-, r-, and s-process element trends in the Galactic thin and thick disks ^*,**

¹ Lund Observatory, Box 43, 221 00 Lund, Sweden e-mail: [sofia;ingemar]@astro.lu.se
² Department of Astronomy, 921 Dennison Building, University of Michigan, Ann Arbor, MI 48109-1090, USA e-mail: tbensby@umich.edu
³ Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: ilyin@aip.de

Received: 25 February 2004
Accepted: 2 December 2004

Abstract

From a detailed elemental abundance analysis of 102 F and G dwarf stars we present abundance trends in the Galactic thin and thick disks for 14 elements (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, Ba, and Eu). Stellar parameters and elemental abundances (except for Y, Ba and Eu) for 66 of the 102 stars were presented in our previous studies (Bensby et al. [CITE], A&A, 410, 527, [CITE], A&A, 415, 155). The 36 stars that are new in this study extend and confirm our previous results and allow us to draw further conclusions regarding abundance trends. The s-process elements Y and Ba, and the r-element Eu have also been considered here for the whole sample for the first time. With this new larger sample we now have the following results: 1) smooth and distinct abundance trends that for the thin and thick disks are clearly separated; 2) the α-element trends for the thick disk show typical signatures from the enrichment of SN Ia; 3) the thick disk stellar sample is in the mean older than the thin disk stellar sample; 4) the thick disk abundance trends are invariant with galactocentric radii (R_m); 5) the thick disk abundance trends appear to be invariant with vertical distance (Z_max) from the Galactic plane. Adding further evidence from the literaure we argue that a merger/interacting scenario with a companion galaxy to produce a kinematical heating of the stars (that make up today's thick disk) in a pre-existing old thin disk is the most likely formation scenario for the Galactic thick disk. The 102 stars have and are all in the solar neighbourhood. Based on their kinematics they have been divided into a thin disk sample and a thick disk sample consisting of 60 and 38 stars, respectively. The remaining 4 stars have kinematics that make them kinematically intermediate to the two disks. Their chemical abundances also place them in between the two disks. Which of the two disk populations these 4 stars belong to, or if they form a distinct population of their own, can at the moment not be settled. The 66 stars from our previous studies were observed with the FEROS spectrograph on the ESO 1.5-m telescope and the CES spectrograph on the ESO 3.6-m telescope. Of the 36 new stars presented here 30 were observed with the SOFIN spectrograph on the Nordic Optical Telescope on La Palma, 3 with the UVES spectrograph on VLT/UT2, and 3 with the FEROS spectrograph on the ESO 1.5-m telescope. All spectra have high signal-to-noise ratios (typically ) and high resolution (, 45 000, and 110 000 for the SOFIN, FEROS, and UVES spectra, respectively).