Monte Carlo Simulation of Sugar Synthesis on Icy Dust Particles Intermittently Irradiated by UV in a Protoplanetary Disk

TL;DR

This study uses Monte Carlo simulations to model sugar synthesis on icy dust particles in protoplanetary disks under UV irradiation, revealing new pathways for organic molecule formation consistent with meteoritic data.

Contribution

It introduces a novel Monte Carlo simulation code for organic synthesis on icy particles, demonstrating alternative pathways for sugar formation in space environments.

Findings

Sugars form after UV irradiation ceases, via decomposition and rearrangement of large molecules.

Sugar abundance depends mainly on the initial H/O atomic ratio, not specific molecules.

Deoxyribose is also synthesized alongside other complex organic molecules.

Abstract

Context. While synthesis of organic molecules in molecular clouds or protoplanetary disks is complex, observations of interstellar grains, analyses of carbonaceous chondrites, and UV photochemistry experiments are rapidly developing and providing constraints on and clues to the complex organic molecule synthesis in space. It motivates us to construct a theoretical synthesis model. Aims. We develop a new code to simulate global reaction sequences of organic molecules to apply it for sugar synthesis by intermittent UV irradiation on the surface of icy particles in a protoplanetary disk. Here we show the first results of our new simulation. Methods. We apply a Monte Carlo method to select reaction sequences from all possible reactions, using the graph-theoretic matrix model for chemical reactions and modeling reactions on the icy particles during UV irradiation. Results. We here obtain the results consistent with the organic molecules in carbonaceous chondrites and obtained by the experiments, however, through a different pathway from the conventional formose reactions previously suggested. During UV irradiation, loosely-bonded O-rich large molecules are continuously created and destroyed. After UV irradiation is turned off, the ribose abundance rapidly increases, through the decomposition of the large molecules with break-ups of O-O bonds and replacements of C-OH by C-H to reach O/C = 1 for sugars. The sugar abundance is regulated mostly by the total atomic ratio H/O of starting materials, but not by their specific molecule forms. Deoxyribose is simultaneously synthesized, and most of the molecules end up with complex C-rich molecules.