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

Free access article

Issue		A&A Volume 469, Number 2, July II 2007


Page(s)		529 - 537
Section		Galactic structure, stellar clusters, and populations
DOI		http://dx.doi.org/10.1051/0004-6361:20066721

A&A 469, 529-537 (2007)
DOI: 10.1051/0004-6361:20066721

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

V. Kravtsov^{1, 2}, G. Alcaíno³, G. Marconi⁴, and F. Alvarado³

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 $\it UBVI$ 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 $\Delta U_{\rm TO}^{\rm BHB}$ 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, $\Delta U_{\rm TO}^{\rm RGB}$ , 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 $\Delta U_{\rm TO}^{\rm RGB}$ on [Fe/H] and have estimated its analytical expression, by assuming a linear relation between the parameter and metallicity. Its slope, $\Delta U_{\rm TO}^{\rm RGB}$ / $\Delta$ [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 M 75 are also discussed.

Key words: Galaxy: globular clusters: general -- Galaxy: globular clusters: individual: M 75 -- stars: population II

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.