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We use far-UV absorption spectra obtained with FUSE towards three late Bstars to study the formation and excitation of H2 in the diffuse ISM. The datainterpretation relies on a model of the chemical and thermal balance inphoton-illuminated gas. The data constrain well the nR product between gasdensity and H2 formation rate on dust grains: nR = 1 to 2.2 e-15 s-1. For eachline of sight the mean effective H2 density n, assumed uniform, is obtained bythe best fit of the model to the observed N(J=1)/N(J=0) ratio, since theradiation field is known. Combining n with the nR values, we find similar H2formation rates for the three stars of about R = 4 e-17 cm3/s.  Because the target stars do not interact with the absorbing matter we canshow that the H2 excitation in the J>2 levels cannot be accounted for by the UVpumping of the cold H2 but implies collisional excitation in regions where thegas is much warmer. The existence of warm H2 is corroborated by the fact thatthe star with the largest column density of CH+ has the largest amount of warmH2.Comment: 7 pages, 4 figures, to be published in Astronomy & Astrophysic

H2 formation and excitation in the diffuse interstellar medium

H₂ formation and excitation in the diffuse interstellar medium