Advanced Evolution in Globular Clusters. the Ultraviolet-Bright Stars in Eight Globular Clusters

Abstract

view Abstract Citations (66) References (64) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Advanced Evolution in Globular Clusters. the Ultraviolet-Bright Stars in Eight Globular Clusters Zinn, Robert Abstract Thirty-eight ultraviolet-bright stars-i.e., stars that are brighter than the horizontal branch (HB) stars and bluer than the giant branch stars-in the globular clusters M3, M10, M13, M15, M53, M92, NGC 4147, and NGC 5466 have been observed spectroscopically at 130 A mm -`. Radial-velocity measurements indicate that 15 of these stars are cluster members. The radial velocity of NGC 5466 (+115 t 4 km ) was measured from the spectrograms of seven red giants. It is suggested that Mayall's radial velocity for M53 should be revised to -89 km s '. The effective temperatures of the ultraviolet-bright stars and the Cepheid variables with periods longer than 1 day have been derived from their broad-band colors or taken from the few published analyses. Their luminosities have been derived from the distance moduli adopted for the clusters and from bolometric corrections taken from the literature. A detailed comparison is made between post-HB evolutionary tracks and the positions of the ultraviolet- bright stars (including the Cepheid variables) in the H-R diagram. For the most part, the theoretical tracks provide adequate explanations for the ultraviolet-bright stars. This comparison has led to the following conclusions: (i) As predicted by the theory, a strong correlation exists between the presence of supra-horizontalbranch stars in a cluster and the presence of HB stars hotter than log = 4.1. (ii) The theoretical relationship between the blueness of the HB and the luminosities of the ultraviolet-bright stars with log LIL0 > 2.3 is supported by observation. (iii) The rare Population II Cepheid variables that have periods between 4 and 12 days are most likely stars that have evolved from the blue end of the HB (log Teff > 4.1). Therefore, their paucity is probably caused by the paucity of hot HB stars. Subject headings: Cepheids and W Virginis stars - globular clusters - stellar evolution Publication: The Astrophysical Journal Pub Date: November 1974 DOI: 10.1086/153197 Bibcode: 1974ApJ...193..593Z full text sources ADS | data products SIMBAD (38)