Variability of Be Stars in Southern Open Clusters

TL;DR

This study investigates the variability and physical characteristics of Be stars in multiple open clusters, revealing that Be stars are fast rotators with no significant differences between transient and stable types.

Contribution

It provides a comprehensive multi-epoch spectroscopic analysis of Be stars across several clusters, introducing a method to estimate H-alpha emission strength without spectroscopy.

Findings

Be stars are faster rotating than normal B-type stars.

No significant physical differences between transient and stable Be stars.

Identification of 12 new transient Be stars.

Abstract

We recently discovered a large number of highly active Be stars in the open cluster NGC 3766, making it an excellent location to study the formation mechanism of Be star disks. To explore whether similar disk appearances and/or disappearances are common among the Be stars in other open clusters, we present here multiple epochs of H-alpha spectroscopy for 296 stars in eight open clusters. We identify 12 new transient Be stars and confirm 17 additional Be stars with relatively stable disks. By comparing the H-alpha equivalent widths to the photometric y - H-alpha colors, we present a method to estimate the strength of the H-alpha emission when spectroscopy is not available. For a subset of 128 stars in four open clusters, we also use blue optical spectroscopy and available Stromgren photometry to measure their projected rotational velocities, effective temperatures, and polar surface gravities. We combine our Be star detections from these four clusters to investigate physical differences between the transient Be stars, stable Be stars, and normal B-type stars with no line emission. Both types of Be stars are faster rotating populations than normal B-type stars, and we find no significant physical differences between the transient and stable Be stars in our sample.