Accretion and the Evolution of T Tauri Disks

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Abstract
Using results and calibrations from a previous paper (Gullbring et al. 1997), we estimate disk accretion rates for preÈmain-sequence stars in the Taurus and Chamaeleon I molecular cloud complexes. The median accretion rate for T Tauri stars of age D1 Myr is D10~8 yr~1 ; the intrinsic scatter at a M _ given age may be as large as 1 order of magnitude. There is a clear decline of mass accretion rates M 0 with increasing age t among T Tauri stars. Representing this decline as we estimate M 0 P t~g, 1.5 [ g [ 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 g 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 l P Rc, corresponds to The limit c D 1 g Z 1.5 c Z 1. corresponds to a roughly constant "" a ÏÏ in the viscosity parameterization ; using Shakura-Sunyaev (1973) current observed disk sizes, we estimate a D 10~2 (on scales D10È100 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 but this result may be strongly a †ected by the presence of binary companion stars. These results M _ , emphasize the need for older samples of stars for studying disk evolution.

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