Neutron-capture elements in extremely metal-poor stars

Abstract

Extremely metal-poor stars exhibit large scatter in the abundance ratios (e.g. [Sr/Fe] and [Ba/Fe], as well as [Sr/Ba]), and sometimes show very large enhancements of neutron-capture elements with respect to iron. However, recent abundance measurements for large samples of metal-poor stars suggest that neutron-capture elements are deficient in general in the lowest metallicity range and the scatter in their abundance ratios is rather small. The metallicity at which the abundance ratios show the largest scatter is dependent on elements: Ba shows largest scatter at [Fe/H] = −3, while Sr shows larger scatter at lower metallicity. Such trends suggest metallicity dependence of the contributions of nucleosynthesis processes that produce light and heavy neutron-capture elements. A clear cut-off is found in the Sr/Ba distribution at [Fe/H] = −3.5. This suggests a metallicity dependence of the process that provides light neutron-capture elements. Recent abundance studies for a large sample of very metal-poor stars found by the Sloan Digital Sky Survey (SDSS) discovered a carbon-enhanced metal-poor star that has [Fe/H] = −3.7 and a large excess of Sr with no detectable Ba. This is a similar feature to that found in the hyper metal-poor star HE 1327–2326 ([Fe/H] = −5.6) and indicates that the progenitors of such carbon-enhanced stars with no excess of heavy neutron-capture elements could be a source of light neutron-capture elements.