The mysterious gamma Cas stars

Abstract:

Long thought to be a prototype of Be stars, the first detected case of such objects, gamma Cas, was found to be actually atypical amongst them! Indeed, it emits very bright and extremely hard X-rays with a luminosity intermediate between that of normal massive stars and that of high-massX-ray binaries. Recent efforts in X-ray monitorings have revealed that such objects are not uncommon hence gamma Cas stars are important ionizing sources, particularly at low metallicities where Be stars abound. The properties of these stars will be reviewed using the latest observational results.