Investigating solid-state CH 3 OH formation with chemical modelling

Astronomy and Astrophysics, vol. 704, pp. A17

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 (CH 3 OH) formation in dark molecular clouds. Aims . A computational study was conducted to investigate the contribution of the radical-molecule H-atom abstraction route in CH 3 OH 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 CH 3 OH 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 H 2 ), 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 CH 3 OH 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.

Authors 4

1. Ko-Yun Huang Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University

2. E. Méndez-Robayo

   Universidad Nacional de Colombia

   Affiliation as printed

   Observatorio Astrónomico Nacional, Universidad Nacional de Colombia

3. S. Viti Aachen Leiden Observatory

   Leiden University · University of Bonn · Leiden Observatory · University College London

   Affiliation as printed

   Department of Physics and Astronomy, University College London

   Leiden Observatory, Leiden University

   Transdisciplinary Research Area (TRA Matter) and Argelander Institut für Astronomie, Universität Bonn

4. Mario A Higuera

   Universidad Nacional de Colombia

   Affiliation as printed

   Observatorio Astrónomico Nacional, Universidad Nacional de Colombia