Investigating the impact of reactions of C and CH with molecular hydrogen on a glycine gas-grain network

TL;DR

This study examines how reactions of atomic carbon and CH with molecular hydrogen influence the chemical composition in a gas-grain network, revealing effects on molecule formation and destruction in collapsing dark clouds.

Contribution

It introduces a sensitivity analysis of reaction parameters affecting molecular abundances, highlighting the impact of hydrogenation reactions on glycine and related species.

Findings

Reactions with H2 alter the hydrogen economy of the network.

Hydrogenation increases simple molecule abundances like CH4, CH3OH, NH3.

Glycine and its precursors decrease in abundance due to hydrogenation.

Abstract

The impact of including the reactions of C and CH with molecular hydrogen in a gas-grain network is assessed via a sensitivity analysis. To this end, we vary 3 parameters, namely, the efficiency for the reaction \ce{C + H2 -> CH2}, and the cosmic ray ionisation rate, with the third parameter being the final density of the collapsing dark cloud. A grid of 12 models is run to investigate the effect of all parameters on the final molecular abundances of the chemical network. We find that including reactions with molecular hydrogen alters the hydrogen economy of the network; since some species are hydrogenated by molecular hydrogen, atomic hydrogen is freed up. The abundances of simple molecules produced from hydrogenation, such as \ce{CH4}, \ce{CH3OH} and \ce{NH3}, increase, and at the same time, more complex species such as glycine and its precursors see a significant decrease in their final abundances. We find that the precursors of glycine are being preferentially hydrogenated, and therefore glycine itself is produced less efficiently.