ZAMS rotational velocities of Be/Oe stars and LGRBs progenitors in the Magellanic Clouds

TL;DR

This study investigates the rotational velocities of Be/Oe stars in the Magellanic Clouds to test models predicting their role as progenitors of long gamma-ray bursts, revealing higher rotation speeds at low metallicity.

Contribution

It provides the first distribution of ZAMS rotational velocities of Be stars in the Magellanic Clouds and compares observed data with theoretical models of LGRB progenitors.

Findings

Be stars in the Magellanic Clouds rotate faster than in the Milky Way.

The distribution of ZAMS rotational velocities varies with mass and metallicity.

Observed rates of LGRBs are consistent with model predictions based on stellar rotation.

Abstract

The Large and Small Magellanic Clouds are priviledged environments to perform tests of theoretical predictions at low metallicity on rotational velocities and stellar evolution. According to theoretical predictions, the rotational velocities of B-type stars are expected to be higher in low metallicity (LMC/SMC) than in high metallicity (MW) environments. To verify the models, we observed with the VLT-FLAMES 523 B and Be stars, which form, at the moment, the largest observed sample of these kind of objects in the MCs. We first determined the stellar fundamental parameters and we found that B and Be stars rotate faster in the MCs than in the MW. We also determined the first distribution of the average ZAMS rotational velocities versus the mass of Be stars. These results indicate that the appearance of Be stars is mass-, metallicity-, stellar evolution-, and star-formation regions-dependant. Moreover, the recent models of Long Gamma Ray Bursts progenitors foresee possible LGRBs progenitors at the SMC's metallicity. We confront these models with the observed (ZAMS rotational velocities, masses) distributions of the fastest rotators (Be and Oe stars) in our sample. Furthermore, we compare the corresponding predicted rates from our study with observed rates of LGRBs.