Vibrational Instability of Metal-Poor Low-Mass Main-Sequence Stars

TL;DR

This study reveals that metal-poor low-mass main-sequence stars exhibit vibrational instabilities in certain g-modes due to nuclear and convective mechanisms, influenced by their metallicity and structural properties.

Contribution

It demonstrates the occurrence of g-mode instabilities in metal-poor low-mass stars caused by the $$-mechanism and convective blocking, highlighting differences from Pop I stars.

Findings

Low-degree low-order g-modes are unstable due to the $$-mechanism.

High order g-modes are unstable via convective blocking in metal-poor stars.

Instability occurs in stars around 1 solar mass with higher effective temperature and luminosity.

Abstract

We find that low-degree low-order g-modes become unstable in metal-poor low-mass stars due to the $\varepsilon$-mechanism of the pp-chain. Since the outer convection zone of these stars is limited only to the very outer layers, the uncertainty in the treatment of convection does not affect the result significantly. The decrease in metallicity leads to decrease in opacity and hence increase in luminosity of a star. This makes the star compact and results in decrease in the density contrast, which is favorable to the $\varepsilon$-mechanism instability. We find also instability for high order g-modes of metal-poor low-mass stars by the convective blocking mechanism. Since the effective temperature and the luminosity of metal-poor stars are significantly higher than those of Pop I stars, the stars showing $\gamma$ Dor-type pulsation are substantially less massive than in the case of Pop I stars. We demonstrate that those modes are unstable for about $1\,M_\odot$ stars in the metal-poor case.