EDP Sciences
Free Access
Volume 391, Number 2, August IV 2002
Page(s) 675 - 680
Section Diffuse matter in space
DOI https://doi.org/10.1051/0004-6361:20020691

A&A 391, 675-680 (2002)
DOI: 10.1051/0004-6361:20020691

H $\mathsf{_2}$ formation and excitation in the diffuse interstellar medium

C. Gry1, 2, F. Boulanger3, C. Nehmé3, G. Pineau des Forêts3, E. Habart3 and E. Falgarone4

1  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

(Received 2 October 2001 / 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 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 $\rm 2.2 \times 10^{-15}\, s^{-1}$. 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 \times 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.

Key words: ISM: molecules -- ISM: clouds -- ISM: lines and bands -- ISM: individual objects: chamaeleon -- ultraviolet: ISM -- stars: individual: HD 102065, HD 108927, HD 96675

Offprint request: C. Gry, cgry@iso.vilspa.esa.es

© ESO 2002