THE R-PROCESS ELEMENT EUROPIUM IN GALACTIC DISK F-DWARF-STARS AND G-DWARF-STARS

Abundances of the r-process element Eu have been derived for 81 nearby field F and G disk dwarfs. Galactic orbital properties and abundances of 15 other elements have previously been derived for these stars by Edvardsson et al. The average logarithmic abundance ratio [Eu/Fe] incr eases from -0.1 to 0.4 with decreasing Fe abundance over the range -0. 9 less than or equal to [Fe/H] less than or equal to +0.3. Scatter in [Eu/Fe] at a given [Fe/H] is consistent with observational and analysi s uncertainties in [Eu/Fe]. The Eu abundances from this study and the published Ba abundances show [Eu/Ba] to increase with stellar age. Thi s is interpreted as indicating a change in the relative number of r-pr ocess and s-process sites over the history of the Galaxy or a change i n the efficiency of the sites. Thus we cannot assume that the ratio of r-process to s-process contributions to heavy element abundances has remained constant, even recently. The Th/Nd stellar chronometer depend s on this ratio being constant; thus, the variation in [Eu/Ba] reporte d here may indicate that the Th/Nd chronometer needs revision. The var iation of [Eu/Fe] with [Fe/H] is found to most closely resemble the va riations of oxygen and the a elements which are thought to be produced primarily in Type II supernovae. When compared to theoretical models for Galactic r-process nucleosynthesis, the abundance trend found for the stars we have studied corresponds most closely to models in which the r-process site is Type II supernovae. Although this correspondence has previously been reported, in the metallicity range observed in th is study the Eu abundances reported here correlate more closely to the predictions based on Type II supernovae models than previous data hav e.