Review about populations of Be stars: stellar evolution of extreme stars

TL;DR

This review summarizes recent advances in understanding Be stars, focusing on their stellar evolution, circumstellar disks, and the impact of various physical factors, highlighting recent observational techniques and future prospects.

Contribution

It provides a comprehensive overview of recent observational and theoretical progress in understanding the physical properties and evolution of Be stars, including their disks and related phenomena.

Findings

High-resolution observations have constrained Be star models.

The effects of metallicity, magnetic fields, and rotation on Be phenomena are discussed.

Future facilities will enhance understanding of Be star evolution.

Abstract

Among the emission-line stars, the classical Be stars known for their extreme properties are remarkable. The Be stars are B-type main sequence stars that have displayed at least once in their life emission lines in their spectrum. Beyond this phenomenological approach some progresses were made on the understanding of this class of stars. With high-technology techniques (interferometry, adaptive optics, multi-objects spectroscopy, spectropolarimetry, high-resolution photometry, etc) from different instruments and space mission such as the VLTI, CHARA, FLAMES, ESPADONS-NARVAL, COROT, MOST, SPITZER, etc, some discoveries were performed allowing to constrain the modelling of the Be stars stellar evolution but also their circumstellar decretion disks. In particular, the confrontation between theory and observations about the effects of the stellar formation and evolution on the main sequence, the metallicity, the magnetic fields, the stellar pulsations, the rotational velocity, and the binarity (including the X-rays binaries) on the Be phenomenon appearance is discussed. The disks observations and the efforts made on their modelling is mentioned. As the life of a star does not finish at the end of the main sequence, we also mention their stellar evolution post main sequence including the gamma-ray bursts. Finally, the different new results and remaining questions about the main physical properties of the Be stars are summarized and possible ways of investigations proposed. The recent and future facilities (XSHOOTER, ALMA, E-ELT, TMT, GMT, JWST, GAIA, etc) and their instruments that may help to improve the knowledge of Be stars are also briefly introduced.