Investigating solid-state C_3OH formation with chemical modelling

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 (Tk), the gas volume density (nH2), the velocity of the C-shock wave (vshock), 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 XH2CO/XCH3OH ratio represents a potential diagnostic tool for this purpose.