Disk Accretion Rates for T Tauri Stars

We present new measurements of disk accretion rates for T Tauri stars in the Taurus molecular cloud complex. Our results are based on intermediate-resolution spectrophotometry from 3200 to 5200 Å, which is used to derive the excess hot continuum emission produced by accretion onto the central star. Previous estimates of T Tauri accretion rates in the literature differ by as much as 1 order of magnitude; our measurements agree better with the lowest estimates, and we discuss the problems and systematic effects that led to the previous disagreement. In particular, we note that the stellar photospheric emission from nonaccreting T Tauri stars exhibits color anomalies compared to main-sequence stars; these anomalies make the estimated extinction depend upon the color index used. We argue that the V-R index is a reasonable compromise to match with optically derived spectral types, and that V-I and V-J are much more likely to be biased by cooler companion stars and starspots. We develop a calibration with which approximate mass accretion rates can be derived for T Tauri stars based on broadband photometry and spectral types, which should enable accretion rates to be estimated for large samples with greater ease.