A Far Ultraviolet Spectroscopic Explorer survey of interstellar molecular hydrogen in translucent clouds

We report the first ensemble results from the Far Ultraviolet Spectroscopic Explorer survey of molecular hydrogen in lines of sight with A(V) greater than or similar to 1 mag. We have developed techniques for fitting computed proles to the low-J lines of H-2, and thus determining column densities for J = 0 and J = 1, which contain greater than or similar to99% of the total H-2. From these column densities and ancillary data we have derived the total H-2 column densities, hydrogen molecular fractions, and kinetic temperatures for 23 lines of sight. This is the first significant sample of molecular hydrogen column densities of similar to10(21) cm(-2), measured through UV absorption bands. We have also compiled a set of extinction data for these lines of sight, which sample a wide range of environments. We have searched for correlations of our H-2-related quantities with previously published column densities of other molecules and extinction parameters. We find strong correlations between H-2 and molecules such as CH, CN, and CO, in general agreement with predictions of chemical models. We also find the expected correlations between hydrogen molecular fraction and various density indicators such as kinetic temperature, CN abundance, the steepness of the far-UV extinction rise, and the width of the 2175 Angstrom bump. Despite the relatively large molecular fractions, we do not see the values greater than 0.8 expected in translucent clouds. With the exception of a few lines of sight, we see little evidence for the presence of individual translucent clouds in our sample. We conclude that most of the lines of sight are actually composed of two or more diffuse clouds similar to those found toward targets like zeta Oph. We suggest a modi cation in terminology to distinguish between a translucent line of sight and a translucent cloud.