Spectroscopic Studies of Very Metal‐poor Stars with the Subaru High Dispersion Spectrograph. III. Light Neutron‐Capture Elements

Abstract

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[Fe/H] −2.4. The abundances of carbon, α-elements, and iron-peak elements determined for these stars confirm the trends found by previous work. One exception is the large over-abundance 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 over-abundances 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.-2 -Most of these stars have extremely low metallicity ([Fe/H] −3). By contrast, the observed scatter in these ratios is much smaller in stars with excesses of heavy neutroncapture 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] −3, while the other process contributes only to the light neutron-capture elements, and affects most stars with [Fe/H] −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 neutron-capture processes, in particular those responsible for the nucleosynthesis of light neutron-capture elements at very low metallicity. CEMP-s stars from samples of objects with metallicity near [FeEven though the abundance ratios between neutron-capture elements and lighter metals like iron exhibit significant scatter, the abundance patterns of heavy neutron-capture elements (Ba-Os) in stars with [Fe/H]< −2.5 agree very well with that of the r-process component in solar system material (e.g., Sneden et al. 1996; Westin et al. 2000;Cayrel et al. 2001). This fact indicates that the neutron-capture elements in metal-poor, non CEMP-s stars have originated from the r-process. From the abundance ratios of Ba/Eu or La/Eu, Burris et al. (2000) and Simmerer et al. (2004) concluded that significant contributions from the (main) s-process appear at [Fe/H]∼ −2.3, though small effect of the s-process is also suggested...

Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans

“…14 (Lai et al 2008;François et al 2007;Honda et al 2004;Aoki et al 2005). Similarly for the [Fe/H] > ∼ −2.…”

“…We compare our results with high-resolution spectroscopic analyses of RGBs with [Fe/H] ≤ −2.5 in the Milky Way halo (Cayrel et al 2004;Cohen et al 2008;Aoki et al 2005Aoki et al , 2007Cohen et al 2006;Lai et al 2008;Honda et al 2004) and in faint (Ursa MajorII and Coma Berenices, Boötes I, Leo IV) and classical (Sextans, Draco, Fornax, Carina, Sculptor) dSphs (Letarte et al 2010;Shetrone et al 2001;Fulbright et al 2004;Koch et al 2008a;Cohen & Huang 2009;Aoki et al 2009;Norris et al 2010;Simon et al 2010;Frebel et al 2010b,a). We distinguish between unmixed and mixed RGBs in order to be able to draw a reliable comparison sample for the elements that are sensitive to mixing.…”

“…Draco Ba abundances of the stars at [Fe/H] > −2.45 are pure r-process ones (Cohen & Huang 2009). Finally, we included (Shetrone et al 2001 Galactic mass is a crucial parameter to sustain the explosions of supernovae (Ferrara & Tolstoy 2000), but also to shape Aoki et al (2005). We also inserted the sample of Milky Way halo stars of Barklem et al (2005), similarly excluding carbon-rich and [Ba/Eu] > 0 (s-process enriched stars).…”

The early days of the Sculptor dwarf spheroidal galaxy

“…In all figures, the grey points show the location of RGB stars in the Milky Way halo (Honda et al 2004;Cayrel et al 2004;Spite et al 2005Spite et al , 2006Aoki et al 2005Aoki et al , 2007Cohen et al 2013Cohen et al , 2006Cohen et al , 2004Lai et al 2008;Yong et al 2013;Ishigaki et al 2013). The Sculptor stars are shown in orange.…”

The Hamburg/ESO R-process enhanced star survey (HERES)

“…Their origin has recently been under active discussion: studies of disk and halo stars by e.g. Aoki et al (2005), Mashonkina et al (2007), François et al (2007) and Montes et al (2007) found that their formation cannot be explained by a simple split into contributions from the "classical" s-process and r-process. Farouqi et al (2008b) give an average observed ratio of Sr/Y = 6.17 ± 1.06, taken from various analyses of halo stars at different metallicities and r-process enhancements.…”