SCYON Abstract

Received on April 24 2007

Multi-Color Photometry of the Galactic Globular Cluster M75 = NGC 6864. A New Sensitive Metallicity Indicator and the Position of the Horizontal Branch in UV

AuthorsV. Kravtsov (1,2), G. Alcaino (3), G. Marconi (4), and F. Alvarado (3)
Affiliation(1) Instituto de Astronomia, UCN, Antofagasta
(2) Sternberg Astronomical Institute, MSU, Moscow
(3) Isaac Newton Institute, Santiago
(4) European Southern Observatory, Santiago
Accepted byAstronomy & Astrophysics
Contactvkravtsov@ucn.cl
URLhttp://arxiv.org/abs/0704.2445
Links

Abstract

We carry out and analyze new multi-color photometry of the Galactic globular cluster (GC) M75 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. Along with the presented photometry of M75, 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. We confirm our earlier finding, and extend it to a larger magnitude range. We demonstrate that Delta UTOBHB 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, Delta UTORGB, 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 Delta UTORGB on [Fe/H] and have estimated its analytical expression, by assuming a linear relation between the parameter and metallicity. Its slope, UTORGB/Delta[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 M75 are also discussed.