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Paper: Heavy Neutron-Capture Elements in Extremely Metal-Poor Stars in Dwarf Galaxies
Volume: 458, Galactic Archaeology: Near-Field Cosmology and the Formation of the Milky Way
Page: 307
Authors: Honda, S.; Aoki, W.; Arimoto, N.; Sadakane, K.
Abstract: Chemical abundance ratios of extremely metal-poor (EMP) stars in dwarf galaxies are key to understanding the early enrichment in such small systems as well as the early formation of the Milky Way (MW) halo structure. Abundance ratios of heavy neutron-capture elements with respect to iron show large star-to-star scatter in EMP stars in the MW halo, and will provide very useful constraints on the formation processes of low-mass stars in the early Galaxy. However, Ba abundances of dwarf galaxy stars in this metallicity range are quite low in general ([Ba/Fe] < –1). An exception is S 15-19 in the Sextans discovered by our previous study that has [Ba/Fe] =+0.5, which is similar to those of r-process-enhanced stars found in the field halo. Our new high resolution spectroscopy for this object for the blue region reveals, however, that no clear excess of r-process elements like Eu appears in this object. Moreover, a significant excess of carbon ([C/Fe] = +1.0) and deficiency of Sr ([Sr/Fe] = –1.4) are found for this object. Taking the variation of radial velocities measured at the two different epochs into consideration, the origin of the excesses of heavy neutron-capture elements in S 15-19 is not the r-process, but is the s-process in an asymptotic giant branch (AGB) star that was the binary companion (primary) of this object. Carbon- and s-process-enhanced material should have been transferred to the surface of S 15-19 across the binary system. As a result, no r-process-enhanced star is known in EMP stars in dwarf galaxies at present. The constraint is still weak because of the limited sample size, and further measurements of neutron-capture elements for dwarf galaxy stars are required.
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