Dark cloud-type chemistry in PDRs with moderate UV field
  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.
