Heavy element abundances in cool dwarf stars: An implication for the evolution of the Galaxy

We present revised strontium, barium and europium abundances for 63 cool st ars with metallicities [Fe/H] ranging from 2.20 to 0.25. The stellar sample has been extracted from Fuhrmann's lists (1998, 2001). It is confined to m ain-sequence and turnoff stars. The results are based on NLTE line formatio n obtained in differential model atmosphere analyses of spectra that have a typical S/N of 200 and a resolution of 40000 to 60000. The element abundan ce ratios reveal a distinct chemical history of the halo and thick disk com pared with that of the thin disk. Europium is overabundant relative to iron and barium in halo and thick disk stars suggesting that during the formati on of these galactic populations high-mass stars exploding as SNe II domina ted nucleosynthesis on a short time scale of the order of 1 Gyr. We note th e importance of [Eu/Mg] determinations for halo stars. Our analysis leads t o the preliminary conclusion that Eu/Mg ratios found in halo stars do not s upport current theoretical models of the r-process based on low-mass SNe; i nstead they seem to point at a halo formation time much shorter than 1 Gyr. A steep decline of [Eu/Fe] and a slight decline of [Eu/Ba] with increasing metallicity have been first obtained for thick disk stars. This indicates the start of nucleosynthesis in the lower mass stars, in SN I and AGB stars , which enriched the interstellar gas with iron and the most abundant s-pro cess elements. From a decrease of the Eu/Ba ratio by similar to0.10... 0.15 dex the time interval corresponding to the thick disk formation phase can be estimated. The step-like change of element abundance ratios at the thick to thin disk transition found in our previous analysis (Mashonkina & Gehre n 2000) is confirmed in this study: [Eu/Ba] and [Eu/Fe] are reduced by simi lar to0.25 dex and similar to0.15 dex, respectively; [Ba/Fe] increases by s imilar to0.1 dex. This is indicative of an intermediate phase before the ea rly stage of the thin disk developed, during which only evolved middle and low mass (<8 M-<circle dot>) stars contributed to nucleosynthesis. Our data provide an independent method to calculate the duration of this phase. The main s-process becomes dominant in the production of heavy elements beyond the iron group during the thin disk evolution. We find that in the thin di sk stars Ba/Fe ratios increase with time from [Ba/Fe] = -0.06 in stars olde r than 8 Gyr to [Ba/Fe] = 0.06 in stars that are between 2 and 4 Gyr old.