LYα Emissions From FUV Irradiated Protoplanetary Disks

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 ${\rm{Ly}}\alpha $ photons. We investigate the impact of stellar ${\rm{Ly}}\alpha $ irradiation on the terrestrial planet region of disks (lesssim1 AU) using an updated thermal-chemical model of a disk atmosphere irradiated by stellar FUV and X-rays. The radiative transfer of ${\rm{Ly}}\alpha $ 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 ${\rm{Ly}}\alpha $ photons deposit their energy deeper in the disk atmosphere than the radially propagating FUV continuum photons. We find that ${\rm{Ly}}\alpha $ has a significant impact on the thermal structure of the atmosphere. Photochemical heating produced by scattered ${\rm{Ly}}\alpha $ 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 ${{\rm{H}}}_{2}$ to vibrational levels from which they can be fluoresced by ${\rm{Ly}}\alpha $ to produce UV fluorescent ${{\rm{H}}}_{2}$ emission. The resulting atmospheric structure may help explain the origin of UV fluorescent ${{\rm{H}}}_{2}$ that is commonly observed from classical T Tauri stars.