The propagation of Lyα in evolving protoplanetary disks

Abstract

We study the role resonant scattering plays in the transport of Lyα 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 Lyα line (80% of the FUV luminosity), we find that resonant scattering further enhances the Lyα density in the deep molecular layers of the disk. Lyα 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 Lyα/FUV- continuum photon density ratio is strongly stratified; FUV-continuum-dominated in the photodissociation layer and Lyα-dominated field in the molecular disk. The enhancement is greatest in the interior of the disk (r 1AU) but is also observed in the outer disk (r 100AU). The majority of the total disk mass is shown to be increasingly Lyα dominated as dust settles toward the midplane. © 2011 The American Astronomical Society. All rights reserved.