THE PROPAGATION OF Lyα IN EVOLVING PROTOPLANETARY DISKS

We study the role resonant scattering plays in the transport of Ly alpha photons in accreting protoplanetary disk systems subject to varying degrees of dust settling. While the intrinsic stellar far-UV (FUV) spectrum of accreting T Tauri systems may already be dominated by a strong, broad Ly alpha line (similar to 80% of the FUV luminosity), we find that resonant scattering further enhances the Ly alpha density in the deep molecular layers of the disk. Lya is scattered downward efficiently by the photodissociated atomic hydrogen layer that exists above the molecular disk. In contrast, FUV-continuum photons pass unimpeded through the photodissociation layer and (forward-) scatter inefficiently off dust grains. Using detailed, adaptive grid Monte Carlo radiative transfer simulations we show that the resulting Ly alpha/FUV-continuum photon density ratio is strongly stratified; FUV-continuum-dominated in the photodissociation layer and Ly alpha-dominated field in the molecular disk. The enhancement is greatest in the interior of the disk (r similar to 1 AU) but is also observed in the outer disk (r similar to 100 AU). The majority of the total disk mass is shown to be increasingly Ly alpha dominated as dust settles toward the midplane.