Iron and nickel diffusion in subdwarf B stars

TL;DR

This study models the diffusion of iron and nickel in subdwarf B stars to assess their impact on stellar pulsations, revealing that nickel accumulates similarly to iron and must be included in models for accurate seismology.

Contribution

It introduces detailed time-dependent diffusion calculations for multiple elements, highlighting the importance of including both iron and nickel in sdB star models.

Findings

Nickel accumulates to levels comparable to iron in sdB envelopes.

Including diffusion of both metals is crucial for accurate pulsation modeling.

The role of other iron-group elements remains to be explored.

Abstract

Pulsations in subdwarf B stars are attributed to radiative levitation of iron-group elements in the stellar envelope. Until now, only iron diffusion is accounted for in stellar models used for sdB seismology. However, nickel has also been suggested as a contributor to the opacity bump that drives the pulsation modes. Stellar models including time-dependent atomic diffusion, as we compute here, are needed to evaluate the importance of different iron-group elements for mode driving. We perform detailed calculations of radiative accelerations of H, He, C, N, O, Ne, Mg, Fe and Ni and include these in Burgers' diffusion equations. We compute the evolution and non-adiabatic pulsations of a typical subdwarf B star. We show that, despite its lower initial abundance, nickel accumulates to comparable mass fractions as iron in the sdB envelope. For accurate determination of pulsation frequencies and mode instability, it is essential that diffusion of both metals are included in stellar models. The role of other iron-group elements remain to be evaluated.