Slowing down atomic diffusion in subdwarf B stars: mass loss or turbulence?

TL;DR

This study investigates whether mass loss or turbulence explains chemical peculiarities in subdwarf B stars by analyzing their seismic properties and matching observed pulsations with theoretical models.

Contribution

First to compute seismic properties of sdB stars with atomic diffusion included during evolution, assessing effects of mass loss and turbulence on pulsations and surface abundances.

Findings

Mass-loss rates needed for observed He abundances are inconsistent with pulsations.

Weak turbulence can explain He anomalies and still allow pulsations.

Pulsations tightly constrain the turbulence models in sdB stars.

Abstract

Subdwarf B stars show chemical peculiarities that cannot be explained by diffusion theory alone. Both mass loss and turbulence have been invoked to slow down atomic diffusion in order to match observed abundances. The fact that some sdB stars show pulsations gives upper limits on the amount of mass loss and turbulent mixing allowed. Consequently, non-adiabatic asteroseismology has the potential to decide which process is responsible for the abundance anomalies. We compute for the first time seismic properties of sdB models with atomic diffusion included consistently during the stellar evolution. The diffusion equations with radiative forces are solved for H, He, C, N, O, Ne, Mg, Fe and Ni. We examine the effects of various mass-loss rates and mixed surface masses on the abundances and mode stability. It is shown that the mass-loss rates needed to simulate the observed He abundances (10^{-14}<=Mdot [Msun/yr]<=10^{-13}) are not consistent with observed pulsations. We find that for pulsations to be driven the rates should be Mdot<=10^{-15} Msun/yr. On the other hand, weak turbulent mixing of the outer 10^{-6} Msun can explain the He abundance anomalies while still allowing pulsations to be driven. The origin of the turbulence remains unknown but the presence of pulsations gives tight constraints on the underlying turbulence model.