2008A&A...483..485N

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. 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, CI, 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. 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 80cm^–3 and a temperature (60-80K) set by the equilibrium between heating and cooling processes. To account for excited (J≥2) H₂ levels column densities, an additional component of warm (∼250K) and dense (n_H≥10⁴cm^–3) gas within 0.03pc 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.