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

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

E. Brocato ¹, V. Castellani ^{1, 2, 3}, G.A. Scotti ², I. Saviane ⁴, G. Piotto ⁴ and F.R. Ferraro <sup>5

1</sup> Osservatorio Astronomico di Collurania, Via M. Maggini, I-64100 Teramo, Italy (brocato,vittorio@astrte.te.astro.it)
² Dipartimento di Fisica, Università di Pisa, Piazza Torricelli 2, I-56100 Pisa, Italy (giovanni@astr1pi.difi.unipi.it)
³ Istituto Nazionale di Fisica Nucleare, LNGS, I-67100 L'Aquila, Italy
⁴ Dipartimento di Astronomia, Università di Padova, Italy (saviane,piotto@pd.astro.it)
⁵ Osservatorio Astronomico, Via Zamboni 33, I-40126 Bologna, Italy (ferraro@astbo3.bo.astro.it)

Received 24 November 1997 / Accepted 19 March 1998

Abstract

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 . 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]. From the location of the HB, we obtain an apparent distance modulus . The R ratio gives Y= 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 dex), 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.

Key words: globular clusters: individual: NGC 6093 stars: evolution Hertzsprung Russell (HR) diagram stars: Population II stars: horizontal-branch

Send offprint requests to: E. Brocato

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Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. The color-magnitude diagram
• 4. The luminosity function
• 5. Population ratios and radial gradients
• 6. Comparison with other clusters.
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998
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