The PDR Structure and Kinematics Around the Compact HII Regions S235 A and S235 C with [CII], [13CII], [OI] and HCO+ Line Profiles
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Maria Kirsanova
Institute of Astronomy, Russian Academy of Sciences
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The aim of the present work is to study structure and gas kinematics in the photodissociation regions (PDRs) around the compact HII regions S235 A and S235 C. We observe the [CII], [13CII] and [OI] line emission, using SOFIA/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] line remains unaffected by these effects. Hence, for dense PDRs, [13CII] emission is a better tracer of gas kinematics. The optical depth of the [CII] line is up to 10 in S235 A. We find an expanding motion of the [CII]-emitting layer of the PDRs into the front molecular wall in both regions. Comparison of the gas and dust columns shows that gas components visible neither in the [CII] 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 HII region + PDR + molecular cloud. Integrated intensities of the [13CII], [CII] and [OI] lines are well-represented by the model, but the models do not reproduce the double-peaked [CII] line profiles due to an insufficient column density of C+. The model predicts that the [OI] 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.

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