The Effect of Rotation on Triggering S Doradus Instabilities in Luminous Blue Variables

TL;DR

This study investigates how stellar rotation influences the triggering of S Doradus variability in luminous blue variables, revealing that rotation enhances instability, mass loss, and the formation of observed nebular structures.

Contribution

It provides a detailed analysis of the impact of rotation on LBV instabilities and predicts observable phenomena like dense disks and bipolar outflows.

Findings

Rotation increases the likelihood of LBV instability at both equator and poles.

Models predict dense equatorial disks or rings around LBVs.

Rotation leads to enhanced mass loss and bipolar outflows.

Abstract

Luminous blue variables are an intermediate stage in the evolution of high-mass stars characterized by extreme mass loss and substantial variability. The stars show large irregular episodic variations on timescales of years to decades in these stars' effective temperatures (called "S Dor variations"). Observations show that these variations are triggered when the stars are in a well-defined strip in the HRD that corresponds to the Modified Eddington Limit, where the atmospheric radiation pressure almost balances gravity. In this work we consider the role that rotation plays in the instability that leads to the triggering of S Dor variations in luminous post-main sequence LBVs. We adopt the existing instability criterion that the effective surface gravity is reduced to 10% of the Newtonian gravity due to radiation pressure in the atmosphere of non-rotating stars. We then specifically describe how rotation impacts this instability. By carrying out numerical simulations of model LBVs at both solar and sub-solar metallicities, we confirm that most LBVs should be unstable at both the equator and the poles, and that rotation exacerbates this effect; some models also produce enhanced mass loss at the pole or equator. Our numerical models also predict dense equatorial disks or rings and high-velocity bipolar outflows, in agreement with existing observations of LBV circumstellar nebulae.