[CII] emission properties of the massive star-forming region RCW 36 in a filamentary molecular cloud

Abstract

Aims. We investigate the properties of [C II] 158 μm emission of RCW 36 in a dense filamentary cloud. Methods. [C II] observations of RCW 36, covering an area of ~30′ × 30′, were carried out with a Fabry-Pérot 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 [C II] intensity with the emission from the large grains and polycyclic aromatic hydrocarbon (PAH). Results. The [C II] 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 [C II]–160 μm relation for RCW 36 shows a higher brightness ratio of [C II]/160 μm than that for RCW 38, while the [C II]–9 μm 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 [C II] emission is spatially well correlated with PAH and cold dust emissions. This means that the observed [C II] emission predominantly comes from photo-dissociation regions. Moreover, the L[C II]∕LFIR ratio shows large variation (10−2–10−3), as compared with the L[C II]/LPAH ratio. In view of the observed tight correlation between L[C II]∕LFIR and the optical depth at λ = 160 μm, the large variation in L[C II]∕LFIR can be simply explained by the geometrical effect, that is, LFIR has contributions from the entire dust-cloud column along the line of sight, while L[C II] has contributions from far-UV illuminated cloud surfaces. Based on the picture of the geometry effect, the enhanced brightness ratio of [C II]/160 μm 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.