Dark cloud-type chemistry in PDRs with moderate UV field

TL;DR

This study investigates the chemical composition of PDRs with moderate UV radiation, revealing the influence of ion-molecular and grain surface chemistry on molecular abundances, which are inherited from earlier dark cloud stages.

Contribution

It provides new insights into the chemical link between molecular abundances and UV radiation in PDRs, emphasizing the roles of ion-molecular and grain surface processes.

Findings

Hydrocarbon abundances peak at molecular clump edges.

Methanol abundance is highest at moderate UV fields, explained by reactive desorption.

The C2H/CH3OH ratio is lower than in other well-known PDRs, similar to hot corinos.

Abstract

We present a study of emission lines of small hydrocarbons C$_2$H and $c$-C$_3$H$_2$, and COMs precursors H$_2$CO and CH$_3$OH in order to better understand the possible chemical link between the molecular abundances and UV radiation field in photodissociation regions (PDRs). We study two PDRs around extended and compact HII regions with $G \leq 50$~Habings in the S235 star-forming complex. We find the highest abundances of both hydrocarbons on the edges of molecular clumps, while $c$-C$_3$H$_2$ is also abundant in the low-density expanding PDR around compact HII region S235\,A. We see the highest methanol column density towards the positions with the UV~field $G\approx 20-30$~Habings and explain them by reactive desorption from the dust grains. The $N_{\rm C_2H}/N_{\rm CH_3OH}$ ratio is lower by a factor of few or the order of magnitude in comparison with the Horsehead and Orion Bar PDRs. The ratio is similar to the value observed in hot corinos in the Perseus cloud. We conclude that ion-molecular and grain surface chemical routes rule the molecular abundances in the PDRs, and the PDRs inherit molecular abundances from the previous dark stage of molecular cloud evolution in spite of massive stars already emitting in optics.