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

TL;DR

This FUSE survey of interstellar molecular hydrogen in the Galactic disk provides detailed measurements of H2 column densities, excitation temperatures, and molecular fractions toward 139 stars, revealing the transition to molecular dominance at certain column densities.

Contribution

The study offers the first extensive FUSE-based survey of H2 in the Galactic disk with updated distances, extending previous work and analyzing excitation states and molecular fractions in relation to dust and UV radiation.

Findings

H2 excitation temperatures suggest thermal coupling for low-J states.

Higher-J states are mainly excited by UV radiative pumping.

Molecular fraction increases significantly at N_H ≥ 10^21 cm^-2.

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 $N_H \sim 5\times10^{21}$ cm$^{-2}$. The lowest rotational states have mean excitation temperatures and rms dispersions, $T_{01} = 88\pm 20$ K and $T_{02} = 77\pm18$ K, suggesting that J = 0,1,2 are coupled to the gas kinetic temperature. Populations of higher-J states exhibit mean excitation temperatures, $T_{24} = 237\pm91$ K and $T_{35} = 304\pm108$ K, produced primarily by UV radiative pumping. Correlations of $f_{H2}$ with E(B-V) and N_H show a transition to $f_{H2} \geq 0.1$ at $N_ H \geq 10^{21}$ 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 $\langle f_{H2} \rangle \geq 0.20$ and $\langle N_H/E(B-V) \rangle = (6.07\pm1.01)\times10^{21}$ cm$^{-2}$ mag$^{-1}$.