Abundance trends in kinematical groups of the Milky Way's disk

Observatoire de Bordeaux, BP 89, 33270 Floirac, France e-mail: soubiran@obs.u-bordeaux1.fr

Received: 18 November 2004
Accepted: 19 March 2005

Abstract

We have compiled a large catalogue of metallicities and abundance ratios from the literature in order to investigate abundance trends of several alpha and iron peak elements in the thin disk and the thick disk of the Galaxy. The catalogue includes 743 stars with abundances of Fe, O, Mg, Ca, Ti, Si, Na, Ni and Al in the metallicity range –1.30 < [Fe/H] < +0.50. We have checked that systematic differences between abundances measured in the different studies were lower than random errors before combining them. Accurate distances and proper motions from Hipparcos and radial velocities from several sources have been retreived for 639 stars and their velocities (U, V, W) and galactic orbits have been computed. Ages of 322 stars have been estimated with a Bayesian method of isochrone fitting. Two samples kinematically representative of the thin and thick disks have been selected, taking into account the Hercules stream which is intermediate in kinematics, but with a probable dynamical origin. Our results show that the two disks are chemically well separated, they overlap greatly in metallicity and both show parallel decreasing alpha elements with increasing metallicity, in the interval –0.80 < [Fe/H] < –0.30. The Mg enhancement with respect to Fe of the thick disk is measured to be 0.14 dex. An even larger enhancement is observed for Al. The thick disk is clearly older than the thin disk with tentative evidence of an AMR over 2–3 Gyr and a hiatus in star formation before the formation of the thin disk. We do not observe a vertical gradient in the metallicity of the thick disk. The Hercules stream has properties similar to that of the thin disk, with a wider range of metallicity. Metal-rich stars assigned to the thick disk and super-metal-rich stars assigned to the thin disk appear as outliers in all their properties.