Vega-like stars

Carsten Dominik 

Leiden Observatory, P.O. Box 9531, NL-2300RA Leiden

Vega-like stars are main-sequence stars with IR excesses usually attributed to the presence of a circumstellar debris disk. According to the results from the IRAS database, approximately 30% or more of all main sequence stars should show such an excess (Aumann & Good 1990, AJ 350, 408; Aumann 1988, AJ 96, 1414) however, a more recent re-evaluation of this problem indicates numbers clearly below 20% (Plets 1997, PHD-thesis, University Leuven). One of the problems of the IRAS studies was that the samples used to study the phenomenon usually were poorly defined. This was also prohibitive to a detailed statistical study of age effects these stars.

We have undertaken the study of a volume-limited sample with ISO. Studying this sample and theoretical models we will show in this contribution that:

• There is an age-excess relation for main sequence stars. We find a fraction of excess stars in our sample of approximately 20%, higher for young stars.

• By example of a particular G type star we show that the inner hole in the disk required to fit the observations needs not only to be created when the stars forms, but also needs to be maintained over the history of the stars evolution.

• We present calculations which indicate that the sun was a Vega-like star when it was young, but that the current amount of material is too little to produce an amount of dust which would be observable with current technology (ISO) from a nearby star.