Role of atomic diffusion in the opacity enhancement inside B-type stars

Abstract

Context. The pulsation frequencies of early B-type stars cannot be reproduced using stellar models with homogeneous abundances. A suitable match requires a dedicated enhancement of the opacity in the layers where its main contributors are the iron-peak elements (the so-called Z-bump), which trigger the oscillations in these stars. Aim. Our aim is to test whether the abundance stratification induced by atomic diffusion in these stellar layers is able to modify the local opacity as needed to account for the asteroseismic observations. Methods. Models representing a typical pulsating B-star were evolved during the main sequence using the Toulouse–Geneva evolution code in an improved version. The migration of the chemicals involves radiative accelerations, which were computed with the single-valued parameter method, and fingering mixing with parameters constrained by three-dimensional simulations. The possible effect of mass-loss was also considered. Results. We show that atomic diffusion modifies the abundance profiles inside the star, leading to an overabundance of the iron-peak elements in the upper part of the envelope. The opacities may become as high as required, provided that fingering mixing, which extends the size of the overabundance zone, is taken into account. A zero-flux of the elements at the surface leading to unphysical accumulations, mass-loss is also required to evolve the model until the end of the main sequence.