Abundances anomalies and meridional circulation in horizontal branch stars
Département de Physique, Université de Montréal, Montréal, PQ, H3C 3J7, Canada e-mail: [paulchar;michaudg;jacques.richer]@umontreal.ca
2 LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
Accepted: 16 February 2009
Context. Photospheric chemical abundances on the horizontal branch (HB) show some striking variations with effective temperature . The most straightforward explanation is that these anomalies develop through diffusion processes, in particular gravitational settling and radiative levitation. However, the abrupt disappearance of strong abundance anomalies as one moves below about 11 000 K on the HB suggests that another factor plays an important role.
Aims. We test an extension to the HB of the diffusion model for main-sequence HgMn stars, where strong anomalies can only develop in the slower rotators. In these rotators the gravitational settling of helium leads to the disappearance of its superficial convection zone, so that chemical separation by radiative levitation can occur all the way to the photosphere.
Methods. More specifically, we calculate the critical rotational velocity at which He settling is prevented by rotationally-induced meridional circulation, in a suite of stellar models spanning the zero-age HB. Helium settling serves as the measure of the atomic diffusion of all species.
Results. Our abundance evolution calculations show that, for models with Teff less than about 11 500 K, corresponding to stars typically observed with the same metal composition as giants, meridional circulation is efficient enough to suppress He settling for rotational velocities, in good agreement with observed values. Once the meridional circulation profile of a star rotating as a near rigid body has been adopted, no adjustable parameter is involved.
Conclusions. The Teff dependence of abundance anomalies observed on the HB can be explained by atomic diffusion transport if one introduces the competition of meridional circulation with the observed Teff dependence of rotation velocity of HB stars.
Key words: diffusion / stars: abundances / stars: horizontal branch / stars: chemically peculiar / stars : interiors
© ESO, 2009