Shell width for column density maps. Top: angle-averaged col. density over radius for run 3S1-hr at 12.56 Myr. From such plots, the shell width has been determined as the radial range where the column density is at least five per cent greater than at large radii (undisturbed gas). The shell width determined in this way is shown in the middle plot as a function of time, and in the bottom plot as a function of outer radius. Black pluses are for run 3S1, red stars for run 3S1-hr. The average shell width does not depend significantly on resolution.