The Stability of Massive Main Sequence Stars as a Function of Metallicity

TL;DR

This study examines how metallicity influences the pulsational stability of massive main sequence stars, revealing that low-metallicity stars are generally stable due to convective damping, while higher-metallicity stars can be pulsationally unstable.

Contribution

It introduces a formulation of convective damping based on simulations and assesses its impact on the stability of massive stars across different metallicities.

Findings

Low-metallicity stars (Z <~ 2 x 10^-3) are stable against pulsations due to convective damping.

High-metallicity stars (Z >~ 2 x 10^-3) can be pulsationally unstable because of the iron-bump opacity mechanism.

Convective damping plays a crucial role in mode stability, especially for short-period oscillations.

Abstract

We investigate the pulsational stability of massive (M >~ 120 Msun) main sequence stars of a range of metallicities, including primordial, Population III stars. We include a formulation of convective damping motivated by numerical simulations of the interaction between convection and periodic shear flows. We find that convective viscosity is likely strong enough to stabilize radial pulsations whenever nuclear-burning (the epsilon-mechanism) is the dominant source of driving. This suggests that massive main sequence stars with Z <~ 2 x 10^-3 are pulsationally stable and are unlikely to experience pulsation-driven mass loss on the main sequence. These conclusions are, however, sensitive to the form of the convective viscosity and highlight the need for further high-resolution simulations of the convection-oscillation interaction. For more metal-rich stars (Z >~ 2 x 10^-3), the dominant pulsational driving arises due to the kappa-mechanism arising from the iron-bump in opacity and is strong enough to overcome convective damping. Our results highlight that even for oscillations with periods a few orders of magnitude shorter than the outer convective turnover time, the "frozen-in" approximation for the convection-oscillation interaction is inappropriate, and convective damping should be taken into account when assessing mode stability.