부산대학교 도서관

Summary: Neutron-capture elements (those with Z>30) are formed in two ways: slow neutron-capture (the s-process) and rapid neutron-capture (the r-process). The s-process is thought to mainly occur in low and intermediate-mass asymptotic giant branch (AGB) stars; the r-process site has not been conclusively identified but probably involves core collapse supernovae or neutron star mergers. A conflict has recently arisen between s-process models and observations, as [Ba/Fe] ratios appear to increase dramatically with decreasing age in open clusters to a degree not predicted by standard s-process models. Other s-process elements do not show the degree of enhancement in young clusters that Ba does. Various solutions have been proposed, including an intermediate process which may disproportionally create Ba, and an increased neutron source in low-mass (M<1.5M) AGB stars. We have assembled and analyzed a sample of 75 stars in 24 open clusters to measure abundances of three light s-process elements (Sr, Y, Zr), three heavy s-process elements (Ba, La, Ce), three r-process elements (Eu, Dy, and Gd), and three elements with significant contributions from both processes (`mixed' elements Mo, Pr, and Nd). Examining [s-process/Fe] trends with age confirms that Ba is unique in its behavior, as it has a large trend with age of -0.05 dex/Gyr but the other s-process elements have more modest trends with cluster age of -0.01 to -0.02 dex/Gyr. All of the s-process elements except Ba fit models based on an enhanced neutron source in low-mass AGB stars; we also do not see in our data the heavy s-process element trends with metallicity predicted by standard models. Problems we encountered with measuring Ba features combined with the uncertainty in predictions of i-process models require caution in interpreting the i-process as a solution to the Ba discrepancy. We do not find strong trends in s-process elements with Galactocentric radius, although there is some evidence of a break in the s-process gradient at ~11 kpc. Open cluster [Eu/Fe] abundances appear to fit with models accounting for r-process contributions from neutron star mergers and jet supernovae, but [r-/H] ratios do not appear to change with Galactocentric radius.