Evolution and appearance of Be stars in SMC clusters

TL;DR

This study investigates the evolution and characteristics of Be stars in Small Magellanic Cloud clusters, examining how factors like metallicity, mass, and age influence their appearance and development.

Contribution

It provides new observational data on Be stars in SMC clusters and develops a model linking their evolution to metallicity and stellar parameters.

Findings

Be stars are more prevalent in low-metallicity environments like the SMC.

A model relating Be star appearance to fractional critical rotation rate is proposed.

Observations include 84 clusters with slitless mode and detailed spectroscopy of NGC 330.

Abstract

Star clusters are privileged laboratories for studying the evolution of massive stars (OB stars). One particularly interesting question concerns the phases, during which the classical Be stars occur, which unlike HAe/Be stars, are not pre-main sequence objects, nor supergiants. Rather, they are extremely rapidly rotating B-type stars with a circumstellar decretion disk formed by episodic ejections of matter from the central star. To study the impact of mass, metallicity, and age on the Be phase, we observed SMC open clusters with two different techniques: 1) with the ESO-WFI in its slitless mode, which allowed us to find the brighter Be and other emission-line stars in 84 SMC open clusters 2) with the VLT-FLAMES multi-fiber spectrograph in order to determine accurately the evolutionary phases of Be stars in the Be-star rich SMC open cluster NGC 330. Based on a comparison to the Milky Way, a model of Be stellar evolution / appearance as a function of metallicity and mass / spectral type is developed, involving the fractional critical rotation rate as a key parameter.