The PDR structure and kinematics around the compact H ii regions S235 A and S235 C with [C ii], [13C ii], [O i], and HCO+ line profiles

• Published in: Monthly notices of the Royal Astronomical Society 2020-09, Vol.497 (3), p.2651-2669
• Main Authors: Kirsanova, M S, Ossenkopf-Okada, V, Anderson, L D, Boley, P A, Bieging, J H, Pavlyuchenkov, Ya N, Luisi, M, Schneider, N, Andersen, M, Samal, M R, Sobolev, A M, Buchbender, C, Aladro, R, Okada, Y
• Format: Article
• Language: English
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/staa2142

ABSTRACT 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.