Modeling of diffuse molecular gas applied to HD 102065 observations

¹ LUTH, UMR 8102, CNRS, Université Paris 7 and Observatoire de Paris, Place J. Janssen, 92195 Meudon, France e-mail: Jacques.Lebourlot@obspm.fr
² Institut d'Astrophysique Spatiale, UMR 8617, CNRS, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
³ LERMA, UMR 8112, CNRS, Observatoire de Paris, France
⁴ Laboratoire d'Astrophysique de Marseille, UMR 6110, CNRS, Université de Provence, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁵ European Space Astronomy Center, RSSD, PO Box 50727, 28080 Madrid, Spain

Received: 28 July 2007
Accepted: 23 January 2008

Abstract

Aims. We model a diffuse molecular cloud present along the line of sight to the star HD 102065. We compare our modeling with observations to test our understanding of physical conditions and chemistry in diffuse molecular clouds.

Methods. We analyze an extensive set of spectroscopic observations which characterize the diffuse molecular cloud observed toward HD 102065. Absorption observations provide the extinction curve, H₂, C I, CO, CH, and CH⁺ column densities and excitation. These data are complemented by observations of C⁺, CO and dust emission. Physical conditions are determined using the Meudon PDR model of UV illuminated gas.

Results. We find that all observational results, except column densities of CH, CH⁺ and H₂ in its excited (J ≥ 2) levels, are consistent with a cloud model implying a Galactic radiation field (G ~0.4 in Draine's unit), a density of 80 cm^-3 and a temperature (60-80 K) set by the equilibrium between heating and cooling processes. To account for excited (J ≥ 2) H₂ levels column densities, an additional component of warm (~250 K) and dense (n_H ≥ 10⁴ cm^-3) gas within 0.03 pc of the star would be required. This solution reproduces the observations only if the ortho-to-para H₂ ratio at formation is ~1. In view of the extreme physical conditions and the unsupported requirement on the ortho-to-para ratio, we conclude that H₂ excitation is most likely to be accounted for by the presence of warm molecular gas within the diffuse cloud heated by the local dissipation of turbulent kinetic energy. This warm H₂ is required to account for the CH⁺ column density. It could also contribute to the CH abundance and explain the inhomogeneity of the CO abundance indicated by the comparison of absorption and emission spectra.