Forming deuterated methanol (and other COMs) in pre-stellar core conditions

The extremely cold (=10K) environment of pre-stellar cores are ideal physical conditions for the isotopic fractionation of hydrogen. It was previously believed that these conditions would restrict the degree of chemical complexity to simpler molecular species. However, there is recent observational evidence (e.g: Jiménez-Serra+2016) that even complex organic molecules (COMs) form in these dark and cold environments. I updated a gas-grain chemical code capable of deuterium chemistry (Sipilä+2015a, Sipilä+2019b) by various mechanisms essential to study the formation of methanol and larger COMs on the surface of icy dust grains. This includes multiple mechanisms for nondiffusive chemistry. Using the improved code, I performed several 1D simulations of the pre-stellar core L1544 and derived column density profiles for methanol and its deuterated isotopologues. I will show that on one hand, when applying a single collision reaction probability (Hasegawa+1992), an increase in the reaction rate is needed to attain observed methanol levels. On the other hand, when applying reaction-diffusion competition (Chang+2007), reactions proceeding by thermal diffusion is enough to reach observed levels. I find that, in contrast to other COMs, the introduced nondiffusive mechanisms play only a secondary role in the formation and deuteration of methanol. In more recent efforts, I am focusing on models for the heavier COMs with the aim to make first predictions about their D/H ratios.