2021A&A...651A..30S

Aims. We investigate the properties of [CII] 158µm emission of RCW 36 in a dense filamentary cloud.
Methods. [CII] observations of RCW 36, covering an area of ∼30'x30', were carried out with a Fabry-Perot spectrometer on board a 100-cm balloon-borne far-infrared (IR) telescope with an angular resolution of 90''. Using AKARI and Herschel images, we compared the spatial distribution of the [CII] intensity with the emission from the large grains and polycyclic aromatic hydrocarbon (PAH).
Results. The [CII] emission is in good spatial agreement with shell-like structures of a bipolar lobe observed in IR images, which extend along the direction perpendicular to the direction of cold dense filament. We found that the [CII]-160µm relation for RCW 36 shows a higher brightness ratio of [CII]/160µm than that for RCW 38, while the [CII]-9µm relation for RCW 36 is in good agreement with that for RCW 38.
Conclusions. Via a spectral decomposition analysis on a pixel-by-pixel basis using IR images, the [CII] emission is spatially well correlated with PAH and cold dust emissions. This means that the observed [CII] emission predominantly comes from photo-dissociation regions. Moreover, the L_[CII]/L_FIR ratio shows large variation (10^–2-10^–3), as 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 λ=160µm, the large variation in L_[CII]/L_FIR can be simply explained by the geometrical effect, that is, 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]/160µm is attributed to the difference in gas structures where massive stars are formed: filamentary (RCW 36) and clumpy (RCW 38) molecular clouds; thus suggesting that RCW 36 is dominated by far-UV illuminated cloud surfaces, as compared with RCW 38.