THE PHYSICS AND CHEMISTRY OF SMALL TRANSLUCENT MOLECULAR CLOUDS - XI - METHANOL

We have made a survey of CH3OH in 27 of our standard sample of 11 cirr us cores and 27 Clemens-Barvainis translucent cores whose structures a nd chemistry have been studied earlier in this series. CH3OH is detect ed in 17 objects, favoring those with larger extinctions. The mean fra ctional abundance is 1(-8), but if the four highest abundance objects are omitted, the mean abundance is 3(-9), the same as in two cold dark clouds. Collision rates remain poorly known for CH3OH, but uncertaint ies in propensity rules are shown not to affect abundances more than 1 0%. The ''geometric'' component of the rates is uncertain by a factor of 2; hence, also, the abundances. The gas-phase chemistry is particul arly simple, formation occurring only via the radiative association re action CH3+ + H2O --> CH3OH2+ + hv followed by electron recombination. We have verified the predictions of this simple model by using the fu ll Standard Model of over 3000 reactions, with conditions suitable for translucent clouds. These gasphase models predict abundances 4 orders of magnitude less than the observed abundances. We have examined grai n surface chemistry in which accreted CO hydrogenates to CH3OH on the surface under the action of UV or cosmic rays and then desorbs in vari ous ways, photodesorption dominating. Despite the uncertainties of the grain processes, they can easily explain the observed abundances and in fact imply much lower desorption efficiencies than are usually adop ted. Methanol is of ''intermediate'' complexity, as are several other species we will study in the next papers, with the goal of testing the boundary between gas-phase and grain chemistry, the latter believed t o be important for the most complex species.