Inferring physical conditions in interstellar clouds of H_2
  We have developed a code that models the formation, destruction, radiative transfer, and vibrational/rotational excitation of H_2 in a detailed fashion. We discuss how such codes, together with FUSE observations of H_2 in diffuse and translucent lines of sight, may be used to infer various physical parameters. We illustrate the effects of changes in the major physical parameters (UV radiation field, gas density, metallicity), and we point out the extent to which changes in one parameter may be mirrored by changes in another. We provide an analytic formula for the molecular fraction, f_H2, as a function of cloud column density, radiation field, and grain formation rate of H_2. Some diffuse and translucent lines of sight may be concatenations of multiple distinct clouds viewed together. Such situations can give rise to observables that agree with the data, complicating the problem of uniquely identifying one set of physical parameters with a line of sight. Finally, we illustrate the application of our code to an ensemble of data, such as the FUSE survey of H_2 in the Large and Small Magellanic Clouds (LMC/SMC), in order to constrain the elevated UV radiation field intensity and reduced grain formation rate of H_2 in those low- metallicity environments. 
Matthew K. Browning, Jason Tumlinson, and J. Michael Shull (Colorado)