Testing the effects of opacity and the chemical mixture on the excitation of pulsations in B stars of the Magellanic Clouds

TL;DR

This study investigates why B-type pulsators are observed in the Magellanic Clouds despite theoretical expectations, focusing on stellar chemistry and opacity physics, and suggests Ni opacity plays a crucial role.

Contribution

It provides new insights into the effects of chemical composition and opacity enhancements on B star pulsations in low-metallicity environments.

Findings

Chemical mixture of B stars in the SMC cannot explain pulsations.

Opacity increases at Ni contribution temperature can drive pulsations.

Ni opacity calculations are crucial for understanding observed pulsators.

Abstract

The B-type pulsators known as \beta Cephei and Slowly Pulsating B (SPB) stars present pulsations driven by the \kappa mechanism, which operates thanks to an opacity bump due to the iron group elements. In low-metallicity environments such as the Magellanic Clouds, \beta Cep and SPB pulsations are not expected. Nevertheless, recent observations show evidence for the presence of B-type pulsator candidates in both galaxies. We seek an explanation for the excitation of \beta Cep and SPB modes in those galaxies by examining basic input physics in stellar modelling: i) the specific metal mixture of B-type stars in the Magellanic Clouds; ii) the role of a potential underestimation of stellar opacities. We first derive the present-day chemical mixtures of B-type stars in the Magellanic Clouds. Then, we compute stellar models for that metal mixture and perform a non-adiabatic analysis of these models. In a second approach, we simulate parametric enhancements of stellar opacities due to different iron group elements. We then study their effects in models of B stars and their stability. We find that adopting a representative chemical mixture of B stars in the Small Magellanic Cloud cannot explain the presence of B-type pulsators there. An increase of the opacity in the region of the iron-group bump could drive B-type pulsations, but only if this increase occurs at the temperature corresponding to the maximum contribution of Ni to this opacity bump. We recommend an accurate computation of Ni opacity to understand B-type pulsators in the Small Magellanic Cloud, as well as the frequency domain observed in some Galactic hybrid \beta Cep-SPB stars.