Volume 527, March 2011
|Number of page(s)||16|
|Section||Interstellar and circumstellar matter|
|Published online||08 February 2011|
Excitation of H2 in photodissociation regions as seen by Spitzer⋆
Institut d’Astrophysique Spatiale (IAS), Université Paris-Sud,
2 Université de Toulouse, UPS, CESR, 9 Avenue du colonel Roche, 31028 Toulouse Cedex 9, France
3 CNRS, UMR5187, 31028 Toulouse, France
4 Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
5 LUTH, Observatoire de Paris-Meudon, Université Paris 7, France
Accepted: 9 November 2010
Aims. We present spectroscopic observations obtained with the infrared Spitzer Space Telescope, which provide insight into the H2 physics and gas energetics in photodissociation regions (PDRs) of low to moderate far-ultraviolet (FUV) fields and densities.
Methods. We analyze data on six well known Galactic PDRs (L1721, California, N7023E, Horsehead, rho Oph, N2023N), sampling a poorly explored range of excitation conditions (χ ~ 5−103), relevant to the bulk of molecular clouds in galaxies. Spitzer observations of H2 rotational lines are complemented with H2 data, including ro-vibrational line measurements, obtained with ground-based telescopes and ISO, to constrain the relative contributions of ultraviolet pumping and collisions to the H2 excitation. The data analysis is supported by model calculations with the Meudon PDR code.
Results. The observed column densities of rotationally excited H2 are observed to be much higher than PDR model predictions. In the lowest excitation PDRs, the discrepancy between the model and the data is about one order of magnitude for rotational levels J ≥ 3. We discuss whether an enhancement in the H2 formation rate or a local increase in photoelectric heating, as proposed for brighter PDRs in former ISO studies, may improve the data-model comparison. We find that an enhancement in the H2 formation rates reduces the discrepancy, but the models still fall short of the data.
Conclusions. This large disagreement suggests that our understanding of the formation and excitation of H2 and/or of PDRs energetics is still incomplete. We discuss several explanations, which could be further tested using the Herschel Space Telescope.
Key words: ISM: molecules / ISM: general / ISM: lines and bands / ISM: clouds / evolution / infrared: ISM
Appendix is only available in electronic form at http://www.aanda.org
© ESO, 2011
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