The Enigma of B-type Pulsators in the SMC

TL;DR

This paper investigates the unexpected presence of B-type pulsators in the SMC, challenging existing theories by analyzing stellar models with revised, SMC-specific chemical compositions and metallicities.

Contribution

It provides a stability analysis of B-type stellar models using a revised chemical composition tailored to the SMC, addressing discrepancies between theory and observations.

Findings

Detection of SPB and β Cep candidates in the SMC despite low metallicity.

Revised chemical composition models show different stability properties.

Challenges previous assumptions about chemical uniformity and scaling in the SMC.

Abstract

Since the early nineties it is accepted that the excitation mechanism of B-type pulsators on the main sequence is due to the opacity peak in the iron-group elements at $T\approx 200,000$ K. The Fe content plays then a major role in the excitation of $\beta$ Cep and SPB pulsations. While theoretical non-adiabatic computations predict no $\beta$ Cep pulsators and only a small number of SPBs for low metallicity environments such as that of the Magellanic Clouds (MCs), recent variability surveys of B stars in the SMC have reported the detection of a significant number of SPB and $\beta$ Cep candidates. Since the SMC is the metal poorest (Z$\approx$0.001-0.004) of the MCs, it constitutes an interesting object for investigating the disagreement between theory and observations. We approach the problem by calling into question some of the hypotheses made in previous studies: given the different chemical evolution of the SMC compared with our local galactic environment, is it appropriate to describe the chemical composition of SMC B stars by scaling the solar mixture to lower $Z$? Is that composition uniform in space and time? In this paper we present the results of a stability analysis of B-type stellar models computed with a revised chemical composition and metallicity specific to the SMC.