Spectroscopic studies of very metal-poor stars with the Subaru High Dispersion Spectrograph. III. Light neutron-capture elements

Elemental abundance measurements have been obtained for a sample of 18 very metal-poor stars using spectra obtained with the Subaru Telescope High Dispersion Spectrograph. Seventeen stars, among which 16 are newly analyzed in the present work, were selected from candidate metal-poor stars identified in the HK survey of Beers and colleagues. The metallicity range covered by our sample is - 3.1 less than or similar to [Fe/H] less than or similar to P - 2.4. The abundances of carbon, alpha-elements, and iron- peak elements determined for these stars confirm the trends found by previous work. One exception is the large overabundance of Mg, Al, and Sc found in BS 16934-002, a giant with [Fe/H] -2.8. Interestingly, this is the most metal- rich star (by about 1 dex in [Fe/H]) known with such large overabundances in these elements. Furthermore, BS 16934-002 does not share the large overabundances of carbon that are associated with the two other, otherwise similar, extremely metal-poor stars CS 22949-037 and CS 29498-043. By combining our new results with those of previous studies, we investigate the distribution of neutron-capture elements in very metal-poor stars, focusing on the production of the light neutron-capture elements (e. g., Sr, Y, and Zr). Large scatter is found in the abundance ratios between the light and heavy neutron-capture elements (e. g., Sr / Ba, Y/ Eu) for stars with low abundances of heavy neutron-capture elements. Most of these stars have extremely low metallicity ([Fe/H] less than or similar to -3). By contrast, the observed scatter in these ratios is much smaller in stars with excesses of heavy neutron-capture elements and with higher metallicity. These results can be naturally explained by assuming that two processes independently enriched the neutron-capture elements in the early Galaxy. One process increases both light and heavy neutron-capture elements and affects stars with [Fe/H] greater than or similar to -3, while the other process contributes only to the light neutron-capture elements and affects most stars with [Fe/H] greater than or similar to -3.5. Interestingly, the Y/Zr ratio is similar in stars with high and low abundances of heavy neutron-capture elements. These results provide constraints on modeling of neutroncapture processes, in particular, those responsible for the nucleosynthesis of light neutron-capture elements at very low metallicity.