Impact of HAC evolution on the formation of small hydrocarbons in the Orion Bar and the Horsehead PDRs

TL;DR

This study models how hydrogenated amorphous carbon grains evolve under UV radiation in PDRs and assesses their role in forming small hydrocarbons, finding their contribution insufficient to explain observed abundances.

Contribution

It introduces a microscopic model of HAC grain evolution under UV radiation and couples it with gas-phase chemistry to evaluate small hydrocarbon formation in PDRs.

Findings

HAC destruction fragments can dominate at low A_V in PDRs.

HAC destruction increases hydrocarbon abundances but doesn't fully explain observations.

Carbonaceous mantle formation inhibits small hydrocarbon production.

Abstract

We study evolution of hydrogenated amorphous carbon (HAC) grains under harsh UV radiation in photo-dissociation regions (PDRs) near young massive stars. Our aim is to evaluate the impact of the HAC grains on formation of observed small hydrocarbons: C$_2$H, C$_2$H$_2$, C$_3$H$^+$, C$_3$H, C$_3$H$_2$, C$_4$H, in PDRs. We developed a microscopic model of the HAC grains based on available experimental results. The model includes processes of photo- and thermodesorption, accretion of hydrogen and carbon atoms and subsequent formation of carbonaceous mantle on dust surface. H$_2$, CH$_4$, C$_2$H$_2$, C$_2$H$_4$, C$_2$H$_6$, C$_3$H$_4$, C$_3$H$_6$, C$_3$H$_8$ are considered as the main fragments of the HAC photo-destruction. We simulated evolution of the HAC grains under the physical conditions of two PDRs, the Orion Bar and the Horsehead nebula. We estimated the production rates of the HAC fragments in gas phase chemical reactions and compared them with the production rates of fragments due to the HAC destruction. The latter rates may dominate under some conditions, namely, at A$_V$=0.1 in both PDRs. We coupled our model with the gas-grain chemical model MONACO and calculated abundances of observed small hydrocarbons. We conclude that the contribution of the HAC destruction fragments to chemistry is not enough to match the observed abundances, although it increases the abundances by several orders of magnitude in the Orion Bar at A$_V$=0.1. Additionally, we found that the process of carbonaceous mantle formation on dust surface can be an inhibitor for the formation of observed small hydrocarbons in PDRs.