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Aims. We model a diffuse molecular cloud present along the line of sight tothe star HD 102065. We compare our modeling with observations to test ourunderstanding of physical conditions and chemistry in diffuse molecular clouds.Methods. We analyze an extensive set of spectroscopic observations whichcharacterize the diffuse molecular cloud observed toward HD 102065. Absorptionobservations provide the extinction curve, H2, C I, CO, CH, and CH+ columndensities and excitation. These data are complemented by observations of CII,CO and dust emission. Physical conditions are determined using the Meudon PDRmodel 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, areconsistent with a cloud model implying a Galactic radiation field (G~0.4 inDraine's unit), a density of 80 cm-3 and a temperature (60-80 K) set by theequilibrium between heating and cooling processes. To account for excited (J>2) H2 levels column densities, an additional component of warm (~ 250K) anddense (nH>10^4 cm-3) gas within 0.03 pc of the star would be required. Thissolution reproduces the observations only if the ortho-to-para H2 ratio atformation is 1. In view of the extreme physical conditions and the unsupportedrequirement on the ortho-to-para ratio, we conclude that H2 excitation is mostlikely to be accounted for by the presence of warm molecular gas within thediffuse cloud heated by the local dissipation of turbulent kinetic energy. Thiswarm H2 is required to account for the CH+ column density. It could alsocontribute to the CH abundance and explain the inhomogeneity of the COabundance indicated by the comparison of absorption and emission spectra.

Modeling of diffuse molecular gas applied to HD 102065 observations