H₂ 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 H 2 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 product between gas density and H 2 formation rate on dust grains: = 1 to . For each line of sight the mean effective H 2 density n , assumed uniform, is obtained by the best fit of the model to the observed ratio, since the radiation field is known. Combining n with the values, we find similar H 2 formation rates for the three stars of about cm 3 s -1 . Because the target stars do not interact with the absorbing matter we can show that the H 2 excitation in the levels cannot be accounted for by the UV pumping of the cold H 2 but implies collisional excitation in regions where the gas is much warmer. The existence of warm H 2 is corroborated by the fact that the star with the largest column density of CH + has the largest amount of warm H 2 .