Viscous and centrifugal instabilities of massive stars

TL;DR

This paper investigates how centrifugal forces, viscous effects, and angular momentum transport can induce pulsations in massive rotating stars, revealing potential new mechanisms for stellar variability.

Contribution

It introduces a comprehensive analysis of viscous and centrifugal instabilities in massive stars, including derivation of oscillation equations and modeling of realistic stellar scenarios.

Findings

Overstable radial modes are predicted in most of the star's life.

Peak growth rates occur during the Hertzsprung-Russell gap crossing.

Viscous instability may drive pulsations and relate to luminous blue variable star instabilities.

Abstract

Massive stars exhibit a variety of instabilities, many of which are poorly understood. We explore instabilities induced by centrifugal forces and angular momentum transport in massive rotating stars. First, we derive and numerically solve linearized oscillation equations for adiabatic radial modes in polytropic stellar models. In the presence of differential rotation, we show that centrifugal and Coriolis forces combined with viscous angular momentum transport can excite stellar pulsation modes, under both low- or high-viscosity conditions. In the low-viscosity limit, which is common in real stars, we demonstrate how to compute mode growth/damping rates via a work integral. Finally, we build realistic rotating $30\,M_\odot$ star models and show that overstable (growing) radial modes are predicted to exist for most of the star's life, in the absence of non-adiabatic effects. Peak growth rates are predicted to occur while the star is crossing the Hertzsprung-Russell gap, though non-adiabatic damping may dominate over viscous driving, depending on the effective viscosity produced by convective and/or magnetic torques. Viscous instability could be a new mechanism to drive massive star pulsations and is possibly related to instabilities of luminous blue variable stars.