Horizontal Branch Models as a Test of Mixing on the RGB

Abstract

We discuss the impact of mixing and rotation along the red-giant branch (RGB) on the properties of horizontal-branch (HB) stars with emphasis on two problems: the nature of the unexpected blue HB population in the metal-rich globular clusters (GCs) NGC 6388 and NGC 6441 and the cause of the low gravities in the blue HB stars. New stellar models indicate that the sloped HBs in NGC 6388 and NGC 6441 might arise from a spread in metallicity, implying that these GCs may be metal-rich analogues of omega Cen. The low gravity problem can be largely explained by the radiative levitation of Fe in the atmospheres of the blue HB stars. We show that the onset of radiative levitation and the drop in HB rotation velocities at Teff ~ 11,000 K coincide with the disappearance of surface convection. The low rotation velocities of the hotter HB stars may be due to the spin down of the surface layers by a weak stellar wind induced by the radiative levitation of Fe. We conclude that the impact of mixing and rotation on the HB remains to be clearly established.