Pulsations as a Driver for LBV Variability

TL;DR

This study explores whether pulsations can trigger LBV star outbursts, finding that while pulsations cause significant temperature and luminosity variations, they alone do not eject stellar material, implying other mechanisms are involved.

Contribution

It provides the first detailed nonlinear hydrodynamic models of LBV pulsations across a wide mass range, linking pulsation behavior to observed variability.

Findings

Pulsations cause large temperature and luminosity variations.

Pulsations alone do not eject stellar outer layers.

Models predict long-period pulsations with high surface velocities.

Abstract

Among the most spectacular variable stars are the Luminous Blue Variables (LBVs), which can show three types of variability. The LBV phase of evolution is poorly understood, and the driving mechanisms for the variability are not known. The most common type of variability, the S Dor instability, occurs on timescales of tens of years. During an S Dor outburst, the visual magnitude of the star increases, while the bolometric magnitude stays approximately constant. In this work, we investigate pulsation as a possible trigger for the S Dor type outbursts. We calculate the pulsations of envelope models using a nonlinear hydrodynamics code including a time-dependent convection treatment. We initialize the pulsation in the hydrodynamic model based on linear non-adiabatic calculations. Pulsation properties for a full grid of models from 20 to 85 M$_{\odot}$ were calculated, and in this paper we focus on the few models that show either long-period pulsations or outburst-like behaviour, with photospheric radial velocities reaching 70-80 km/s. At the present time, our models cannot follow mass loss, so once the outburst event begins, our simulations are terminated. Our results show that pulsations alone are not able to drive enough surface expansion to eject the outer layers. However, the outbursts and long-period pulsations discussed here produce large variations in effective temperature and luminosity, which are expected to produce large variations in the radiatively driven mass-loss rates.