Far-ultraviolet spectra of the nuclei of globular clusters M 30, M 54, M 70.

Abstract

IUE spectra in the low dispersion mode have been collected from the nuclei of the three galactic globular clusters NGC 6681 (= M70), NGC 6715 (= M54), and NGC 7099 (= M30). High UV fluxes are obtained from M54 which had been found earlier to be a real characteristic of the cluster population, suggesting a possible connection of this cluster with EB clusters as defined by van Albada et al. (1981). Although the B and EB clusters anticorrelate as groups with metallicity, the flux level in M70 suggests that within the EB group UV colors and metallicity are again correlated. A point-like UV source has been detected at the center of M30. Separation between the central source and the diffuse cluster background radiation has been achieved through a multi-Gaussian best-fit procedure. The diffuse component shows the behavior of B-type clusters, in accordance with ANS data. The point-like source is shown to be a star in a post-HB phase crossing the HR diagram towards the asymptotic giant branch.