H 2 formation and excitation in the diffuse interstellar medium

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: n R = 1 to 2.2 10s. 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 = 410cms. 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.