Diffusion in the Atmospheres of Blue Horizontal-Branch Stars.

We investigate the effects of diffusion in the atmospheres of hot horizontal-branch stars using a model atmosphere code including diffusion self-consistently. Equilibrium stratifications (i.e., for which the diffusion velocity equals zero in each layer) are computed for models of effective temperatures between 10,000 and 25,000 K. The stratified models provide much better agreement with many observational features [jump in the (u, u-y) color-magnitude diagram, gaps, lower spectroscopic gravities] in comparison with classical horizontal-branch models. The observed abundance anomalies are also consistent with the amounts that can be supported in the atmospheres.