Enhanced formation of interstellar complex organic molecules on carbon monoxide ice

TL;DR

This study models how carbon monoxide ice influences the formation of complex organic molecules in space, revealing that CO ice can enhance certain molecules' production depending on temperature and cosmic ray conditions.

Contribution

It introduces a chemical model that accounts for variable binding energies on CO ice, showing how CO coverage affects complex organic molecule formation in prestellar cores.

Findings

CO ice increases formation of O-bearing complex organic molecules.

Enhanced diffusion of CH₃/CH₂ on CO ice boosts molecule production.

Temperature and cosmic rays critically influence chemical enhancement.

Abstract

We investigate the role of carbon monoxide ice in the chemical evolution of prestellar cores using astrochemical rate equation models. We constrain the ratios of the binding energies on CO ice and H$_{2}$O ice for a series of adsorbates deemed important in diffusive chemistry on H$_{2}$O ices. We later include these ratios in our chemical reaction network model, where the binding and diffusion energies of icy species vary as a function of the surface composition. When the surface coverage of CO increases, the model shows an enhancement of O-bearing complex organic molecules, especially those formed from the intermediate products of CO hydrogenation (e.g. HCO) and CH$_{3}$/CH$_{2}$. Because the binding energy of CH$_{3}$/CH$_{2}$ is in the right range, its diffusion rate increases significantly with CO coverage. At $T>$14 K and with less influence, enhanced diffusion of HCO also contributes to the increase of the abundances of COM. We find, however, that chemistry is not always enhanced on CO ice and that the temperature and cosmic ray ionization rate of each astronomical object is crucial for this particular chemistry, revealing a highly non-trivial behavior that needs to be addressed on a per-case basis. Our results are highly relevant in the context of interstellar ice observations with JWST.