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Abstract

The Eu abundances from this study and the published Ba abundances show [Eu/Ba] to increase with stellar age. This is interpreted as indicating a change in the relative number of r-process and s-process sites over the history of the Galaxy or a change in 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 depends on this ratio being constant; thus, the variation in [Eu/Ba] reported here may indicate that the Th/Nd chronometer needs revision.

The variation of [Eu/Fe] with [Fe/H] is found to most closely resemble the variations of oxygen and the α 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 this study the Eu abundances reported here correlate more closely to the predictions based on Type II supernovae models than previous data have.

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