A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Galactic Disk

Abstract

We report results from a FUSE survey of interstellar molecular hydrogen (H2) in the Galactic disk toward 139 O-type and early B-type stars at Galactic latitudes $|b|<10^{\circ }$, with updated photometric and parallax distances. The H2 absorption is measured using the far-ultraviolet Lyman and Werner bands, including strong R(0), R(1), and P(1) lines from rotational levels $J=0$ and $J=1$ and excited states up to $J=5$ (sometimes $J=6$ and 7). For each sight line, we report column densities $N_{H2}$, $N_{HI}$, $N(J)$, $N_H=N_{HI}+2N_{H2}$, and molecular fraction, $f_{H2}=2N_{H2}/N_H$. Our survey extends the 1977 Copernicus H2 survey up to $N_H\sim 5\times 10^{21}$ cm${}^{-2}$. The lowest rotational states have mean excitation temperatures and rms dispersions, $T_{01}=88\pm 20$ K and $T_{02}=77\pm 18$ K, suggesting that J = 0,1,2 are coupled to the gas kinetic temperature. Populations of higher-J states exhibit mean excitation temperatures, $T_{24}=237\pm 91$ K and $T_{35}=304\pm 108$ K, produced primarily by UV radiative pumping. Correlations of $f_{H2}$ with E(B-V) and N_H show a transition to $f_{H2}\ge 0.1$ at $N_H\ge 10^{21}$ cm${}^{-2}$ and $E(B-V)>0.2$, interpreted with an analytic model of H2 formation-dissociation equilibrium and attenuation of the far-UV radiation field by self-shielding and dust opacity. Results of this disk survey are compared to previous FUSE studies of H2 in translucent clouds, at high Galactic latitudes, and in the Magellanic Clouds. Using updated distances to the target stars, we find average sight-line values $⟨f_{H2}⟩\ge 0.20$ and $⟨N_H/E(B-V)⟩=(6.07\pm 1.01)\times 10^{21}$ cm${}^{-2}$ mag${}^{-1}$.

Accepted to ApJ. 40 pages, 11 figures, 5 tables (four will be machine readable in journal). Revised text in Sections 3.4 and 3.5

References in corpus (16)

• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• HI-to-H2 Transitions and H I Column Densities in Galaxy Star-Forming Regions
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• Molecular hydrogen in high-redshift Damped Lyman-alpha systems: The VLT/UVES database
• Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
• A new, large-scale map of interstellar reddening derived from HI emission
• The Far Ultraviolet Spectroscopic Explorer Survey of OVI Absorption in the Disk of the Milky Way
• Destruction of Interstellar Dust in Evolving Supernova Remnant Shock Waves
• X-shooter observations of strong H${}_2$-bearing DLAs at high redshift
• The Magellan Uniform Survey of Damped Lyman alpha Systems II: Paucity of Strong Molecular Hydrogen Absorption
• Observational Properties of Rotationally Excited Molecular Hydrogen In Translucent Lines of Sight
• Modeling of diffuse molecular gas applied to HD 102065 observations
• Physical conditions in diffuse interstellar medium of local and high redshift galaxies: measurements based on excitation of H${}_2$ rotational and CI fine-structure levels
• Rapid ortho-to-para nuclear spin conversion of H2 on a silicate dust surface
• Revisiting the Temperature of the Diffuse ISM with CHESS Sounding Rocket Observations

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