Maps derived by MCMC fitting with a constant β = 1.8, using Herschel 250–500 μm maps at 40″ resolution. Background is not subtracted from the maps. (Left) Optical depth τ₂₅₀ with contours of column density N_H2 derived from ¹³CO observations of Russeil et al. (2003) drawn at levels 4, 3, 2, and 1 × 10²¹ cm^-2. (Right) Colour temperature T with contours of the τ₂₅₀ map drawn at levels 0.0035, 0.0029, and 0.0017. Sources B-1, B-2, B-3, and B-4 are marked with crosses (Getsources coordinates) and circles (WISE coordinates).