Spectral opacities κ(ρ,T,ν). Each data point represents a set of spectral opacities for a given density and temperature. The table has been compiled from three different opacity collections. At low temperatures (below 1500 K), the dust opacities are from Semenov et al. (2003) and are completed by the dust grains opacities of Draine (2003b) at the high frequency end (red circles). For temperatures between 1500 and 3200 K, the opacities for molecular gas based on the Ferguson et al. (2005) calculations are used (green crosses). Finally, for temperatures above 3200 K, the atomic gas opacities are from the OP project (Badnell et al. 2005, blue squares). Two coloured areas, grey and yellow, show the approximate range of densities and temperatures typically reached during the first and second stages of the collapse of a cloud core, respectively.