Monte Carlo simulation of complex organic molecules synthesis by UV irradiation in protoplanertary disks

The elucidation of the formation of complex organic molecules (COMs) is an important key to understanding chemical evolution in the universe. One of the formation mechanism of COMS is the radical reactions on the surface of ice dust exposed to ultraviolet light, cosmic rays, and other high energy sources. However, the synthetic pathways and the specific types of COMs produced remain unclear. In this study, a Monte Carlo simulation for chemical reactions was newly developed to investigate the COMs synthesis driven by UV irradiation. This method significantly reduces computational costs by employing bold approximations for activation energies. This enables calculations involving large molecules, extended timescales, and various parameters that are not covered by conventional quantum chemical calculations. In addition, while reaction network models require the reaction to be given in advance, this method is a forward search that predicts the reaction path from the reactants without assuming the reaction, and thus can explore extremely complex reaction networks. Using this novel method, the surface reactions of ice dust in a protoplanetary disk are simulated. These reactions are induced by exposure to ultraviolet radiation when the dust is temporarily lifted into the upper layers. Results show that the initial molecules (composed of formaldehyde, ammonia, water, and other carbon sources) produced highly complex molecules through continuous radical reactions during UV irradiation. Amino acids and sugars were also found to be synthesized by random bond rearrangements and not conventional building-up-type reactions such as the Strecker reaction and formose reaction. Their productions, with a wide range of initial C/H and O/H ratios, were calculated, leading to the derivation of semi-analytical solutions for predicting the relative yields. Finally, from the parameter dependence of these results, the conditions where our simulations would be valid will be discussed during this conference.