Published online by Cambridge University Press: 22 October 2009
Thus far, our impressions regarding the evolutionary timescales for young circumstellar disks have been based on small number statistics. Over the past decade, however, in addition to precision study of individual star/disk systems, substantial observational effort has been invested in obtaining less detailed data on large numbers of objects in young star clusters. This has resulted in a plethora of information now enabling statistical studies of disk evolutionary diagnostics. Along an ordinate, one can measure disk presence or strength through indicators such as ultraviolet/blue excess or spectroscopic emission lines tracing accretion, infrared-excess tracing dust, or millimeter flux-measuring mass. Along an abscissa, one can track stellar age. While bulk trends in disk indicators versus age are evident, observational errors affecting both axes, combined with systematic errors in our understanding of stellar ages, both cloud and bias any such trends. Thus, detailed understanding of the physical processes involved in disk dissipation and of the relevant timescales remains elusive. Nevertheless, a clear effect in current data that is unlikely to be altered by data analysis improvements is the dispersion in disk lifetimes. Inner accretion disks are traced by near-infrared emission. Moderating a generally declining trend in near-infrared continuum excess and excess frequency with age over <1 to 8 ± 4 Myr, is the fact that a substantial fraction of rather young (<1 Myr old) stars apparently have already lost their inner accretion disks, while a significant number of rather old (8–16 Myr) stars apparently still retain them. By the age of 3–8 Myr, evidence for inner accretion disks for the vast majority of stars (~90%) ceases to be apparent. Terrestrial zone dust is traced by mid-infrared emission where sufficient sensitivity and uniform data collection are only now being realized with data return from the Spitzer Space Telescope. Constraints on mid-disk dissipation and disk-clearing trends with radius are forthcoming.