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Astron. Astrophys. 335, 929-942 (1998)

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1. Introduction

NGC 6093 (M 80) is a bright, moderately metal poor globular cluster in the southern hemisphere for which only old photographic photometry for the more luminous cluster stars is available to date (Harris & Racine 1974, hereafter HR74). However, the cluster appears to be an interesting object in many respects. As a first point, the estimated metallicity [Fe/H] = -1.68 (Zinn and West 1984) appears very similar to the cluster M 3 ([Fe/H]= -1.65, Zinn and West 1984) 1. In spite of this similarity, the two clusters have quite different Horizontal Branch (HB) morphology, with M 80 closely resembling M 13-like clusters where HB stars are essentially confined to the blue side of the RR Lyrae instability region. Thus M 80 appears a cluster where the "second parameter" affecting the HB distribution in galactic globular clusters (GGC) can be investigated.

Our interest in this cluster is furtherly increased by the evidence that M 80 is one of the densest globular clusters in the Galaxy, with an estimated central density as large as [FORMULA]=5.68 (Djorgovski 1993). Indeed, there are increasing evidences for which high central density GGCs appear correlated with the occurrence of a peculiar population of very hot HB stars, forming a HB "blue tail" extended to very large effective temperatures (see e.g. Buonanno et al. 1985, Djorgovski & Piotto 1993, Fusi Pecci et al. 1993, Castellani 1994, Buonanno et al. 1997). Moreover, a very blue HB morphology for M 80 was already predicted on the basis of high ultraviolet emissions observed with IUE spectra (Caloi et al. 1980, 1984) and has been recently confirmed by high resolution HST observations presented by Shara and Drissen (1995). According to this scenario, M 80 appears an ideal target to investigate the quoted correlation.

In this paper we present the results of such an investigation. In the next section, observations and data reduction will be presented. In Sect. 3 we present the color-magnitude diagram (CMD), and discuss all the parameters which can be derived from it. Sect. 4 will be devoted to the luminosity function and to a comparison with theoretical models. Population ratios and population gradients will be presented in Sect. 5 together with a short discussion on the possible origin of the blue HB tail. Sect. 6 will deal with a discussion on the CMD morphology. We will show the comparison with a sample of clusters with similar metallicities, discussing at the light of present theoretical knowledge the possible origin of some puzzling differences. The discussion will be finally extended to the CMDs of a large sample of GGCs, disclosing unexpected evidences which might suggest a possible revision of the present evaluation of the ranking in metallicity of GGCs.

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998