Variability of be stars in southern open clusters

Abstract

We recently discovered a large number of highly active Be stars in the open cluster NGC3766, 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 Hα 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 Hα equivalent widths to the photometric y - Hα colors, we present a method to estimate the strength of the Hα emission when spectroscopy is not available. For a subset of 128 stars in four open clusters, we also use blue optical spectroscopy and available Strömgren 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. © 2009 The American Astronomical Society.