2020MNRAS.497.2651K

The aim of this work is to study structure and gas kinematics in the photodissociation regions (PDRs) around the compact H II regions S235 A and S235 C. We observe the [C II], [¹³C II], and [O I] line emission, using SOFIA/upGREAT, and complement them by data of HCO⁺ and CO. We use the [¹³C II] line to measure the optical depth of the [C II] emission, and find that the [C II] line profiles are influenced by self-absorption, while the [¹³C II] line remains unaffected by these effects. Hence, for dense PDRs, [¹³C II] emission is a better tracer of gas kinematics. The optical depth of the [C II] line is up to 10 in S235 A. We find an expanding motion of the [C II]-emitting layer of the PDRs into the front molecular layer in both regions. Comparison of the gas and dust columns shows that gas components visible neither in the [C II] nor in low-J CO lines may contribute to the total column across S235 A. We test whether the observed properties of the PDRs match the predictions of spherical models of expanding H II region + PDR + molecular cloud. Integrated intensities of the [¹³C II], [C II], and [O I] lines are well represented by the model, but the models do not reproduce the double-peaked [C II] line profiles due to an insufficient column density of C⁺. The model predicts that the [O I] line could be a more reliable tracer of gas kinematics, but the foreground self-absorbing material does not allow using it in the considered regions.