Evolutionary time scales for circumstellar disks associated with intermediate- and solar-type stars

The frequency with which massive, optically thick disks form around solar-type and intermediate mass stars is determined. Criteria are developed for diagnosing massive, optically thick disks, and the fraction of young, optically visible stars surrounded by such disks is determined. The range of survival times for massive, optically thick disks is determined by determining the fraction of stars surrounded by such disks as a function of stellar age. The existence of the probable descendents of massive, optically thick disks - optically thin dust disks, perhaps analogous to the structures surrounding Vega and Beta Pictoris - is established. The existence of structures in transition between those which are optically thick from the stellar surface outward, and those which are optically thin throughout is established.