H₂ formation and excitation in the diffuse interstellar medium

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

Corresponding author: C. Gry, cgry@iso.vilspa.esa.es

Received: 2 October 2001
Accepted: 6 May 2002

Abstract

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.