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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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