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Title:
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
Authors:
Kravtsov, V.; Alcaíno, G.; Marconi, G.; Alvarado, F.
Affiliation:
AA(Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile ; Sternberg Astronomical Institute, University Avenue 13, 119899 Moscow, Russia ), AB(Isaac Newton Institute of Chile, Ministerio de Educación de Chile, Casilla 8-9, Correo 9, Santiago, Chile ), AC(ESO - European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile ), AD(Isaac Newton Institute of Chile, Ministerio de Educación de Chile, Casilla 8-9, Correo 9, Santiago, Chile )
Publication:
Astronomy and Astrophysics, Volume 469, Issue 2, July II 2007, pp.529-537 (A&A Homepage)
Publication Date:
07/2007
Origin:
EDP Sciences
Astronomy Keywords:
Galaxy: globular clusters: general, Galaxy: globular clusters: individual: M 75, stars: population II
DOI:
10.1051/0004-6361:20066721
Bibliographic Code:
2007A&A...469..529K

Abstract

Aims:We carry out and analyze new multi-color photometry of the Galactic globular cluster (GC) M 75 in UBVI 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]{ZW}<-1.1 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 Δ U_TO^BHB 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 arcsec telescope and 1.3 m Warsaw telescope of the Las Campanas Observatory. At the same time, Δ U_TO^RGB, 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 Δ U_TO^RGB on [Fe/H] and have estimated its analytical expression, by assuming a linear relation between the parameter and metallicity. Its slope, Δ U_TO^RGB/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.

Based on observations with the 1.3 m Warsaw telescope at Las Campanas Observatory. Individual photometry measurements are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/469/529

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