11:00 AM - 11:15 AM
[PCG22-08] Monte Carlo simulation of complex organic molecules synthesis by UV irradiation
Keywords:Complex organic molecule, Protoplanetary disk, Theoretical chemical model
Complex organic molecules (COMs) have been widely observed in protoplanetary disks, molecular clouds. One of the formation mechanisms of COMs is radical reactions occurring on the icy grain surface driven by UV irradiation. While many experiments have reported that various organic molecules, including biomolecules, can be synthesized under such ice conditions, most of the reaction processes involved in COMs synthesis are not clear due to the complexity of the reaction network of radical reactions induced by irradiation, as well as the challenge of in-situ analysis of experimental products. Therefore, numerical simulations that are complementary to experiments are necessary to elucidate the process of COMs synthesis.
In this study, we developed a new chemical reaction simulation scheme using a Monte Carlo method. To explore the complex reaction network of COMs synthesis, our model was designed to eliminate the need to prepare reaction pathways in advance and to keep computational costs low. This approach allows for a global investigation of COMs synthesis reactions and leads to a comprehensive understanding of the COMs formation process from a wide range of parameter surveys. In this study, we focused on the COMs synthesis on the surface of ice dust in a protoplanetary disk, and aimed to understand the types of COMs produced and their formation mechanisms.
Some of the dust in a protoplanetary disk is transported to the upper layer by gas turbulence and exposed to intense ultraviolet radiation from the central star. At that time, the molecules on the ice surface become radicals, promoting organic synthesis. However, much of the dust would then sink back into the disk and be shielded from the UV light. Assuming this situation, the calculation was performed in two phases: "UV phase," in which the ice dust is exposed to UV light and photodissociation occurs, and "post-UV phase," in which the irradiation ends and photodissociation does not proceed. The temperature was set to T = 100 K, and the chemical reactions within the initial set of molecules (methanol, formaldehyde, ammonia, and water molecules) were simulated.
Results showed that photodissociation induced by UV irradiation and subsequent radical-radical reactions randomly from the covalent bonds in the initial molecules, producing a wide variety of bond types and functional groups. Consequently, highly complex molecules, such as amino acids and sugars, were synthesized under a broad range of initial conditions. The calculations using 57 different sets of initial molecules showed that the final abundance of amino acids is generally more than 10 times higher than that of sugars, while their final abundances have extremely similar dependence on the atomic ratios of the initial molecules, both peaking at C/H∼0.1-0.3 and O/H∼0.3-0.5. In this study, a semi-analytical formula that predicts the final abundances of amino acids and sugars from the atomic ratios of the initial molecules was derived, identifying the mechanisms that control the production of amino acids and sugars.
In this study, we developed a new chemical reaction simulation scheme using a Monte Carlo method. To explore the complex reaction network of COMs synthesis, our model was designed to eliminate the need to prepare reaction pathways in advance and to keep computational costs low. This approach allows for a global investigation of COMs synthesis reactions and leads to a comprehensive understanding of the COMs formation process from a wide range of parameter surveys. In this study, we focused on the COMs synthesis on the surface of ice dust in a protoplanetary disk, and aimed to understand the types of COMs produced and their formation mechanisms.
Some of the dust in a protoplanetary disk is transported to the upper layer by gas turbulence and exposed to intense ultraviolet radiation from the central star. At that time, the molecules on the ice surface become radicals, promoting organic synthesis. However, much of the dust would then sink back into the disk and be shielded from the UV light. Assuming this situation, the calculation was performed in two phases: "UV phase," in which the ice dust is exposed to UV light and photodissociation occurs, and "post-UV phase," in which the irradiation ends and photodissociation does not proceed. The temperature was set to T = 100 K, and the chemical reactions within the initial set of molecules (methanol, formaldehyde, ammonia, and water molecules) were simulated.
Results showed that photodissociation induced by UV irradiation and subsequent radical-radical reactions randomly from the covalent bonds in the initial molecules, producing a wide variety of bond types and functional groups. Consequently, highly complex molecules, such as amino acids and sugars, were synthesized under a broad range of initial conditions. The calculations using 57 different sets of initial molecules showed that the final abundance of amino acids is generally more than 10 times higher than that of sugars, while their final abundances have extremely similar dependence on the atomic ratios of the initial molecules, both peaking at C/H∼0.1-0.3 and O/H∼0.3-0.5. In this study, a semi-analytical formula that predicts the final abundances of amino acids and sugars from the atomic ratios of the initial molecules was derived, identifying the mechanisms that control the production of amino acids and sugars.