Far Ultraviolet Spectroscopic Explorer observations of diffuse interstellar molecular hydrogen

We describe a moderate-resolution Far Ultraviolet Spectroscopic Explorer (F USE) minisurvey of H-2 in the Milky Way and Magellanic Clouds using four ho t stars and four active galactic nuclei as background sources. FUSE spectra of nearly every stellar and extragalactic source exhibit numerous absorpti on 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 detec table H-2 and could be the "Lockman Hole of molecular gas," of importance f or QSO absorption-line studies. We measure H-2 column densities in low rota tional states (J = O and 1) to derive rotational and/or kinetic temperature s of diffuse interstellar gas. The higher J abundances can constrain models of the UV radiation fields and gas densities. In three optically thick clo uds 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 C opernicus sample composed primarily of targets in the disk. We find no evid ence for great differences in the abundance or state of excitation of H-2 b etween sight lines in the Galaxy and those in the SMC and LMC. In the futur e, we will probe the distribution and physical parameters of diffuse molecu lar gas in the disk and halo and in the lower metallicity environs of the L MC and SMC.