Accretion and the Evolution of T Tauri Disks

Abstract

Using results and calibrations from a previous paper (Gullbring et al. 1997), we estimate disk accretion rates for pre-main-sequence stars in the Taurus and Chamaeleon I molecular cloud complexes. The median accretion rate for T Tauri stars of age ~1 Myr is ~10-8 Mȯ yr-1; the intrinsic scatter at a given age may be as large as 1 order of magnitude. There is a clear decline of mass accretion rates M dot with increasing age t among T Tauri stars. Representing this decline as M dot ~t^{- eta } , we estimate 1.5 ~ 1.5 corresponds to gamma >~ 1. The limit gamma ~ 1 corresponds to a roughly constant " alpha " in the Shakura-Sunyaev (1973) viscosity parameterization; using current observed disk sizes, we estimate alpha ~ 10-2 (on scales ~10-100 AU). Much of the observed variation in mass accretion rates can be accounted for by varying initial disk masses between 0.01 and 0.2 Mȯ, but this result may be strongly affected by the presence of binary companion stars. These results emphasize the need for older samples of stars for studying disk evolution.