CCD photometry of M 80 and the ranking of the C-M diagrams of galactic globulars

We present deep B and V CCD photometry of the intermediate metallicity, high central density galactic globular cluster M 80. 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 M 80 has been compared with the corresponding diagrams of other clusters with similar metallicity (within ±0.1 dex), and with different metallicity. Both comparisons reveal clear differences, which might be interpreted as uncertainties in color calibrations of the colormagnitude diagrams or might reflect differences in the “global” metallicity of the galactic globular clusters.