Analysis of old very metal rich stars in the solar neighbourhood^⋆,⋆⋆

M. Trevisan¹, B. Barbuy¹, K. Eriksson², B. Gustafsson², M. Grenon³ and L. Pompéia⁴

¹ Universidade de São Paulo, Rua do Matão 1226, 05508-900 São Paulo, Brazil
e-mail: trevisan@astro.iag.usp.br; barbuy@astro.iag.usp.br
² Departament of Astronomy and Space Physics, Uppsala University, Box 515, 75120 Uppsala, Sweden
e-mail: kjell.eriksson@fysast.uu.se, bengt.gustafsson@astro.uu.se
³ Observatoire de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
e-mail: michel.grenon@unige.ch
⁴ Universidade do Vale do Paraíba, Av. Shishima Hifumi 2911, São José dos Campos, 12244-000 São Paulo, Brazil
e-mail: pompeia@univap.br

Received: 3 November 2010
Accepted: 2 September 2011

Abstract

Context. A sample of mostly old metal-rich dwarf and turn-off stars with high eccentricity and low maximum height above the Galactic plane has been identified. From their kinematics, it was suggested that the inner disk is their most probable birthplace. Their chemical imprints may therefore reveal important information about the formation and evolution of the still poorly understood inner disk.

Aims. To probe the formation history of these stellar populations, a detailed analysis of a sample of very metal-rich stars is carried out. We derive the metallicities, abundances of α elements, ages, and Galactic orbits.

Methods. The analysis of 71 metal-rich stars is based on optical high-resolution échelle spectra obtained with the FEROS spectrograph at the ESO 1.52-m Telescope at La Silla, Chile. The metallicities and abundances of C, O, Mg, Si, Ca, and Ti were derived based on LTE detailed analysis, employing the MARCS model atmospheres.

Results. We confirm the high metallicity of these stars reaching up to [Fe i/H]  = 0.58, and the sample of metal-rich dwarfs can be kinematically subclassified in samples of thick disk, thin disk, and intermediate stellar populations. All sample stars show solar α-Fe ratios, and most of them are old and still quite metal rich. The orbits suggest that the thin disk, thick disk and intermediate populations were formed at Galactocentric distances of  ~8 kpc,  ~6 kpc, and  ~7 kpc, respectively. The mean maximum height of the thick disk subsample of Z_max ~ 380 pc, is lower than for typical thick disk stars. A comparison of α-element abundances of the sample stars with bulge stars shows that the oxygen is compatible with a bulge or inner thick disk origin. Our results suggest that models of radial mixing and dynamical effects of the bar and bar/spiral arms might explain the presence of these old metal-rich dwarf stars in the solar neighbourhood.