Discovery of Blue Hook Stars in the Massive Globular Cluster M54.pdf

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Discovery of Blue Hook Stars in the Massive Globular Cluster M54.pdf

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Discovery of Blue Hook Stars in the Massive Globular Cluster M54

a r X i v : a s t r o - p h / 0 3 1 0 7 4 6 v 2 1 2 N o v 2 0 0 3 Discovery of Blue Hook Stars in the Massive Globular Cluster M54 Alfred Rosenberg Instituto de Astrof??sica de Canarias, Via La?ctea s/n, E-38205 La Laguna, Tenerife, Spain alf@ll.iac.es Alejandra Recio-Blanco Dip. di Astronomia, Universita? di Padova, Vicolo dell’Osservatorio 2, I-35122 Padova, Italy recio@pd.astro.it and Macarena Garc??a-Mar??n Dpto. Astrof??sica Molecular e Infrarroja, Instituto de Estructura de la Materia (DAMIR/IEM-CSIC), Serrano 113bis, E-28006 Madrid, Spain maca@damir.iem.csic.es 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, ω Centauri (NGC 5139), NGC 6273, and NGC 6388. Those clusters, like M54, are among the most luminous in our Galaxy, indi- cating 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 Teff = 27000 K could be interpreted within the late helium flash theoretical scenario, a possible explanation for the origin of those stars. Subject headings: Stars: horizontal branch – Galaxy: globular clusters: individual: NGC 6715 – H-R diagram. 1. Introduction The horizontal branch (HB) hosts stars with a helium-burning core of about 0.5 M⊙, and a hydrogen-burning shell. The masses of the hydrogen envelopes vary from more than 0.2 M⊙ – 2 – to less than 0.02 M⊙. Furthermore, the less massive the hydrogen envelope is, the hotter is the corresponding HB star. In the case of a star cluster, we find a color spread of the HB stars which