Far Ultraviolet Spectroscopic Explorer observations of diffuse interstellar molecular hydrogen

We describe a moderate-resolution Far Ultraviolet Spectroscopic Explorer (FUSE) minisurvey of H-2 in the Milky Way and Magellanic Clouds using four hot stars and four active galactic nuclei as background sources. FUSE spectra of nearly every stellar and extragalactic source exhibit numerous absorption lines from the H-2 Lyman and Werner bands between 912 and 1120 Angstrom. One extragalactic sight line (PKS 2155-304) with low N(H I) shows no detectable H-2 and could be the Lockman Hole of molecular gas, of importance for QSO absorption-line studies. We measure H-2 column densities in low rotational states (J = O and 1) to derive rotational and/or kinetic temperatures of diffuse interstellar gas. The higher J abundances can constrain models of the UV radiation fields and gas densities. In three optically thick clouds toward extragalactic sources, we find n(H) approximate to 30-50 cm(-3) and cloud thicknesses similar to 2-3 pc. The rotational temperatures for H-2 at high Galactic latitude, (T-01) = 107 +/- 17 K (seven sight lines) and 120 +/- 13 K (three optically thick clouds), are higher than those in the Copernicus sample composed primarily of targets in the disk. We find no evidence for great differences in the abundance or state of excitation of H-2 between sight lines in the Galaxy and those in the SMC and LMC. In the future, we will probe the distribution and physical parameters of diffuse molecular gas in the disk and halo and in the lower metallicity environs of the LMC and SMC.