2021ApJ...911...55S

We report results from a Far Ultraviolet Spectrographic Explorer (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°, with updated photometric and parallax distances. H₂ absorption is measured using the far-UV 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), and N_H = N_HI + 2N_H2 and the molecular fraction f_H2 = 2N_H2/N_H. Our survey extends the 1977 Copernicus H₂ survey up to N_H ≃ 5 x 10²¹ cm^–2. The lowest rotational states have excitation temperatures and rms dispersions <T₀₁> = 88 ± 20 K and <T₀₂> = 77 ± 18 K, suggesting that J = 0, 1, 2 are coupled to the gas kinetic temperature. Populations of higher-J states exhibit mean excitation temperatures <T₂₄> = 237 ± 91 K and <T₃₅> = 304 ± 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 >= 0.1 at N_H >= 10²¹ 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> = 0.20 and <N_H/E(B - V)> = 6.07 x 10²¹ cm^–2 mag^–1.