Enrichment of heavy elements in the red giant S 15-19 in the Sextans dwarf spheroidal galaxy

Abstract

We determined the chemical abundances of the extremely metal-poor (EMP) star S 15-19 ([Fe=H]= -3.0) in the Sextans dwarf galaxy. While heavy neutron-capture elements (e.g., Ba) are generally deficient in EMP stars in dwarf galaxies, this object was shown to have an exceptional over-abundance of Ba ([Ba=Fe] ∼ +0.5) by a previous study, 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, however, reveals 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 a deficiency of Sr ([Sr=Fe] = -1.4) are found for this object. Taking the variation of the 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. These results are compared with carbon-enhanced metal-poor stars in the field halo.