Discovery of Blue Hook Stars in the Massive Globular Cluster M54

Abstract

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, Omega 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)= 27000 K could be interpreted within the late helium flash theoretical scenario, a possible explanation for the origin of those stars.

arXiv comment10 pages, 2 figures, accepted for publication in the Astrophysical Journal

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• The fraction of second generation stars in Globular Clusters from the analysis of the Horizontal Branch
• M 54 + Sagittarius = omega Centauri
• Multivariate Analysis of Globular Cluster's Horizontal Branch Morphology: searching for the second parameter
• Kinematic Decoupling of Globular Clusters with Extended Horizontal-Branch
• The Infrared Ca II triplet as metallicity indicator
• The central density cusp of the Sagittarius dwarf spheroidal galaxy
• The ubiquitous nature of the Horizontal Branch second U-jump: A link with the Blue Hook scenario?
• The presence of intermediate-mass black holes in globular clusters and their connection with extreme horizontal branch stars
• Dust, pulsation, chromospheres and their role in driving mass loss from red giants in Galactic globular clusters
• The hot horizontal-branch stars in omega Centauri
• On the Origin of Bimodal Horizontal-Branches in Massive Globular Clusters: The Case of NGC 6388 and NGC 6441
• A Chemical Composition Survey of the Iron-Complex Globular Cluster NGC 6273 (M 19)
• On the remote Galactic globular cluster NGC 2419
• On the Horizontal Branch of the Galactic Globular NGC2808
• The Blue Hook Populations of Massive Globular Clusters
• Hot Horizontal Branch stars in omega Centauri: clues about their origin from the cluster Color Magnitude Diagram
• Spectroscopy of horizontal branch stars in Omega Centauri
• The evolutionary status of the blue hook stars in Omega Centauri
• The population of variable stars in M54 (NGC6715)
• Evolved Stars in the Core of the Massive Globular Cluster NGC 2419
• Blue Hook Stars in Globular Clusters
• The MODEST questions: challenges and future directions in stellar cluster research
• SHOTGLAS II. MUSE spectroscopy of blue horizontal branch stars in the core of $\omega$ Centauri and NGC 6752
• Spectroscopy of blue horizontal branch stars in NGC 6656 (M22)
• AstroSat study of the globular cluster NGC 2298: probable evolutionary scenarios of hot HB stars
• Many new variable stars discovered in the core of the globular cluster NGC 6715 (M54) with EMCCD observations
• Horizontal Branch Stars and the Ultraviolet Universe