  Complex organic molecules (COMs) have been widely observed in molecular
clouds and protostellar environments. One of the formation mechanisms of COMs
is radical reactions on the icy grain surface driven by UV irradiation. While
many experiments have reported that various COMs can be synthesized under such
ice conditions, the majority of the reaction processes are unclear.
Complementary numerical simulations are necessary to unveil the synthetic
process behind the formation of COMs. In this study, we develop a chemical
reaction simulation using a Monte Carlo method. To explore the complex reaction
network of COM synthesis, the model was designed to eliminate the need to
prepare reaction pathways and to keep computational costs low. With this
simulation, we investigate the chemical reactions occurring on icy dust
surfaces during and after UV irradiation, assuming a protoplanetary disk
environment. We aim to reveal the types of organic molecules produced in a disk
and the formation mechanisms of COMs, in particular, amino acids and sugars.
The results show that photodissociation and subsequent radical-radical
reactions cause random rearrangement of the covalent bonds in the initial
molecules composed of methanol, formaldehyde, ammonia, and water. Consequently,
highly complex molecules such as amino acids and sugars were produced in a wide
range of the initial conditions. We found that the final abundances of amino
acids and sugars have extremely similar dependence on the atomic ratios of the
initial molecules, which peak at C/H~0.1-0.3 and O/H~0.3-0.5, although the
amino acids abundance is usually more than ten times higher than that of
sugars. To understand this dependence, a semi-analytical formula was derived.
Additionally, parameter surveys have suggested that the decomposition reactions
of amino acids and sugars undergo a rapid transition within the threshold of a
given parameter.
