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Appendix A: CH column densities

Table A.1:   CH column densities.

Appendix B: PDR models

Figure B.1 presents models produced using the Meudon PDR code, a steady-state, depth-dependent model of a plane-parallel cloud (Le Petit et al. 2006; Goicoechea & Le Bourlot 2007; Gonzalez Garcia et al. 2008). Our chemical network is based on a modified version of the Ohio State University (OSU) gas-phase network updated for photochemical studies. We have computed the thermal and chemical structure of diffuse clouds with A_V<5, -5000 cm^-3, cosmic ray ionization rates of 10^-17 s^-1, 10^-16 s^-1, and 10^-15 s^-1, and illuminated only at one side by a UV field, , between 1 and 10 times the mean interstellar radiation field (in Draine units). The [CN]/[CH], [HCO⁺]/[CH] and [C₂H]/[CH] ratios are sensitive to the gas density as suggested by Cardelli et al. (1991). For the bulk of the diffuse matter producing absorption features, the observations are consistent with gas densities between 100 and 1000 cm^-3. Figure B.1 also shows the predicted scaling between CH and H₂ column densities in the parameter space appropriate for diffuse clouds. An approximately similar trend is predicted for C₂H when A_V>0.01.

Figure B.1: Left: predicted variation of the CH (black line) and CCH (red dashed line) column density as a function of the H2 column density for three models, computed with  s-1: and  cm-3 (top), and  cm-3 (middle) and and  cm-3 (bottom). Right: Variation of the CN/CH (black lines) and CCH/CH (red dashed lines) column density ratios for the same models. The gas density is defined as n(H) +2n(H2). The observed ranges are indicated.

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