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L. A. Hillenbrand (Caltech), M. R. Meyer (U. Arizona)
It is fairly common consensus that circumstellar disks are near-ubiquitously present around stars at birth. These disks are the sites of planetary formation, and with detection of exo-solar planets now occurring routinely, understanding of the evolutionary link between young circumstellar disks and planets is more critical than ever. Dimunition of accretion disk activity and transition from optically thick accretion disks to optically thin post-planet-building disks occurs during the early pre-main sequence phase. Observationally, there are at present no statistically significant quantitative constraints on accretion disk lifetimes as a function of stellar mass, although data provided by 2MASS and SIRTF will form the basis for significant progress over the next decade. We have attempted to constrain accretion disk lifetimes by compiling optical and near-infrared photometric and spectroscopic data for a large sample (N > 2500) of relatively nearby young stellar objects, from which near-infrared excesses and stellar ages and masses can be derived. Our analysis of the near-infrared excess trends with stellar age and stellar mass supports a conclusion of large dispersions at any given mass in accretion disk lifetimes. There are rather young (<1 Myr) stars which already have lost their disks, and also rather old (>10 Myr) stars which still retain their inner accretion disks, as has been known for some time. Nevertheless, a generally declining trend of disk fraction with increasing stellar age is suggested, with relatively few (but indeed some) disks present around low-mass stars after 15 Myr.