Observational constraints on dust disklifetimes: Implications for planet formation

摘要

Thus far, our impressions regarding the evolutionary timescales for young circumstellar diskshave been based on small number statistics. Over the past decade, however, in addition to preci-sion study of individual star/disk systems, substantial observational effort has been invested inobtaining less detailed data on large numbers of objects in young star clusters. This has resultedin 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 ul-traviolet/blue excess or spectroscopic emission lines tracing accretion, infrared-excess tracingdust, or millimeter flux-measuring mass. Along an abscissa, one can track stellar age. Whilebulk 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 anysuch trends. Thus, detailed understanding of the physical processes involved in disk dissipationand of the relevant timescales remains elusive. Nevertheless, a clear effect in current data thatis unlikely to be altered by data analysis improvements is the dispersion in disk lifetimes. Inneraccretion disks are traced by near-infrared emission. Moderating a generally declining trend innear-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 theirinner accretion disks, while a significant number of rather old (8-16 Myr) stars apparently stillretain them. By the age of 3-- 8 Myr, evidence for inner accretion disks for the vast majorityof stars (~90%) ceases to be apparent. Terrestrial zone dust is traced by mid-infrared emissionwhere sufficient sensitivity and uniform data collection are only now being realized with datareturn from the Spitzer Space Telescope. Constraints on mid-disk dissipation and disk-clearingtrends with radius are forthcoming.