Comparison between wind velocity, v_∞, and the degree of limb brightening of the Hα and [N II] spectral lines, defined as the ratio of the peak brightness to that at lines of sight near the star (r = 0). The boundary between D-type and R-type ionization fronts is at v_∞ ≈ 22 km s^-1. The red lines are for Hα and the blue for [N II]. Panel a) shows the raw simulation data, and panel b) when smoothed to a spatial resolution of 1 arcsec. The lines in panel b) follow the legend in panel a).