evolution on the main sequence and proceeds as a red giant. From a study of IRAS observations of some 100 bright giants, mostly of spectral type G, he concluded that infrared (IR) excesses are much rarer for such stars than for their progenitor A-type main-sequence stars. These observations suggest that the increased luminosity of the stars has drastically contributed to evaporate the remaining cometary bodies in the circumstellar disks. On the other hand, Matese and Whitmire (1989) have suggested that the remaining small fragments could provide cores whereupon gas from the stellar wind can condense to dust grains, which would lead to an enhanced visibility of the dust debris disks of some giants.

Jura's search was limited to bright giants, for which IR excesses can reliably be determined from the IRAS Point Source Catalogue Version 2 (PSC). Since the IRAS Faint Source Catalogue Version 2 (FSC), has become available, it has become possible to extend the search for IR excesses around late-type giants to a much larger sample. A first glance at the FSC shows that a significant number of excess stars indeed exists (Sect. 2), so that we embarked on a systematic study of this phenomenon. This study is described in Sect.3. During our search, we became aware of an independent study by Zuckerman et al. (1995), which contains several objects in common with the present study, but which differs from it on several points we allude to in the text.

Online publication: June 5, 1998

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