Discovery of Blue Hook Stars in the Massive Globular Cluster M54

We present BV photometry centered on the globular cluster M54 (NGC 6715). The color-magnitude diagram clearly shows a blue horizontal branch extending anomalously beyond the zero-age horizontal-branch theoretical models. These kinds of horizontal-branch stars (also called ``blue hook'' stars), which go beyond the lower limit of the envelope mass of canonical horizontal-branch hot stars, have so far been known to exist in only a few globular clusters: NGC 2808, ω Centauri (NGC 5139), NGC 6273, and NGC 6388. Those clusters, like M54, are among the most luminous in our Galaxy, indicating a possible correlation between the existence of these types of horizontal-branch stars and the total mass of the cluster. A gap in the observed horizontal branch of M54 around T_eff=27,000 K could be interpreted within the late helium flash theoretical scenario, which is a possible explanation for the origin of blue hook stars.