Age dependence of the Vega phenomenon: Observations

We study the time dependency of Vega-like excesses using infrared studies obtained with the imaging photopolarimeter ISOPHOT on board the Infrared Space Observatory. We review the different studies published on this issue and critically check and revise ages and fractional luminosities in the different samples. The conclusions of our study differ significantly from those obtained by other authors (e.g., Holland and coworkers; Spangler and coworkers), who suggested that there is a global power law governing the amount of dust seen in debris disks as a function of time. Our investigations lead us to conclude that (1) for stars at most ages, a large spread in fractional luminosity occurs, but (2) there are few very young stars with intermediate or small excesses; (3) the maximum excess seen in stars of a given age is about f(d) approximate to 10(-3), independent of time; and (4) Vega-like excess is more common in young stars than in old stars.