Open clusters towards the Galactic centre: chemistry and dynamics

A VLT spectroscopic study of NGC 6192, NGC 6404, NGC 6583^⋆,⋆⋆

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: laura@arcetri.astro.it; randich;@arcetri.astro.it; galli@arcetri.astro.it
² Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuna Mackenna 4860, Casilla 306, Santiago 22, Chile
e-mail: mzoccali@astro.puc.cl
³ ESO, Alonso de Cordova 3107, Vitacura, Santiago de Chile, Chile
e-mail: gcarraro@eso.org; ljilkova@eso.org
⁴ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic
⁵ Dipartimento di Astronomia, Universitá di Padova, Vicolo Osservatorio 3, 35122 Padova, Italy
⁶ Dipartimento di Fisica, Universitá di Perugia, via Pascoli, 06123 Perugia, Italy
e-mail: enrico.maiorca@pg.infn.it; busso@fisica.unipg.it
⁷ Istituto Nazionale di Fisica Nucleare, sezione di Perugia, via Pascoli, 06123 Perugia, Italy

Received: 14 July 2010
Accepted: 18 August 2010

Abstract

Context. In the framework of the study of the Galactic metallicity gradient and its time evolution, we present new high-resolution spectroscopic observations obtained with FLAMES and the fiber link to UVES at VLT of three open clusters (OCs) located within  ~7 kpc from the Galactic centre (GC): NGC 6192, NGC 6404, NGC 6583. We also present new orbit determination for all OCs with Galactocentric distances (R_GC) ≤ 8 kpc and metallicity from high-resolution spectroscopy.

Aims. We aim to investigate the slope of the inner disc metallicity gradient as traced by OCs and to discuss its implication for the chemical evolution of our Galaxy.

Methods. We have derived memberships of a group of evolved stars for each clusters, obtaining a sample of 4, 4, and 2 member stars in NGC 6192, NGC 6404, and NGC 6583, respectively. Using standard LTE analysis we derived stellar parameters and abundance ratios for the iron-peak elements Fe, Ni, Cr, and for the α-elements Al, Mg, Si, Ti, Ca. We calculated the orbits of the OCs currently located within 8 kpc from the GC, and discuss their implication on the present-time radial location.

Results. The average metallicities of the three clusters are all oversolar: [Fe/H] =  + 0.12 ± 0.04 (NGC 6192),  + 0.11 ± 0.04 (NGC 6404),  + 0.37 ± 0.03 (NGC 6583). They are in qualitative agreement with their Galactocentric distances, because they are all internal OCs, and expected to be more metal rich than the solar neighbourhood. The abundance ratios of the other elements over iron [X/Fe] are consistent with solar values.

Conclusions. The clusters we have analysed confirm, together with other OC and Cepheid data, a steep gradient in the inner disc, a signature of an evolutionary rate that is different from the one in the outer disc.