Signatures of Hot Molecular Hydrogen Absorption from Protoplanetary Disks: I. Non-thermal Populations

Keri Hoadley; Kevin France; Nicole Arulanantham; R. O. Parke Loyd; Nicholas Kruczek

doi:10.3847/1538-4357/aa7fc1

paper

Signatures of Hot Molecular Hydrogen Absorption from Protoplanetary Disks: I. Non-thermal Populations

arXiv:1707.03449 · doi:10.3847/1538-4357/aa7fc1

Abstract

The environment around protoplanetary disks (PPDs) regulates processes which drive the chemical and structural evolution of circumstellar material. We perform a detailed empirical survey of warm molecular hydrogen (H $_{2}$ ) absorption observed against H I-Ly $α$ (Ly $α$ : $λ$ 1215.67 Å) emission profiles for 22 PPDs, using archival Hubble Space Telescope (HST) ultraviolet (UV) spectra from the Cosmic Origins Spectrograph (COS) and the Space Telescope Imaging Spectrograph (STIS) to identify H $_{2}$ absorption signatures and quantify the column densities of H $_{2}$ ground states in each sightline. We compare thermal equilibrium models of H $_{2}$ to the observed H $_{2}$ rovibrational level distributions. We find that, for the majority of targets, there is a clear deviation in high energy states (T $_{e x c}$ $≳$ 20,000 K) away from thermal equilibrium populations (T(H $_{2}$ ) $≳$ 3500 K). We create a metric to estimate the total column density of non-thermal H $_{2}$ (N(H $_{2}$ ) $_{n L T E}$ ) and find that the total column densities of thermal (N(H $_{2}$ )) and N(H $_{2}$ ) $_{n L T E}$ correlate for transition disks and targets with detectable C IV-pumped H $_{2}$ fluorescence. We compare N(H $_{2}$ ) and N(H $_{2}$ ) $_{n L T E}$ to circumstellar observables and find that N(H $_{2}$ ) $_{n L T E}$ correlates with X-ray and FUV luminosities, but no correlations are observed with the luminosities of discrete emission features (e.g., Ly $α$ , C IV). Additionally, N(H $_{2}$ ) and N(H $_{2}$ ) $_{n L T E}$ are too low to account for the H $_{2}$ fluorescence observed in PPDs, so we speculate that this H $_{2}$ 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 Ly $α$ efficiently pumps H $_{2}$ levels with T $_{e x c}$ $\geq$ 10,000 K out of thermal equilibrium.

20 pages + Appendix (7 pages), 13 figures + Appendix figure sets (4), accepted for publication to ApJ

Signatures of Hot Molecular Hydrogen Absorption from Protoplanetary Disks: I. Non-thermal Populations

Abstract

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