BG equilibrium temperature as a function of the radiation field intensity. The hatched area corresponds to the range of equilibrium temperatures span by the entire BGs size distribution. The solid lines represent the equilibrium temperature for the most abundant size bin. The hatched area and the solid line are only plotted for the values of χ/χ₀ that are given by Fig. 12. Black is for LamC grains, red is for aSil grains as described in Compiègne et al. (2011). The radiation field spectral shape is that mention in the text with a) no extinction, b) A(FUV) = 1 mag and the Lyman continuum photons removed.