Spectrally resolved CII emission in M~33 ({\tt HerM33es}): Physical
  conditions and kinematics around BCLMP 691
  This work presents high spectral resolution observations of the \CII\ line at
158 \micron, one of the major cooling lines of the interstellar medium, taken
with the HIFI heterodyne spectrometer on the Herschel satellite. In BCLMP 691,
an \HII\ region far north (3.3 kpc) in the disk of M 33, the \CII\ and CO line
profiles show similar velocities within $0.5 \kms$, while the \HI\ line
velocities are systematically shifted towards lower rotation velocities by
$\sim 5\kms$. Observed at the same $12"$ angular resolution, the \CII\ lines
are broader than those of CO by about 50% but narrower than the \HI\ lines. The
\CII\ line to far-infrared continuum ratio suggests a photoelectric heating
efficiency of 1.1%. The data, together with published models indicate a UV
field $G_0 \sim 100$ in units of the solar neighborhood value, a gas density
$n_H \sim 1000 \cc$, and a gas temperature $T\sim 200$ K. Adopting these
values, we estimate the C$^+$ column density to be $N_{C^+} \approx 1.3 \times
10^{17} \cmt$. The \CII\ emission comes predominantly from the warm neutral
region between the \HII\ region and the cool molecular cloud behind it. From
published abundances, the inferred C$^+$ column corresponds to a hydrogen
column density of $N_H \sim 2 \times 10^{21} \cmt$. The CO observations suggest
that $N_H = 2 N_{H_2} \sim 3.2 \times 10^{21} \cmt$ and 21cm measurements, also
at $12"$ resolution, yield $N_\HI \approx 1.2 \times 10^{21} \cmt$ within the
\CII\ velocity range. Thus, some H$_2$ not detected in CO must be present, in
agreement with earlier findings based on the SPIRE 250 -- 500 $\mu$m emission.
