Observational Properties of Rotationally Excited Molecular Hydrogen In Translucent Lines of Sight

Abstract

The {\it Far Ultraviolet Spectroscopic Explorer} ({\it FUSE}) has allowed precise determinations of the column densities of molecular hydrogen ($\Hmol$) in Galactic lines of sight with a wide range of pathlengths and extinction properties. However, survey studies of lines of sight with greater extinction have been mostly restricted to the low-$J$ states (lower total angular momentum) in which most molecular hydrogen is observed. This paper presents a survey of column densities for the molecular hydrogen in states of greater rotational excitation ($J\ge 2$) in Galactic lines of sight with $\log{\NHmol} \gtrsim 20$. This study is comprehensive through the highest excited state detectable in each line of sight. J=5 is observed in every line of sight, and we detect J=7 in four lines of sight, J=8 in one line of sight, and vibrationally excited $\Hmol$ in two lines of sight. We compared the apparent $b$-values and velocity offsets of the higher-$J$ states relative to the dominant low-$J$ states and we found no evidence of any trends that might provide insight into the formation of higher-$J$ $\Hmol$, although these results are the most affected by the limits of the {\it FUSE} resolution. We also derive excitation temperatures based on the column densities of the different states. We confirm that at least two distinct temperatures are necessary to adequately describe these lines of sight, and that more temperatures are probably necessary. Total $\Hmol$ column density is known to be correlated with other molecules; we explore if correlations vary as a function of $J$ for several molecules, most importantly CH and CH${}^{+}$. Finally, we briefly discuss interpretations of selected lines of sight by comparing them to models computed using the Meudon PDR code.

58 total preprint pages (33 text & references), 10 figures (6 B&W, 4 color), 12 tables

References in corpus (5)

• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
• Excitation mechanisms in newly discovered H2-bearing Damped Lyman-alpha clouds: systems with low molecular fractions
• Directional Radiation and Photodissociation Regions in Molecular Hydrogen Clouds
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• Interstellar Extinction and Elemental Abundances: Individual Sight Lines
• The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus
• Broadened profiles of Diffuse Interstellar Bands
• Interstellar Bow Shocks around Fast Stars Passing through the Local Interstellar Medium
• Vibrationally Excited Molecular Hydrogen Near Herschel 36