Gas surface density for a viscous disc with external photo-evaporation from neighbouring stars. The gas surface density is shown as a function of disc radius and time. A variety of parameter configurations is used for this study: Along the x-axis of this figure, α is varied from α = 10⁻⁴ to α = 10⁻², whereas on the y-axis, different sets of disc masses and radii are used, as indicated in the panels. The colour coding gives the time evolution. The discs with 0.01 solar masses are dispersed by external photo-evaporation within the first 0.5 Myr and thus show now evolution in the plot.