Taking Another Look: Light n-Capture Element Abundances in Metal-Poor Halo Stars

Elements are produced in stars through a variety of processes; some are well known, others are still the object of active research. The elements zirconium (Zr) and yttrium (Y) reside in the mass range where there is uncertainty about the production mechanism at early time. The rapid n-capture process (r-process) was believed to be responsible for the production, but no study has been able to successfully use the r-process to reproduce the abundance signature for elements in this mass range for metal-poor halo stars. It has been suggested that there may be an undiscovered component to the r-process. We have performed new abundance calculations for the elements Y and Zr in six metal-poor halo giants, using high-quality spectra, and in the case of Zr, new atomic data. Our results are consistent with previous authors' analysis of these stars, but with reduced values for the uncertainty, due to new laboratory data and better quality observational data. Comparison of the lighter n- capture elements' abundances to those of the heavier s-process element barium (Ba) show that there is no correlation between the production of the light elements to the heavy elements. This indicates that there seem to be a multiplicity of processes operating early in the history of the Galaxy. Our results suggest that the main r-process is responsible for production of the elements from Ba onward in atomic number, while some as yet unknown process (or processes) is responsible for the production of the lighter n-capture elements.