Gas surface density Σ in a protoplanetary disk as a function of distance and time. The surface density is plotted in intervals of 2  ×  10⁴ years. The uppermost line shows a state shortly after the beginning of disk evolution. The lowermost line is the profile when the calculation is stopped. The two panels differ from each other by the boundary condition that is used at the inner edge of the disk at 0.1 AU, as described in the text. All other parameters are identical and given as M_∗ = 1   M_⊙, α = 7  ×  10^-3 and Σ₀ = 200 g/cm².