Abundances anomalies and meridional circulation in horizontal branch stars

Abstract

Context. Photospheric chemical abundances on the horizontal branch (HB) show some striking variations with effective temperature (T eff ). 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 T eff 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 T eff 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 T eff dependence of rotation velocity of HB stars.

Horizontal branch evolution, metallicity, and sdB stars

“…The rotation velocities are shown on the lower panel of Fig. 14. They are all smaller than 10 km s −1 , which is too small to interfere with atomic diffusion at those T eff according to Quievy et al (2009). The uncertainty on T eff is also shown in that panel.…”

“…The overabundances are explained by atomic diffusion driven by radiative accelerations in stars with T eff > 11 000 K, and the sudden break in anomalies at 11 000 K was shown to be related to observed rotation velocities (Quievy et al 2009). Given the relatively large observational error bars, the anomalies appeared compatible with a simple diffusion model involving only one parameter, the mass of the outer region mixed by turbulence.…”

“…In Fig. 4 of Quievy et al (2009), the T eff interval from 10 000 to 12 000 K is the interval over which anomalies are expected in only a fraction of the stars. In M 3, M 13, and NGC 288 (left panel), all stars with T eff > 11 000 K rotate slowly and have [Fe/H] within the range expected over most of the lifetime of a cluster with the original metallicity of the cluster.…”

“…According to Fig. 4 of Quievy et al (2009), the 13 500 and 15 000 K stars with rotation velocities of 30-40 km s −1 may or may not rotate fast enough for atomic diffusion to be affected, depending on whether meridional circulation penetrates into the surface convection zone or not. Because both stars may have a solar metallicity to start with, little can be inferred from their relatively large rotation velocities.…”

AmFm and lithium gap stars

“…Hence, when building stellar models, one should be cautious when replacing meridional circulation, which is an advective process, by turbulent mixing. A careful study of the atomic diffusion of metals within the context of meridional circulation, such as the one carried out for helium in Quievy et al (2009), could help determine the implications of such an approximation. This could perhaps lead to asteroseismic tests which could distinguish between models using rotationally induced turbulence (Talon et al 2006) and those using meridional circulation (Charbonneau & Michaud 1988), which are both used to explain the disappearance of the AmFm character for rotation velocities greater than 100 km s −1 .…”