EDP Sciences
Free access
Issue
A&A
Volume 391, Number 2, August IV 2002
Page(s) 675 - 680
Section Diffuse matter in space
DOI http://dx.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 )

Abstract
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

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access.

An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account.
    In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.