The PDR structure and kinematics around the compact HII regions S235A and S235C with [CII], [13CII], [OI] and HCO+ line profiles. (arXiv:2007.15708v1 [astro-ph.GA])

The PDR structure and kinematics around the compact HII regions S235A and S235C with [CII], [13CII], [OI] and HCO+ line profiles. (arXiv:2007.15708v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kirsanova_M/0/1/0/all/0/1">M. S. Kirsanova</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ossenkopf_Okada_V/0/1/0/all/0/1">V. Ossenkopf-Okada</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Anderson_L/0/1/0/all/0/1">L. D. Anderson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Boley_P/0/1/0/all/0/1">P. A. Boley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bieging_J/0/1/0/all/0/1">J. H. Bieging</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pavlyuchenkov_Y/0/1/0/all/0/1">Ya. N. Pavlyuchenkov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Luisi_M/0/1/0/all/0/1">M. Luisi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schneider_N/0/1/0/all/0/1">N. Schneider</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Andersen_M/0/1/0/all/0/1">M. Andersen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Samal_M/0/1/0/all/0/1">M. R. Samal</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sobolev_A/0/1/0/all/0/1">A. M. Sobolev</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Buchbender_C/0/1/0/all/0/1">C. Buchbender</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aladro_R/0/1/0/all/0/1">R. Aladro</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Okada_Y/0/1/0/all/0/1">Y. Okada</a>

The aim of the present work is to study structure and gas kinematics in the
photodissociation regions (PDRs) around the compact HII regions S235A and
S235C. 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 S235A. We find an expanding motion of the [CII]-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 [CII] nor in
low-J CO lines may contribute to the total column across S235A. 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.

The aim of the present work is to study structure and gas kinematics in the
photodissociation regions (PDRs) around the compact HII regions S235A and
S235C. 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 S235A. We find an expanding motion of the [CII]-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 [CII] nor in
low-J CO lines may contribute to the total column across S235A. 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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