Metallicity dependence of turbulent pressure and macroturbulence in stellar envelopes

TL;DR

This study investigates how metallicity influences turbulent pressure and macroturbulence in stellar envelopes, revealing metallicity-dependent variations that support a link between turbulent pressure and observed macroturbulence in hot massive stars.

Contribution

It provides the first detailed analysis of metallicity effects on turbulent pressure in stellar models across a wide mass range, connecting these effects to macroturbulence observations.

Findings

Turbulent pressure is lower in hot, high-luminosity models with Magellanic Cloud metallicities compared to solar metallicity.

In low-metallicity models in the cool HR diagram region, turbulent pressure is not reduced.

Observations of Magellanic Cloud stars support a connection between macroturbulence and turbulent pressure.

Abstract

Macroturbulence, introduced as a fudge to reproduce the width and shape of stellar absorption lines, reflects gas motions in stellar atmospheres. While in cool stars, it is thought to be caused by convection zones immediately beneath the stellar surface, the origin of macroturbulence in hot stars is still under discussion. Recent works established a correlation between the turbulent-to-total pressure ratio inside the envelope of stellar models and the macroturbulent velocities observed in corresponding Galactic stars. To probe this connection further, we evaluated the turbulent pressure that arises in the envelope convective zones of stellar models in the mass range 1-125 Msun based on the mixing-length theory and computed for metallicities of the Large and Small Magellanic Cloud. We find that the turbulent pressure contributions in models with these metallicities located in the hot high-luminosity part of the Hertzsprung-Russel (HR) diagram is lower than in similar models with solar metallicity, whereas the turbulent pressure in low-metallicity models populating the cool part of the HR-diagram is not reduced. Based on our models, we find that the currently available observations of hot massive stars in the Magellanic Clouds appear to support a connection between macroturbulence and the turbulent pressure in stellar envelopes. Multidimensional simulations of sub-surface convection zones and a larger number of high-quality observations are necessary to test this idea more rigorously.