Horizontal branch evolution, metallicity, and sdB stars

Context. Abundance anomalies have been observed in field sdB stars and in nearly all horizontal branch (HB) stars of globular clusters with T eff > 11 000 K, whatever be the cluster metallicity. Aims. We aim to determine the abundance variations that are expected in sdB stars and in HB stars of metallicities Z ≥ 10- 4 and investigate what the observed abundances teach us about hydrodynamical processes competing with atomic diffusion. Methods. Complete stellar evolution models, including the effects of atomic diffusion and radiative acceleration, have been computed from the zero age main-sequence for metallicities of Z 0 = 0.0001,0.001,0.004 and 0.02. On the HB the masses were selected to cover the T eff interval from 7000 to 37 000 K. Some 60 evolutionary HB models were calculated. The calculations of surface abundance anomalies during the horizontal branch depend on one parameter, the surface mixed mass. Results. For sdB stars with T eff 11 000 K in all observed clusters, independent of metallicity, we found that most observed abundance anomalies (even up to ∼× 200) were compatible, within error bars, with expected abundances. A mixed mass of ∼10 ―7 M ⊙ was determined by comparison with observations. Conclusions. Observations of globular cluster HB stars with T eff > 11 000 K and of sdB stars with T eff < 37 000 K suggest that most observed abundance anomalies can be explained by element separation driven by radiative acceleration occuring at a mass fraction of ∼10 ―7 M ⊙ . Mass loss or turbulence appear to limit the separation between 10 ―7 M ⊙ and the surface.