Monte Carlo Simulation Of Chemical Evolution Of The Protoplanetary Disk

Current astrochemical models for the collapse of prestellar core to form protoplanetary disk are not complete because many important physical processes are absent in these models.Therefore,we proposed a three phase astrochemical model,which is composed of gas phase,active layer and partially active ice mantle in which species can diffuse and react with each other.We simulated the chemical evolution of molecular clouds in the process these collapse to form protoplanetary disks.Moreover,we simulated the chemical evolution of a simple protoplanetary disk model.Based on the two-phase model,our model includes interstitial species and the reactions between these interstitial species.The small molecules produced by photodissociation of ice or interstitial molecules occupy interstitial binding sites.Abundant radicals are produced inside ice mantle in the cold stage of molecular cloud evolution.When temperature increases so that radicals can diffuse inside ice mantle,these radicals can recombine to form complex organic molecules because the barriers of the radical recombination reactions are small.The complex organic molecules desorb into gas phase as the temperature further increases and the protoplanetary disks gradually are formed.The self-shielding effects of H₂ and CO are included in our models.A new Monte Carl algorithm is used to accelerate Monte Carlo simulation.Compared our model results with previous model results,our new model can produce more abundant complex organic molecules such as methyl formate and dimethyl ether when the temperature is high enough so that radicals can diffuse and form complex organic molecules.Moreover,more radicals such as hydroxyl are formed in the cold stage of molecular clouds.Our models successfully solved the problem that complex organic molecules are underproduced in most current astrochemical models,so our models are more suitable for the chemical evolution of collapsing molecular clouds and protoplanetary disks.