Accretion and the Evolution of T Tauri Disks

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 M_☉ 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 with increasing age t among T Tauri stars. Representing this decline as ∝ t^η, we estimate 1.5 ≲ η ≲ 2.8; the large uncertainty is due to the wide range of accretion rates at a given age, the limited age range of the sample, and errors in estimating stellar ages and accretion luminosities. Adopting values of η near the low end of this range, which are more likely given probable errors and the neglect of birthline age corrections, masses accreted during the T Tauri phase are roughly consistent with disk masses estimated from millimeter-wave dust emission. Similarity solutions for evolving, expanding disks are used to investigate observational constraints on disk properties employing a minimum of parameters. For an assumed power-law form of the disk viscosity with radius ν ∝ R^γ, η ≳ 1.5 corresponds to γ ≳ 1. The limit γ ~ 1 corresponds to a roughly constant "α" in the Shakura-Sunyaev (1973) viscosity parameterization; using current observed disk sizes, we estimate α ~ 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 M_☉, 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.