2002A&A...391..675G

We use far-UV absorption spectra obtained with FUSE towards three late B stars to study the formation and excitation of H₂ in the diffuse ISM. The data interpretation relies on a model of the chemical and thermal balance in photon-illuminated gas. The data constrain well the n R product between gas density and H₂ formation rate on dust grains: nR=1 to 2.2x10^–15s^–1. For each line of sight the mean effective H₂ density n, assumed uniform, is obtained by the best fit of the model to the observed N(J=1)/N(J=0) ratio, since the radiation field is known. Combining n with the n R values, we find similar H₂ formation rates for the three stars of about R=4x10^–17cm³/s. Because the target stars do not interact with the absorbing matter we can show that the H₂ excitation in the J>2 levels cannot be accounted for by the UV pumping of the cold H₂ but implies collisional excitation in regions where the gas is much warmer. The existence of warm H₂ is corroborated by the fact that the star with the largest column density of CH⁺ has the largest amount of warm H₂.