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 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 Ṁ with increasing age t among T Tauri stars. Representing this decline as Ṁ~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 ~ Rγ, 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- March 1998
- DOI:
- 10.1086/305277
- Bibcode:
- 1998ApJ...495..385H
- Keywords:
-
- ACCRETION;
- ACCRETION DISKS;
- STARS: CIRCUMSTELLAR MATTER;
- STARS: FORMATION;
- STARS: PRE-MAIN-SEQUENCE;
- Accretion;
- Accretion Disks;
- Stars: Circumstellar Matter;
- Stars: Formation;
- Stars: Pre-Main-Sequence