FUV Irradiated Disk Atmospheres: Lyα and the Origin of Hot H2 Emission

FUV Irradiated Disk Atmospheres: Lyα and the Origin of Hot H2 Emission

Authors:

Ádámkovics et al

Abstract:

Protoplanetary disks are strongly irradiated by a stellar FUV spectrum that is dominated by Lyα photons. We investigate the impact of stellar Lyα irradiation on the terrestrial planet region of disks (≲1AU) using an updated thermal-chemical model of a disk atmosphere irradiated by stellar FUV and X-rays. The radiative transfer of Lyα is implemented in a simple approach that includes scattering by H I and absorption by molecules and dust. Because of their non-radial propagation path, scattered Lyα photons deposit their energy deeper in the disk atmosphere than the radially propagating FUV continuum photons. We find that Lyα has a significant impact on the thermal structure of the atmosphere. Photochemical heating produced by scattered Lyα photons interacting with water vapor and OH leads to a layer of hot (1500 - 2500 K) molecular gas. The temperature in the layer is high enough to thermally excite the H2 to vibrational levels from which they can be fluoresced by Lyα to produce UV fluorescent H2 emission. The resulting atmospheric structure may help explain the origin of UV fluorescent H2 that is commonly observed from classical T Tauri stars.