Quiescent and active phases in Be stars: a WISE snapshot of Young Galactic Open Clusters

Abstract

Through the modeling of near-infrared photometry of star-plus disk systems with the codes bedisk/beray, we successfully describe the Wide-Field Infrared Survey Explorer (WISE) photometric characteristics of Be stars in five young open clusters, NGC 663, NGC 869, NGC 884, NGC 3766, and NGC 4755, broadly studied in the literature. WISE photometry allows previously known Be stars to be detected and to find new Be candidates which could be confirmed spectroscopically. The location of Be stars in the WISE color-magnitude diagram, separates them in two groups; active (Be stars hosting a developed circumstellar disk) and quiescent objects (Be stars in a diskless phase), and this way, we can explore how often stars are observed in these different stages. The variability observed in most active variable Be stars is compatible with a disk dissipation phase. We find that 50% of Be stars in the studied open clusters are in an active phase. We can interpret this as Be stars having a developed circumstellar disk one-half of the time. The location of Be stars with a developed disk in the color-magnitude diagram require mass loss rates in agreement with values recently reported in the literature. For these objects, we expect to have a tight relation between the equivalent width of the Hα line and the mass of the disk, if the inclination is known. Also, near-infrared photometry of Be stars in stellar clusters has the potential of being useful to test whether there is a preferential viewing angle.