Formation of Complex Organic Molecules in Prestellar Cores: The Role of Non-Diffusive Grain Chemistry

TL;DR

This study models the formation of complex organic molecules in prestellar cores, highlighting the importance of non-diffusive grain chemistry and chemical desorption processes, successfully reproducing observed abundances in L1544.

Contribution

It introduces a non-diffusive astrochemical model that accurately reproduces COM abundances and their spatial distribution in a prestellar core, incorporating new laboratory-verified chemical pathways.

Findings

Simultaneous reproduction of gas-phase and ice-phase COM abundances in L1544.

Radical-radical surface reactions proceed efficiently via non-diffusive mechanisms.

Chemical desorption significantly contributes to gas-phase COM formation.

Abstract

We present the results of astrochemical modeling of complex organic molecules (COMs) in the ice and gas of the prestellar core L1544 with the recently updated MONACO rate equations-based model. The model includes, in particular, non-diffusive processes, new laboratory verified chemical routes for acetaldehyde and methane ice formation and variation of H and $\rm H_2$ desorption energies depending on the surface coverage by $\rm H_2$ molecules. For the first time, we simultaneously reproduce the abundances of several oxygen-bearing COMs in the gas phase, the approximate location of the peak of methanol emission, as well as the abundance of methanol in the icy mantles of L1544. Radical-radical reactions on grains surface between species such as $\rm CH_3$, $\rm CH_3O$ and $\rm HCO$ efficiently proceed non-diffusively. COMs are delivered to the gas phase via chemical desorption amplified by the loops of H-addition/abstraction surface reactions. However, gas-phase chemical reactions as well provide a noticeable input to the formation of COMs in the gas, but not to the COMs solid-state abundances. This particularly applies for CH$_3$CHO and CH$_3$OCH$_3$. The simulated abundances of COMs in the ice are in the range 1\%--2\% (for methyl formate ice) or $\sim$~0.1\% (for CH$_3$CHO and CH$_3$OCH$_3$) with respect to the abundance of H$_2$O ice. We stress a similarity between the simulated abundances of icy COMs in L1544 and the abundances of COMs in the gas phase of hot cores/corinos. We compare our non-diffusive model with the diffusive model and provide constraints for the species' diffusion-to-desorption energy ratios.