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J. A. Simmerer, C. Sneden (University of Texas Austin), V. M. Woolf (Armagh Observatory, Northern Ireland), D.L. Lambert (University of Texas Austin)
Throughout the history of the Galaxy, the composition of neutron-capture elements (those with atomic numbers greater than 30) has been dictated mainly by two nucleosynthetic processes: the s-process and the r-process. Elements generated by the s(low)-process are believed to be formed in the He-burning of low to intermediate mass stars, while r(apid)-process elements likely originate in massive star supernovae. In the most metal-poor stars, those elements with large s-process components are deficient with respect to those with large r-process components. This is consistent with expected nucleosynthetic output from the massive stars that probably dominated the element forming processes early on in the Galaxy. Typically the measurement of the s-/r-process has been carried out with the elements Ba and Eu, since the former is produced almost entirely in the s-process and the latter in the r-process. Here we use the s-process element La to measure the s-process, since new and highly accurate atomic data have become available for La and Eu, making the results much more precise than they have been. Our sample includes ~200 stars in the range range -2.5<[Fe/H]<-1, for which we have gathered new high resolution (R~60,000) and signal-to-noise (S/N>100) spectra, and from which we derive La/Eu. We have also derived La/Eu from the data of Woolf et al. (1995) in order to cover a higher metallicity range. Preliminary results indicate that La/Eu rises steadily, if not entirely smoothly, over the range -2>[Fe/H]>0, with the s-process apparently beginning to "turn on" near [Fe/H] ~-1.8. The higher metallicity data of Woolf et al. indicates that the rise to the solar La/Eu is gradual, but may show evidence of scatter.