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I. Song, J.-P. Caillault (University of Georgia), D. Barrado y Navascués (MPIfA), J. R. Stauffer (CfA)
A systematic program to determine the ages of Vega--like stars has been carried out. There have been very few published age estimates, and improving the quantity and the quality of these estimates is extremely important, since they may play an important role in understanding whether planet formation is common. If the ages of Vega--like stars are all very young, then this suggests that planet formation is common and that the range of dust optical depths is an age indicator; if the ages are spread out over a few hundred Myr, then planet formation would be determined by \emph{random initial conditions}.
While it is difficult to determine directly the ages of individual A--type Vega--like stars, if they posses a low mass binary companion then it is possible to employ indirect methods, such as standard age--dating techniques for low--mass stars. As part of our program, we analyzed more than 100 low mass binary companion candidates (based on proper motions, radial velocities, parallaxes, separations, etc.) but found that none of them was a bona-fide companion to the early--type Vega--like stars.
However, we have determined directly the ages of eight late--type Vega--like stars. With the exception of the very unusual pre--main sequence star system HD 98800, the other seven Vega--like stars are the same age as the Hyades cluster or older. Also, we have attempted to determine the ages of A--type Vega--like stars directly via Strömgren photometry; most of them are younger than 400 Myr. For the early--type stars, the correlation of age and IR excess, plus the generally young ages for these prototypical objects, suggest that many A stars form with disks and that the disk excess decreases with age due to some evolutionary process. For the late--type Vega--like stars, the much older derived ages at least suggests that the time-scale for disk evolution is longer.
The author(s) of this abstract have provided an email address for comments about the abstract: song@physast.uga.edu