Photoevaporation of Circumstellar Disks around Young Stars

We examine the ability of photoevaporative disk winds to explain the low-velocity components observed in the forbidden line spectra of low-mass T Tauri stars. Using the analytic model of Shu, Johnstone, & Hollenbach (1993) and Hollenbach et al. (1994) as a basis, we examine the characteristics of photoevaporative outflows with hydrodynamic simulations. General results from the simulations agree well with the analytic predictions, although some small differences are present. Most importantly, the flow of material from the disk surface develops at smaller radii than in the analytic approximations and the flow-velocity from the disk surface is only one-third the sound speed. A detailed presentation of observational consequences of the model is given, including predicted line widths, blue-shifts, and integrated luminosities of observable sulfur and nitrogen emission lines. We demonstrate that these predictions are in agreement with current observational data on the low-velocity forbidden line emission of ionized species from T Tauri stars. This is in contrast with magnetic wind models, which systematically under-predict these forbidden line luminosities. However, the present model cannot easily account for the luminosities of neutral oxygen lines in T Tauri stars