Dark cloud-type chemistry in photodissociation regions with moderate ultraviolet field

Abstract

ABSTRACT We present a study of emission lines of small hydrocarbons C2H and c-C3H2, and COMs precursors H2CO and CH3OH 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 H ii regions with G ≤ 50 Habings in the S235 star-forming complex. We find the highest abundances of both hydrocarbons on the edges of molecular clumps, while c-C3H2 is also abundant in the low-density expanding PDR around compact H ii region S235 A. We see the highest methanol column density towards the positions with the UV field G ≈ 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.