a) Column density map of Vela C derived from Herschel observations at 160, 250, 350 and 500 μm with a resolution of 36″ (see Hill et al. 2011). The peak column density is  ~10²³ cm^-2, although the column density range on this image is 2 × 10²¹–5 × 10²² cm^-2 to emphasize both the high density filamentary structure and the more diffuse dust around. The “low density areas” are minima at a few 10²¹ cm^-2 on the East and West sides of the H ii region. The RCW 36 area is indicated by a dashed box. White contours represent the radio continuum emission at 5 GHz. b) Zoom on the RCW 36 bipolar nebula as seen by Herschel at 70, 160 and 250 μm (in blue, green, red, respectively). c) Three-colour image of RCW 36 showing areas with column density from 2 × 10²¹ to 5 × 10²² cm^-2 (red), temperature from 14 to 40 K (green) and Hα (blue). Hα emission is within a white ellipse, with a major axis oriented East-West. The column density exhibits a ring-like structure (white dashed-dotted ellipse), that is part of the Centre-Ridge. The hourglass or bipolar shape is suggested with a bi-lobed curve shown in blue.