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The aim of the present work is to study structure and gas kinematics in thephotodissociation regions (PDRs) around the compact HII regions S235A andS235C. We observe the [CII], [13CII] and [OI] line emission, usingSOFIA/upGREAT and complement them by data of HCO+ and CO. We use the [13CII]line to measure the optical depth of the [CII] emission, and find that the[CII] line profiles are influenced by self-absorption, while the [13CII] lineremains unaffected by these effects. Hence, for dense PDRs, [13CII] emission isa better tracer of gas kinematics. The optical depth of the [CII] line is up to10 in S235A. We find an expanding motion of the [CII]-emitting layer of thePDRs into the front molecular layer in both regions. Comparison of the gas anddust columns shows that gas components visible neither in the [CII] nor inlow-J CO lines may contribute to the total column across S235A. We test whetherthe observed properties of the PDRs match the predictions of spherical modelsof expanding HII region + PDR + molecular cloud. Integrated intensities of the[13CII], [CII] and [OI] lines are well-represented by the model, but the modelsdo not reproduce the double-peaked [CII] line profiles due to an insufficientcolumn density of C+. The model predicts that the [OI] line could be a morereliable tracer of gas kinematics, but the foreground self-absorbing materialdoes not allow using it in the considered regions.

The PDR structure and kinematics around the compact HII regions S235A and S235C with [CII], [13CII], [OI] and HCO+ line profiles