2025A&A...704A..17H

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 $\frac{X_{\mathrm{H}_{2}\mathrm{CO}}}{X_{\mathrm{CH}_{3}\mathrm{OH}}}$ ratio represents a potential diagnostic tool for this purpose.