Formation of COMs through CO hydrogenation on interstellar grains
  Glycoaldehyde, ethylene glycol, and methyl formate are complex organic
molecules that have been observed in dark molecular clouds. Because there is no
efficient gas-phase route to produce these species, it is expected that a
low-temperature surface route existst that does not require energetic
processing. CO hydrogenation experiments at low temperatures showed that this
is indeed the case. Glyoxal can form through recombination of two HCO radicals
and is then further hydrogenated. Here we aim to constrain the methyl formate,
glycolaldehyde, and ethylene glycol formation on the surface of interstellar
dust grains through this cold and dark formation route. We also probe the
dependence of the grain mantle composition on the initial gas-phase composition
and the dust temperature. A full CO hydrogenation reaction network was built
based on quantum chemical calculations for the rate constants and branching
ratios. This network was used in combination with a microscopic kinetic Monte
Carlo simulation to simulate ice chemistry, taking into account all positional
information. After benchmarking the model against CO-hydrogenation experiments,
simulations under molecular cloud conditions were performed. COMs are formed in
all interstellar conditions we studied, even at temperatures as low as 8 K.
This is because the HCO + HCO reaction can occur when HCO radicals are formed
close to each other and do not require to diffuse. Relatively low abundances of
methyl formate are formed. The final COM abundances depend more on the H-to-CO
ratio and less on temperature. Only above 16 K, where CO build-up is less
efficient, does temperature start to play a role. Molecular hydrogen is
predominantly formed through abstraction reactions on the surface. Our
simulations are in agreement with observed COM ratios for mantles that have
been formed at low temperatures.
