1998A&A...335..929B

We present deep B and V CCD photometry of the intermediate metallicity, high central density galactic globular cluster M80. More than 10000 stars have been measured, reaching a limiting magnitude V≃23. The color magnitude diagram shows a population of very hot horizontal branch (HB) stars, stretching down to V magnitudes fainter than those of the turn off stars, to the limiting level of the present photometry. Using the giant branch as metallicity indicator, we derive [Fe/H]=-1.71±0.20. From the location of the HB, we obtain an apparent distance modulus (m-M)_V=15.58±0.12. The R ratio gives Y=0.25±0.05 and shows a radial trend, with the HB stars in the blue tail more concentrated than the red giant stars. We show also evidences that the hot HB stars are ``bona fide'' normal He burning stars. The color-magnitude diagram of M80 has been compared with the corresponding diagrams of other clusters with similar metallicity (within ±0.1dex), and with different metallicity. Both comparisons reveal clear differences, which might be interpreted as uncertainties in color calibrations of the color-magnitude diagrams or might reflect differences in the ``global'' metallicity of the galactic globular clusters.