OPTICAL VEILING, DISK ACCRETION, AND THE EVOLUTION OF T-TAURI STARS

High-resolution spectra of 31 K7-M1 T Tauri stars (TTS) in the Taurus-Auriga molecular cloud demonstrate that most of these objects exhibit substantial excess emission at 5200 Å. Extrapolations of these data consistent with low-resolution spectrophotometry indicate that the extra emission is comparable to the stellar luminosity in many cases. If this continuum emission arises in the boundary layers of accreting disks, ≳ 30% of all TTS may be accreting material at a rate which is sufficiently rapid to alter their evolution from standard Hayashi tracks. We estimate that roughly 10% of the final stellar mass is accreted in the TT phase. This amount of material is comparable to the minimum gravitationally unstable disk mass estimated by Larson and we speculate that the TT phase represents the final stages of disk accretion driven by gravitational instabilities.