Equilibrium Simplified Chemistries for H2O and CO in three-phase astrochemical models

TL;DR

This paper develops and validates simplified equilibrium chemical models for CO and H2O in astrochemical environments, enabling fast and accurate abundance calculations in complex, evolving three-phase gas-grain systems.

Contribution

The study provides updated analytical equilibrium solutions for CO and H2O that account for ice desorption and inert reservoirs, matching detailed models within a factor of two.

Findings

Analytical solutions closely match detailed models.

Solutions remain accurate during rapid dynamical changes.

Recommended for fast abundance calculations in cold molecular environments.

Abstract

Astrochemical models can be greatly simplified, with obvious computational advantages, if the reaction networks for key species can be reduced to a bare minimum. In addition, if chemical equilibrium holds, then simple analytical solutions can be formulated. These have particular advantages in the application to complex models evolving over multi-point spatial grids. In this study, the equilibrium solutions to highly simplified chemical networks for CO and H2O have been re-assessed with particular attention to the formulation of the ice desorption rates in the context of 'three-phase' gas-grain astrochemical models. The analytical solutions have also been updated to account for the chemically inert reservoir of molecules below the surface ice layers, and to include the effects of reactive desorption. We find that a very close match is obtained to the results from detailed three-phase models of the time-dependent astrochemistry, and the abundances are typically accurate to within a factor of two over the entire range of densities and extinction that are applicable to dense clouds and young star-forming regions. In addition, these solutions give accurate results over most of the range of conditions even for systems undergoing rapid dynamical evolution. Although there are some caveats of applicability, we therefore recommend that these solutions be used in models of cold molecular environments where the rapid calculation of the abundances of CO, H2O and atomic coolants is helpful.