Investigating solid-state CH₃OH formation with chemical modelling

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Observatorio Astrónomico Nacional, Universidad Nacional de Colombia, Teusaquillo, Bogotá, Bogota, Colombia
³ Transdisciplinary Research Area (TRA Matter) and Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ Department of Physics and Astronomy, University College London, Gower Street, London WC1 E6BT, UK

^★ Corresponding author: ejmendezr@unal.edu.co

Received: 27 June 2025
Accepted: 22 September 2025

Abstract

Context. Recent Monte Carlo simulations and laboratory studies of interstellar ices have proposed an alternative pathway involving the radical-molecule H-atom abstraction reaction in the overall mechanism of methanol (CH₃OH) formation in dark molecular clouds.

Aims. A computational study was conducted to investigate the contribution of the radical-molecule H-atom abstraction route in CH₃OH formation in interstellar ices, both in non-shocked and shocked environments, and to examine how the physical conditions of the interstellar medium (ISM) affect the overall CH₃OH synthesis pathway.

Methods. A set of chemical models were run using the gas-grain chemical code UCLCHEM to systematically explore methanol synthesis in various physical scenarios, including non-shock and low- and high-velocity C-shocks.

Results. This work demonstrates for the first time that, under non-shock and shocked-influenced environments, the primary reaction leading to the formation of methanol in the inner layers of interstellar ices is indeed the radical-molecule H-atom abstraction route. However, such a route is dependent on the gas kinetic temperature (T_k), the gas volume density (n_H2), the velocity of the C-shock wave (v_shock), and the cosmic ray ionisation rate (ζ). Furthermore, gaseous formaldehyde may trace C-type shocks and serve to differentiate methanol formation pathways in low-velocity C-shocked environments, as its abundance varies more significantly than that of CH₃OH with the inclusion of the H-atom abstraction reaction in UCLCHEM. Thus, the X_H2CO/X_CH3OH ratio represents a potential diagnostic tool for this purpose.