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.
Highlights
A large number of early B-type stars show oscillations, gravity as well as pressure modes, triggered by the κ-mechanism involving iron-peak elements (Daszynska-Daszkiewicz et al 2017, and references therein)
The stellar evolution had to be stopped at 0.95 Myr – long before the age of ν Eri – because the numerical fraction of Fe in the superficial convection zone became too large for the test atom approximation to remain valid
Our results showed that radiative diffusion could create an opacity enhancement in the Z-bump, provided fingering mixing is at play to extend the overabundances over the whole Z-bump
Summary
A large number of early B-type stars show oscillations, gravity as well as pressure modes, triggered by the κ-mechanism involving iron-peak elements (Daszynska-Daszkiewicz et al 2017, and references therein). In a pioneering work, Bourge & Alecian (2006) suggested that this overabundance could result from radiative diffusion inside the star This result, was based on a rather simple diffusion model that ignored the feedback of the abundances on the radiative accelerations. As a first step, to focus on models representing the star ν Eri, which is a bright early-B star at the end of its main sequence (MS) life, with an age between 15.7 and 18.9 Myr (Pamyatnykh et al 2004, Table 1) Precise asteroseismology of this star has been performed using ground-based observations (Pamyatnykh et al 2004) as well as space missions (BRITE; Daszynska-Daszkiewicz et al 2017, hereafter DPW17)
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