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

Abstract

ABSTRACT We study evolution of hydrogenated amorphous carbon (HAC) grains under harsh UV radiation in photodissociation regions (PDRs) near young massive stars. Our aim is to evaluate the impact of the HAC grains on formation of observed small hydrocarbons: C2H, C2H2, C3H+, C3H, C3H2, C4H, in PDRs. We developed a microscopic model of the HAC grains based on available experimental results. The model includes processes of photo and thermo-desorption, accretion of hydrogen and carbon atoms and subsequent formation of carbonaceous mantle on dust surface. H2, CH4, C2H2, C2H4, C2H6, C3H4, C3H6, C3H8 are considered as the main fragments of the HAC photodestruction. 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 AV = 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 AV = 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.