[CII] emission properties of the massive star-forming region RCW36 in a
  filamentary molecular cloud
  Aims: To investigate properties of [CII]158 $\mu$m emission of RCW36 in a
dense filamentary cloud. Methods: [CII] observations of RCW36 covering an area
of ~30 arcmin$\times$30 arcmin were carried out with a Fabry-P\'{e}rot
spectrometer aboard a 100-cm balloon-borne far-infrared (IR) telescope with an
angular resolution of 90 arcsec. By using AKARI and Herschel images, the
spatial distribution of the [CII] intensity was compared with those of emission
from the large grains and PAH. Results: The [CII] emission is spatially in good
agreement with shell-like structures of a bipolar lobe observed in IR images,
which extend along the direction perpendicular to the direction of a cold dense
filament. We found that the [CII]--160 $\mu$m relation for RCW36 shows higher
brightness ratio of [CII]/160 $\mu$m than that for RCW 38, while the [CII]--9
$\mu$m relation for RCW36 is in good agreement with that for RCW38.
Conclusions: The [CII] emission spatially well correlates with PAH and cold
dust emissions. This means that the observed [CII] emission dominantly comes
from PDRs. Moreover, the L_[CII]/L_FIR ratio shows large variation compared
with the L_[CII]/L_PAH ratio. In view of the observed tight correlation between
L_[CII]/L_FIR and the optical depth at $\lambda$=160 $\mu$m, the large
variation in L_[CII]/L_FIR can be simply explained by the geometrical effect,
viz., L_FIR has contributions from the entire dust-cloud column along the line
of sight, while L_[CII] has contributions from far-UV illuminated cloud
surfaces. Based on the picture of the geometry effect, the enhanced brightness
ratio of [CII]/160 $\mu$m is attributed to the difference in gas structures
where massive stars are formed: filamentary (RCW36) and clumpy (RCW38)
molecular clouds and thus suggests that RCW36 is dominated by far-UV
illuminated cloud surfaces compared with RCW38.
