Evolution towards the critical limit and the origin of Be stars

TL;DR

This study models the rotational evolution of single stars to understand how they reach critical rotation speeds, shedding light on the origin of Be stars and how metallicity influences this process.

Contribution

It provides a comprehensive set of stellar models across various masses, metallicities, and rotation rates to analyze the mechanisms driving surface acceleration during the Main Sequence.

Findings

Higher frequency of fast rotators predicted in clusters aged 10-25 Myr.

Metallicity significantly affects the ability of stars to reach critical rotation.

Models align with observed ages of most Be stars.

Abstract

We determine which are the mechanisms accelerating the surface of single stars during the Main Sequence evolution. We have computed 112 stellar models of four different initial masses between 3 and 60 M_sun, at four different metallicities between 0 and 0.020, and with seven different values of the ratio Omega/Omega_crit between 0.1 and 0.99. For all the models, computations were performed until either the end of the Main Sequence evolution or the reaching of the critical limit. The evolution of surface velocities during the Main Sequence lifetime results from an interplay between meridional circulation (bringing angular momentum to the surface) and mass loss by stellar winds (removing it). The dependence on metallicity of these two mechanisms plays a key role in determining for each metallicity, a limiting range of initial masses (spectral types) for stars able to reach or at least approach the critical limit. Present models predict a higher frequency of fast rotating stars in clusters with ages between 10 and 25 Myr. This is the range of ages where most of Be stars are observed.