Multi-color photometry of the Galactic globular cluster M 75 = NGC 6864. A new sensitive metallicity indicator and the position of the horizontal branch in UV ^*,**

¹ Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile e-mail: vkravtsov@ucn.cl
² Sternberg Astronomical Institute, University Avenue 13, 119899 Moscow, Russia
³ Isaac Newton Institute of Chile, Ministerio de Educación de Chile, Casilla 8-9, Correo 9, Santiago, Chile e-mail: inewton@terra.cl, falvarad@eso.org
⁴ ESO - European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile e-mail: gmarconi@eso.org

Received: 9 November 2006
Accepted: 22 March 2007

Abstract

Aims.We carry out and analyze new multi-color photometry of the Galactic globular cluster (GC) M 75 in and focus on the brighter sequences of the color–magnitude diagram (CMD), with particular emphasis on their location in U-based CMD. Specifically, we study the level both of the horizontal (HB) and red giant branches (RGB) relative to the main-sequence turnoff (TO) in the U magnitude.

Methods.Along with the presented photometry of M 75, we use our collection of photometric data on GCs belonging to the metal-poor range, [Fe/H] dex, obtained from observations with different equipment, but calibrated by standard stars situated in the observed cluster fields.

Results.We confirm our earlier finding, and extend it to a larger magnitude range. We demonstrate that  expressing the difference in U magnitude between the TO point and the level of the blue HB, near its red boundary, of the metal-poor GCs observed with the EMMI camera of the NTT/ESO telescope is about 0.4–0.5 mag smaller as compared to GCs observed with the 100″ telescope and 1.3 m Warsaw telescope of the Las Campanas Observatory. At the same time, , the difference in U magnitude between the TO and RGB inflection (brightest) points, does not show such an apparent dependence on the characteristics of U filters used, but it depends on cluster metallicity. We have shown, for the first time, the dependence of the parameter  on [Fe/H] and have estimated its analytical expression, by assuming a linear relation between the parameter and metallicity. Its slope, /Δ[Fe/H] ~ 1.2 mag/dex, is approximately a factor of two steeper than that of the dependence of the RGB bump position in the V magnitude on metallicity. The asymptotic giant branch (AGB) clump and features of the RGB luminosity function (LF) of M 75 are also discussed.