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 n R product between gas density and H-2 formation rate on dust grains: n R = 1 to 2.2 +/- 10(15) s(-1). 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 N(J = 1)/N(J = 0) ratio, since the radiation field is known. Combining n with the n R values, we find similar H-2 formation rates for the three stars of about R = 4 x 10(17) 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 J > 2 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.