Variation of a) mid-plane temperature and b) pressure with distance from the star at different evolutionary epochs (0.2 Ma and from 0.5 Ma to 3 Ma in steps of 0.5 Ma) in a model for the evolution of the solar nebula (one-zone α-model). They define the outer boundary conditions for a planetesimal embedded in an accretion disk. In the left part the regions are indicated where the disappearance of an important absorber results in an extended region of nearly constant mid-plane temperature.