All Tables

Table A.1: Reactions and rate coefficients adopted in our chemical model. The parameters $\alpha$ , $\beta$ , and $\gamma$ define the rate coefficients k cm³ s^-1 at temperature T through the relation k = J $(a_{\rm g}, T)\gamma (T/300)^{\alpha } {\rm exp}(-\beta /T)$ ; J allows for Coulomb focusing in reactions of positive ions and negatively charged grains (Draine & Sutin 1987, Eq. (3.4)); $a_{\rm g}$ = 0.1 $\mu$ m is adopted in the table. The rates s^-1 of reactions induced directly by cosmic rays (crp) or indirectly by the secondary electrons, which generate H₂fluorescence photons (secpho), are given by $\gamma \zeta$ , where $\zeta$ is the rate of cosmic ray ionization of H₂. In the first three reactions (formation of H₂ or HD on grain surfaces), $\gamma$ denotes the fraction of such reactions which form the specified product; the rate is calculated internally in the program from the grain parameters. Key reactions are in bold face. Numbers in parentheses are powers of 10.