The Physics and Chemistry of Small Translucent Molecular Clouds. XI. Methanol

Abstract

We have made a survey of CH3OH in 27 of our standard sample of 11 cirrus cores and 27 Clemens-Barvainis translucent cores whose structures and chemistry have been studied earlier in this series. CH3OH is detected in 17 objects, favoring those with larger extinctions. The mean fractional 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 uncertainties in propensity rules are shown not to affect abundances more than 10%. The "geometric" component of the rates is uncertain by a factor of 2; hence, also, the abundances. The gas-phase chemistry is particularly simple, formation occurring only via the radiative association reaction {CH}^{+}3+{H}2{O}-->{CH}3{OH}^{+}2+h upsilon followed by electron recombination. We have verified the predictions of this simple model by using the full Standard Model of over 3000 reactions, with conditions suitable for translucent clouds. These gas-phase models predict abundances 4 orders of magnitude less than the observed abundances. We have examined grain surface chemistry in which accreted CO hydrogenates to CH3OH on the surface under the action of UV or cosmic rays and then desorbs in various 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 adopted. 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 to be important for the most complex species.