Discovery of high-velocity EHB stars in the globular cluster ωCentauri(NGC 5139)

摘要

We report the discovery of 45 high-velocity extreme horizontal branch(EHB) stars in the globular cluster Omega Centauri(NGC 5139).The tangential velocities of these EHB stars are determined to be in the range 93~313 km s^(-1),with an average uncertainty of ~27 km s^(-1).The central escape velocity of the cluster is determined to be in the range 60~105 km s^(-1).These EHB stars are significantly more concentrated toward the cluster core compared with other cluster members.The formation mechanisms of these EHB stars are discussed.Our conclusions can be summarized as follows:(1) A comparison of the tangential velocities of these EHB stars to the centra]escape velocity of the cluster shows that most if not all of these EHB stars are unbound to the cluster;(2) These EHB stars obtained high velocities in the central cluster region no longer than ~1 Myr ago and may be subsequently ejected from the cluster in the next ~1 Myr;(3) If the progenitors of these EHB stars were single stars,then they may have experienced a fast mass-loss process.If the progenitors were in close binaries,then they may have formed through disruptions by the intermediate-mass black hole in the cluster center.