skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

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

Abstract

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 (H{sub 2}) absorption observed against H i-Ly α (Ly α : λ 1215.67) emission profiles for 22 PPDs, using archival Hubble Space Telescope ultraviolet (UV) spectra to identify H{sub 2} absorption signatures and quantify the column densities of H{sub 2} ground states in each sightline. We compare thermal equilibrium models of H{sub 2} to the observed H{sub 2} rovibrational level distributions. We find that, for the majority of targets, there is a clear deviation in high-energy states ( T {sub exc} ≳ 20,000 K) away from thermal equilibrium populations ( T (H{sub 2}) ≳ 3500 K). We create a metric to estimate the total column density of non-thermal H{sub 2} ( N (H{sub 2}){sub nLTE}) and find that the total column densities of thermal ( N (H{sub 2})) and N (H{sub 2}){sub nLTE} correlate for transition disks and targets with detectable C iv-pumped H{sub 2} fluorescence. We compare N (H{sub 2}) and N (H{sub 2}){sub nLTE} to circumstellar observables and find that N (H{sub 2}){sub nLTE} correlates with X-ray and far-UV luminosities,more » but no correlations are observed with the luminosities of discrete emission features (e.g., Ly α , C iv). Additionally, N (H{sub 2}) and N (H{sub 2}){sub nLTE} are too low to account for the H{sub 2} fluorescence observed in PPDs, so we speculate that this H{sub 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{sub 2} levels with T {sub exc} ≥ 10,000 K out of thermal equilibrium.« less

Authors:
; ; ; ;  [1]
  1. Laboratory for Atmospheric and Space Physics (LASP), University of Colorado Space Science Building (SPSC), 3665 Discovery Drive, Boulder, CO 80303 (United States)
Publication Date:
OSTI Identifier:
22679886
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 846; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; AUGER ELECTRON SPECTROSCOPY; COMPARATIVE EVALUATIONS; CORRELATIONS; COSMIC DUST; DENSITY; DISTRIBUTION; FLUORESCENCE; HYDROGEN; LUMINOSITY; LYMAN LINES; PLANETS; PROTOPLANETS; SPACE; STARS; TELESCOPES; THERMAL EQUILIBRIUM; ULTRAVIOLET RADIATION; ULTRAVIOLET SPECTRA; X RADIATION

Citation Formats

Hoadley, Keri, France, Kevin, Arulanantham, Nicole, Loyd, R. O. Parke, and Kruczek, Nicholas, E-mail: keri.hoadley@colorado.edu. Signatures of Hot Molecular Hydrogen Absorption from Protoplanetary Disks. I. Non-thermal Populations. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA7FC1.
Hoadley, Keri, France, Kevin, Arulanantham, Nicole, Loyd, R. O. Parke, & Kruczek, Nicholas, E-mail: keri.hoadley@colorado.edu. Signatures of Hot Molecular Hydrogen Absorption from Protoplanetary Disks. I. Non-thermal Populations. United States. doi:10.3847/1538-4357/AA7FC1.
Hoadley, Keri, France, Kevin, Arulanantham, Nicole, Loyd, R. O. Parke, and Kruczek, Nicholas, E-mail: keri.hoadley@colorado.edu. Fri . "Signatures of Hot Molecular Hydrogen Absorption from Protoplanetary Disks. I. Non-thermal Populations". United States. doi:10.3847/1538-4357/AA7FC1.
@article{osti_22679886,
title = {Signatures of Hot Molecular Hydrogen Absorption from Protoplanetary Disks. I. Non-thermal Populations},
author = {Hoadley, Keri and France, Kevin and Arulanantham, Nicole and Loyd, R. O. Parke and Kruczek, Nicholas, E-mail: keri.hoadley@colorado.edu},
abstractNote = {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 (H{sub 2}) absorption observed against H i-Ly α (Ly α : λ 1215.67) emission profiles for 22 PPDs, using archival Hubble Space Telescope ultraviolet (UV) spectra to identify H{sub 2} absorption signatures and quantify the column densities of H{sub 2} ground states in each sightline. We compare thermal equilibrium models of H{sub 2} to the observed H{sub 2} rovibrational level distributions. We find that, for the majority of targets, there is a clear deviation in high-energy states ( T {sub exc} ≳ 20,000 K) away from thermal equilibrium populations ( T (H{sub 2}) ≳ 3500 K). We create a metric to estimate the total column density of non-thermal H{sub 2} ( N (H{sub 2}){sub nLTE}) and find that the total column densities of thermal ( N (H{sub 2})) and N (H{sub 2}){sub nLTE} correlate for transition disks and targets with detectable C iv-pumped H{sub 2} fluorescence. We compare N (H{sub 2}) and N (H{sub 2}){sub nLTE} to circumstellar observables and find that N (H{sub 2}){sub nLTE} correlates with X-ray and far-UV luminosities, but no correlations are observed with the luminosities of discrete emission features (e.g., Ly α , C iv). Additionally, N (H{sub 2}) and N (H{sub 2}){sub nLTE} are too low to account for the H{sub 2} fluorescence observed in PPDs, so we speculate that this H{sub 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{sub 2} levels with T {sub exc} ≥ 10,000 K out of thermal equilibrium.},
doi = {10.3847/1538-4357/AA7FC1},
journal = {Astrophysical Journal},
issn = {0004-637X},
number = 1,
volume = 846,
place = {United States},
year = {2017},
month = {9}
}