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

Detailed models for the density and temperature profiles of gas and dust inprotoplanetary disks are constructed by taking into account X-ray andultraviolet (UV) irradiation from a central T Tauri star, as well as dust sizegrowth and settling toward the disk midplane. The spatial and sizedistributions of dust grains in the disks are numerically computed by solvingthe coagulation equation for settling dust particles. The level populations andline emission of molecular hydrogen are calculated using the derived physicalstructure of the disks. X-ray irradiation is the dominant heating source of thegas in the inner disk region and in the surface layer, while the far UV heatingdominates otherwise. If the central star has strong X-ray and weak UVradiation, the H2 level populations are controlled by X-ray pumping, and theX-ray induced transition lines could be observable. If the UV irradiation isstrong, the level populations are controlled by thermal collisions or UVpumping, depending on the properties of the dust grains in the disks. As thedust particles evolve in the disks, the gas temperature at the disk surfacedrops because the grain photoelectric heating becomes less efficient, while theUV radiation fields become stronger due to the decrease of grain opacity. Thismakes the H2 level populations change from local thermodynamic equilibrium(LTE) to non-LTE distributions, which results in changes to the line ratios ofH2 emission. Our results suggest that dust evolution in protoplanetary diskscould be observable through the H2 line ratios. The emission lines are strongfrom disks irradiated by strong UV and X-rays and possessing small dust grains;such disks will be good targets in which to observe H2 emission.