  There is now considerable evidence that horizontal branch (HB) stars hotter
than about 11,500 K experience an enormous enhancement of their photospheric
iron abundance due to radiative levitation. In globular clusters, the
photospheric iron abundance can reach values of [Fe/H] ~ +0.3, or up to two
orders of magnitude higher than the cluster iron abundance. Model atmospheres
which take into account the iron overabundance are needed for understanding the
appearance of the HB in globular cluster color-magnitude diagrams (CMDs), for
the derivation of accurate luminosities, gravities and masses, and for the
ultraviolet spectral synthesis of old, metal-poor stellar populations.
