Dust spectral indices as a function of temperature. a) β_FIR versus T_d for the whole 24° × 8° region. b) β_FIR versus T_d for points where τ₃₅₃ ≥ 4 × 10^-4. c) β_mm versus T_d for points where τ₃₅₃ ≥ 4 × 10^-4. The triangle and square in panels b) and c) indicate the values obtained by fitting the emissivities predicted by the TLS model (Paradis et al. 2011) for T_d = 17 and 25 K (see Sect. 6.1.2). The colour scale is logarithmic and it represents the density of points. The contours show the densities for the cumulated fractions, given by the values in each panel, of the data points, from red to yellow.