Molecular Hydrogen Emission from Protoplanetary Disks. II. Effects of X‐Ray Irradiation and Dust Evolution

Abstract

Detailed models for the density and temperature profiles of gas and dust in\nprotoplanetary disks are constructed by taking into account X-ray and\nultraviolet (UV) irradiation from a central T Tauri star, as well as dust size\ngrowth and settling toward the disk midplane. The spatial and size\ndistributions of dust grains in the disks are numerically computed by solving\nthe coagulation equation for settling dust particles. The level populations and\nline emission of molecular hydrogen are calculated using the derived physical\nstructure of the disks. X-ray irradiation is the dominant heating source of the\ngas in the inner disk region and in the surface layer, while the far UV heating\ndominates otherwise. If the central star has strong X-ray and weak UV\nradiation, the H2 level populations are controlled by X-ray pumping, and the\nX-ray induced transition lines could be observable. If the UV irradiation is\nstrong, the level populations are controlled by thermal collisions or UV\npumping, depending on the properties of the dust grains in the disks. As the\ndust particles evolve in the disks, the gas temperature at the disk surface\ndrops because the grain photoelectric heating becomes less efficient, while the\nUV radiation fields become stronger due to the decrease of grain opacity. This\nmakes the H2 level populations change from local thermodynamic equilibrium\n(LTE) to non-LTE distributions, which results in changes to the line ratios of\nH2 emission. Our results suggest that dust evolution in protoplanetary disks\ncould be observable through the H2 line ratios. The emission lines are strong\nfrom disks irradiated by strong UV and X-rays and possessing small dust grains;\nsuch disks will be good targets in which to observe H2 emission.\n