H2 Rovibrational Excitation in Protoplanetary Disks and Its Effects on the Chemistry

The effect of H-2 rovibrational excitation on the chemistry of protoplanetary disks is studied using a framework that solves for the disk physical and chemical structure and includes a detailed calculation of H-2 level populations. Chemistry with rovibrationally excited H-2 is found to be important for the formation of several commonly observed species in disks and this work demonstrates the need to accurately treat photon-dominated-region chemistry in disks if we are to make inferences on the chemical state of the disk during planet formation epochs. This is found to be even more critical for molecules like C2H, CN, or HCN that are commonly used to infer changes in the elemental disk C/O and N/O ratios, with implications for planetesimal formation and the composition of exoplanet atmospheres. Computed vertical column densities with the full H-2 population calculation are increased by similar to 1-2 orders of magnitude for molecules such as CN and HCN/HNC compared to calculations with no treatment of excited H-2. For the commonly used pseudo-level approximation, the computed columns of these molecules are overestimated by a factor of similar to 3-5 when compared to the full model. We further note that the computed abundance for these molecules strongly depends on the strength of the FUV photons at energies that pump H-2 (i.e., 11-13.6 eV), which is not well constrained in disks, and that rate constants as a function of H-2 rovibrational levels for the key reaction N + H-2 -> NH are needed for a more accurate assessment of CN/HCN chemistry but are currently unavailable.