Herschel/HIFI observations of [CII] and [13CII] in PDRs
  Recently, we introduced detailed isotopic chemistry into the KOSMA-tau model
for photon-dominated regions (PDRs) to give theoretical predictions for the
abundance of the carbon isotopologues as a function of PDR parameters. Combined
with radiative transfer computations for specific geometries, we estimated the
possible intensity ratio of the [CII]/[13CII] lines. Here, we compare these
predictions with new observations.
  We performed Herschel/HIFI observations of the [CII] 158micron line in a
number of PDRs. In all sources we observed at least two hyperfine components of
the [13CII] transition allowing to determine the [CII]/[13CII] intensity ratio,
after some revision of the intrinsic hyperfine ratios. Comparing the intensity
ratios with the results from the updated KOSMA-tau model, we identify cases
dominated by chemical fractionation and cases dominated by the optical depth of
the main isotopic line.
  An observable enhancement of the [CII]/[13CII] intensity ratio due to
chemical fractionation depends mostly on geometry and velocity structure, and
less on the gas density and radiation field. In our observations the
[CII]/[13CII] ratio for the integrated line intensity was always dominated by
the optical depth of the main isotopic line. However, an enhanced intensity
ratio is found for particular velocity components in a few sources: the
red-shifted material in the ultracompact HII region Mon R2, the wings of the
turbulent profile in the Orion Bar, and possibly a blue wing in NGC7023. The
mapping of the [13CII] lines in the Orion Bar allows to derive a C+ column
density map confirming the temperature stratification of the C+ layer, in
agreement with the chemical stratification of the Bar. The C+ column densities
for all sources show that at the position of the [CII] peak emission, a
dominant fraction of the gas-phase carbon is in the form of C+.
