Barium and europium abundances in cool dwarf stars and nucleosynthesis of heavy elements

We revise barium abundances in 29 cool stars with metallicities [Fe/H] rang ing from -2.20 to 0.07 and europium abundances in 15 stars with [Fe/H] from -1.52 to 0.07. The sample has been extracted from Fuhrmann's lists (1998, 2000) and confined to main-sequence and turnoff stars with only one subgian t added. The results are based on differential NLTE model atmosphere analys es of spectra that have a typical SM of 200 and a resolution of 40000 or 60 000. The statistical equilibrium of Eu II is first investigated with a mode l atom containing 32 levels of Full plus the ground state of Eu III. NLTE e ffects decrease the equivalent widths of the Eu 11 lines compared with LTE resulting in positive NLTE abundance corrections which are below 0.08 dex f or all the stars investigated. The solar barium abundance log epsilon (Ba,. ) = 2.21 and the europium abundance log epsilon (Eu,.) = 0.53 are found fro m the Ba Ir and Eu 11 solar Aux line profile fitting, and they coincide wit hin error bars with meteoritic abundances of Grevesse et al. (1996). Here t he usual scale with log epsilon (H) = 12 is used. The isotopic ratio Eu-151 : Eu-153 = 55. 45 is obtained from solar disk center intensity profile fitt ing of the Eu II lambda 4129 Angstrom line. We report here for the first ti me that the elemental ratios [Ba/Fe], [Eu/Fe] and [Eu/Ba] show a different behaviour for stars of different Galactic populations. For the halo stars t he [Ba/Fe] ratios are approximately solar, europium is overabundant relativ e to iron and barium with the mean Values [Eu/Fe] = 0.62 and [Eu/Ba] = 0.64 . For thick disk stars it is found that a) barium is slightly underabundant relative to iron by about 0.1 dex; b) europium is overabundant relative to iron with the [Eu/Fe] ratios between 0.30 and 0.44; and c) europium is ove rabundant relative to barium with a mean value of [Eu/Ba] = 0.49 +/- 0.03. A step-like change in the [Eu/Ba] and [Ba/Fe] ratios occurs at the thick to thin disk transition; so, nearly solar elemental ratios [Ba/Fe], [Eu/Fe] a nd [Eu/Ba] are found for the thin disk stars. These data suggest that a) th e halo and thick disk stellar population formed quickly during an interval comparable with the evolution time of an AGE progenitor of 3 to 4 M., and t he r-process dominated heavy element production at that epoch; b) there was a hiatus in star formation before the early stage of the thin disk develop ed. The even-to-odd Ba isotope ratios estimated from hyperfine structure (H FS) affecting the Ba Ir resonance line in the halo and thick disk stars fav our a significant contribution of Ba-138 to barium for a pure r-process, an d this is supported by the recent data of Arlandini et al. (1999).