Interstellar Hydrocarbons: Degradation Chemistry in Diffuse Clouds

TL;DR

This paper explores a new 'top-down' degradation chemistry model for explaining the presence of simple hydrocarbons in diffuse interstellar clouds, supported by experimental and observational data.

Contribution

It introduces a novel degradation scheme for hydrocarbon formation in diffuse clouds, contrasting with traditional synthesis models, and demonstrates its feasibility through chemical modeling.

Findings

Degradation scheme can produce hydrocarbons consistent with observations

Experimental data supports hydrocarbon release from HAC decomposition

Preliminary models show promising results for the top-down approach

Abstract

Observations of diffuse clouds showed that they contain a number of simple hydrocarbons (e.g. CH, C$_2$H, (l- and c-)C$_3$H$_2$, and C$_4$H) in abundances that may be difficult to understand on the basis of conventional gas-phase chemical models. Recent experimental results revealed that the photo-decomposition mechanism of hydrogenated amorphous carbon (HAC) and of solid hexane releases a range of hydrocarbons into the gas, containing up to 6 C-atoms for the case of HAC decomposition. These findings motivated us to introduce a new potential input to interstellar chemistry; the "top-down" or degradation scheme, as opposed to the conventional "build-up" or synthesis scheme. In this work, we demonstrate the feasibility of the top-down approach in diffuse clouds using gas-grain chemical models. In order to examine this scheme, we derived an expression to account for the formation of hydrocarbons when HACs are photo-decomposed after their injection from grain mantles. Then, we calculated the actual formation rate of these species by knowing their injected fraction (from experimental work) and the average rate of mantle carbon injection into the ISM (from observations). Our preliminary results are promising and reveal that the degradation scheme can be considered as an efficient mechanism for the formation of some simple hydrocarbons in diffuse clouds. However, an actual proof of the efficiency of this process and its rate constants would require comprehensive experimental determination.