Black-and-white version of Fig. 4. Gray-coded concentrations in two HB stars of the same metallicity after 25 Myr on the HB. Left panel with a T_eff of 14 000 K (0.59    M_⊙) and right panel of 30 100 K (0.51    M_⊙). The scale of the radius is linear, but the logarithmic value of the mass coordinate above a number of points, log ΔM/M_∗, is shown to the left of the horizontal black line. The concentration scale is given in the right insert. Small circles near the top of the left panel mark the extent of the surface convection zone, while similar circles near the center of both models mark the central convection zone. The small inset in between the two panels shows the high T_eff star on the radius scale of the low T_eff star. For  −7 < log ΔM/M_∗ <  −4 the concentration is quite different for many species. It is surprisingly so for C and O for log ΔM/M_∗ >  −2.