How Efficient is Rotational Mixing in Massive Stars ?

TL;DR

This study evaluates the efficiency of rotational mixing in massive stars by comparing stellar evolution models with detailed observational data from the Magellanic Clouds, highlighting the roles of magnetic fields and initial chemical compositions.

Contribution

The paper introduces a grid of stellar models that incorporate magnetic fields and varying initial conditions, providing new insights into mixing processes in massive stars.

Findings

Magnetic fields are crucial for angular momentum transport.

Chemical mixing should be neglected to match observations.

Initial abundances significantly affect model calibration at low metallicities.

Abstract

The VLT-Flames Survey for Massive Stars (Evans05,Evans06) provides recise measurements of rotational velocities and nitrogen surface abundances of massive stars in the Magellanic Clouds. Specifically, for the first time, such abundances have been estimated for stars with significant rotational velocities. This extraordinary data set gives us the unique possibility to calibrate rotationally and magnetically induced mixing processes. Therefore, we have computed a grid of stellar evolution models varying in mass, initial rotational velocity and chemical composition. In our models we find that although magnetic fields generated by the Spruit-Taylor dynamo are essential to understand the internal angular momentum transport (and hence the rotational behavior), the corresponding chemical mixing must be neglected to reproduce the observations. Further we show that for low metallicities detailed initial abundances are of prime importance, as solar-scaled abundances may result in significant calibration errors.