2007ApJ...655..940L

We have calculated synthetic interstellar cloud models to investigate the formation and destruction of high-J molecular hydrogen in photodissociation regions. The effects of five physical parameters (the incident ultraviolet [UV] intensity, H₂column density, cloud temperature, total density, and H₂formation rate coefficient) on the populations of H₂rotational levels are explored. We have found that N(4)/N(0) is proportional to the incident UV intensity I_UVand the H₂molecular fraction f is simply related to the ratio of I_UVand the hydrogen density n_H, implying a new method to derive I_UVand n_Hwith the observational parameters N(4)/N(0) and f, assuming an H₂formation rate R. High-resolution FUSE spectra of H₂toward three translucent sight lines (HD 110432, HD 192639, and HD 185418) in the Milky Way and 24 diffuse sight lines in the SMC and LMC are referenced to obtain N(4)/N(0) and f. Using our method, we are able to derive I_UV∼10-40 for the translucent sight lines, when R is assumed to be 10 times higher than the Galactic value. Synthetic models for the Magellanic sight lines suggest that I_UV≥10, n_H≤100/cm3, and R∼3x10^–16 for the low H₂column density sight lines (N[H₂]≤10¹⁸/cm2) and I_UV≥10, n_H≤1000/cm3, and R∼3x10^–17 for the high H₂column density sight lines (N[H₂]≥10¹⁹/cm2).