Helium Emission from Classical T Tauri Stars: Dual Origin in Magnetospheric Infall and Hot Wind

High-resolution emission-line profiles of He I and He II in 31 classical T Tauri stars are analyzed with the aim of probing the environs of the star-disk interface in accreting low-mass young stars. The diagnostic power of the helium lines lies in their high-excitation potentials, which restrict their formation to a region either of high temperature or close proximity to a source of ionizing radiation. The He I profiles are decomposed into kinematic components that support the paradigm of magnetically controlled accretion from the disk onto the stellar surface but also require a significant contribution from a hot wind. A narrow component, seen in 28/31 stars, is characterized by relatively uniform line widths and centroid velocities among all the helium lines. Our analysis supports previous conclusions that this feature is consistent with formation in the decelerating postshock gas at the magnetosphere footpoint. A broad component, seen in 22/31 stars, displays a diversity of kinematic properties. Our analysis suggests that in many stars the He I broad component is itself composite. At one extreme are stars where the broad component is redshifted in excess of 8 km s-1, as would occur if helium emission arises primarily from polar angles less than 54.°7 in the funnel flow. At the other extreme are stars where the broad component is blueshifted in excess of -30 km s-1, requiring an origin in outflowing gas. The additional occurrence of maximum blue wing velocities exceeding -200 km s-1 in 14 stars leads us to argue that hot winds are present in about half of our sample. The relation between the narrow component and the optical veiling differs between the stars with or without a hot helium wind, suggesting that when the hot wind is present the luminosity and temperature of the accretion shock are reduced. A comparison of broad component helium emission with standard outflow indicators leads us to suggest that there are two sources of inner wind in T Tauri accretion disk systems: one a hot polar/coronal wind that prevails in stars with high veiling, and the other a more widespread cool disk wind that is likely launched at the magnetosphere/disk boundary.