Molecular cloud chemistry and the importance of dielectronic recombination

Dielectronic recombination (DR) of singly charged ions is a reaction pathway that is commonly neglected in chemical models of molecular clouds. In this study we include state-of-the-art DR data for He⁺, C ⁺, N⁺, O⁺, Na⁺, and Mg⁺ in chemical models used to simulate dense molecular clouds, protostars, and diffuse molecular clouds. We also update the radiative recombination (RR) rate coefficients for H⁺, He⁺, C⁺, N⁺, O⁺, Na⁺, and Mg⁺ to the current state-of-the-art values. The new RR data have little effect on the models. However, the inclusion of DR results in significant differences in gas-grain models of dense, cold molecular clouds for the evolution of a number of surface and gas-phase species. We find differences of a factor of 2 in the abundance for 74 of the 655 species at times of 10⁴-10⁶ yr in this model when we include DR. Of these 74 species, 16 have at least a factor of 10 difference in abundance. We find the largest differences for species formed on the surface of dust grains. These differences are due primarily to the addition of C⁺ DR, which increases the neutral C abundance, thereby enhancing the accretion of C onto dust. These results may be important for the warm-up phase of molecular clouds when surface species are desorbed into the gas phase. We also note that no reliable state-of-the-art RR or DR data exist for Si ⁺, P⁺, S⁺, Cl⁺, and Fe⁺. Modern calculations for these ions are needed to better constrain molecular cloud models.