Volume 391, Number 2, August IV 2002
|Page(s)||675 - 680|
|Published online||02 August 2002|
H2 formation and excitation in the diffuse interstellar medium
ISO Data Center, ESA Research and Scientific Support Department, PO Box 50727, 28080 Madrid, Spain
2 Laboratoire d'Astrophysique de Marseille, BP 8, 13376 Marseille Cedex 12, France
3 Institut d'Astrophysique Spatiale, Université Paris Sud, Bat. 121, 91405 Orsay Cedex, France
4 Ecole Normale Superieure, Laboratoire de Radioastronomie, 24 rue Lhomond, 75231 Paris Cedex 05, France
Corresponding author: C. Gry, firstname.lastname@example.org
Accepted: 6 May 2002
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 product between gas density and H2 formation rate on dust grains: = 1 to . 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 ratio, since the radiation field is known. Combining n with the values, we find similar H2 formation rates for the three stars of about cm3 s-1. Because the target stars do not interact with the absorbing matter we can show that the H2 excitation in the 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.
Key words: ISM: molecules / ISM: clouds / ISM: lines and bands / ISM: individual objects: chamaeleon / ultraviolet: ISM / stars: individual: HD 102065, HD 108927, HD 96675
© ESO, 2002
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