Correlations of optical and infrared excesses in T Tauri stars

We report the results of a survey of excess optical continuum emission (veiling) in a sample of 35 K7-M1 pre-main-sequence stars in Taurus-Auriga. Stars with detectable veiling emission always show significant near-infrared K-L excesses. This result agrees with the prediction of a simple accretion disk model, where a boundary layer produces the veiling and the infrared excess comes from the disk. The K - N colors of T Tauri stars fall into two distinct groups: stars in the group with small or absent K-N excesses have no detectable veiling emission, while objects with large K - N excesses usually have detectable veiling. This result also agrees with the prediction of the disk model, where a large K-N excess indicates an optically thick disk, and an optically thick disk is required to generate a mass accretion rate large enough to produce detectable veiling. Stars with strong H alpha and [O I] emission also have veiling, suggesting a connection between accretion and outflow. These latter correlations are not strong, possibly the result of radiative transfer, cooling, and ionization effects in H alpha and [O I].