2017ApJ...846....6H

The environment around protoplanetary disks (PPDs) regulates processes that drive the chemical and structural evolution of circumstellar material. We perform a detailed empirical survey of warm molecular hydrogen (H₂) absorption observed against H I-Lyα (Lyα: λ1215.67) emission profiles for 22 PPDs, using archival Hubble Space Telescope ultraviolet (UV) spectra to identify H₂ absorption signatures and quantify the column densities of H₂ ground states in each sightline. We compare thermal equilibrium models of H₂ to the observed H₂ rovibrational level distributions. We find that, for the majority of targets, there is a clear deviation in high-energy states (T_exc >= 20,000 K) away from thermal equilibrium populations (T(H₂) >= 3500 K). We create a metric to estimate the total column density of non-thermal H₂ (N(H₂)_nLTE) and find that the total column densities of thermal (N(H₂)) and N(H₂)_nLTE correlate for transition disks and targets with detectable C IV-pumped H₂ fluorescence. We compare N(H₂) and N(H₂)_nLTE to circumstellar observables and find that N(H₂)_nLTE correlates with X-ray and far-UV luminosities, but no correlations are observed with the luminosities of discrete emission features (e.g., Lyα, C IV). Additionally, N(H₂) and N(H₂)_nLTE are too low to account for the H₂ fluorescence observed in PPDs, so we speculate that this H₂ may instead be associated with a diffuse, hot, atomic halo surrounding the planet-forming disk. We create a simple photon-pumping model for each target to test this hypothesis and find that Lyα efficiently pumps H₂ levels with T_exc >= 10,000 K out of thermal equilibrium.