Emission-line diagnostics of T tauri magnetospheric accretion. II. Improved model tests and insights into accretion physics

We present new radiative transfer models of magnetospheric accretion in T Tauri stars. Hydrogen and Na I line profiles were calculated, including line damping and continuum opacity for a grid of models spanning a large range of infall rates, magnetospheric geometries, and gas temperatures. We also calculated models for rotating magnetospheres and show that for typical T Tauri rotation rates, the line profiles are not significantly affected. We show that line-damping wings can produce significant high-velocity emission at Ha, and to a lesser extent in higher Balmer lines, in much better agreement with observations than previous models. We present comparisons to specific objects spanning a wide range of accretion activity and find that in most cases the models successfully reproduce the observed emission profile features. Blueshifted absorption components cannot be explained without including a wind outside of the magnetosphere, and true P Cygni Balmer line profiles in the few objects with extreme accretion activity indicate both absorption and emission from a wind. We constrain the range of gas temperatures required to explain observational diagnostics like profile shapes, line ratios, and continuum emission. The exact heating mechanism remains unclear but is probably linked to the accretion process itself. In order to explain observed correlations between line emission and accretion luminosity, we find that the size of the emitting region must be correlated with the accretion rate. We suggest that such a correlation may manifest itself in reality via nonaxisymmetric accretion, where the number and/or width of discrete funnel flows increase with increasing accretion rate, a scenario also indicated by accretion shock models.