Horizontal branch evolution, metallicity, and sdB stars^⋆

¹ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
² Département de Physique, Université de Montréal, Montréal, PQ, H3C 3J7, Canada
e-mail: michaudg@astro.umontreal.ca; jacques.richer@umontreal.ca
³ Université Montpellier II, GRAAL, CNRS, UMR 5024, place Eugène Bataillon, 34095 Montpellier, France
e-mail: Olivier.Richard@univ-montp2.fr

Received: 25 October 2010
Accepted: 8 February 2011

Abstract

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 Z₀ = 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 < 37   000 K and for HB 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.