Forming deuterated methanol in pre-stellar core conditions

The formation mechanisms for most complex organic molecules (COMs) are still debated. Either COMs form mostly on the surface of dust grains or mostly by reactions between simpler hydrogenation products upon their desorption into the gas phase. Methanol, the simplest of the O-bearing COMs, plays a key role in both scenarios. Our aim is to improve the suitability of our models for the formation and deuteration of COMs in the extremely cold conditions of pre-stellar cores, where chemical reactions between heavier reactants on the surface of dust grains are hindered by the reactant's immobility. Initially, we focused our efforts on CH_3OH and its singly deuterated isotopologue CH_2DOH. We updated a gas-grain chemical code capable of deuterium chemistry by including various non-diffusive reaction mechanisms: Eley-Rideal reactions, photodissociation-induced reactions, and three-body reactions. Moreover, we added the reaction H_2CO + CH_3O ̊ightarrow CH_3OH + HCO to our chemical network, which was found to contribute significantly to methanol formation in both microscopic kinetic Monte Carlo simulations and laboratory experiments. We performed several 1D simulations of the pre-stellar core L1544, where we derived column density profiles for CH_3OH and CH_2DOH and compared our model results with more conventional modelling approaches and available gas-phase observations. We show that multiple models with different parameter sets provide column density profiles that are in reasonable agreement with the observed values. On the one hand, when applying a single collision reaction probability, either an increase in the reaction rate by the occurrence of diffusion by quantum tunneling or a lowered diffusion-to-binding energy ratio (E_ d /E_ b = 0.2) for thermal diffusion is needed to match the observed methanol levels. On the other hand, when applying reaction-diffusion competition, reactions proceeding by thermal diffusion with a conservative diffusion-to-binding energy ratio (E_ d /E_ b = 0.55) are sufficient to reach observed column densities. We find that, in contrast to other COMs, the introduced non-diffusive mechanisms play only a secondary role in the formation and deuteration of methanol. Additionally, we find only a negligible contribution from H_2CO + CH_3O ̊ightarrow CH_3OH + HCO.