Enrichment of Heavy Elements in the red giant S15-19 in the Sextans Dwarf Spheroidal Galaxy

TL;DR

This study analyzes the chemical composition of the EMP star S15-19 in the Sextans dwarf galaxy, revealing s-process enrichment likely from a binary companion, contrasting previous assumptions of r-process origin.

Contribution

It provides new high-resolution spectroscopic evidence that the heavy element enrichment in S15-19 is due to s-process transfer from a binary AGB star, challenging prior r-process interpretations.

Findings

No clear r-process element excess, such as Eu, was detected.

S15-19 shows significant carbon enhancement ([C/Fe]= +1.0).

Heavy element enrichment is attributed to s-process transfer from a binary companion.

Abstract

We determined chemical abundances of the Extremely Metal-Poor (EMP) star S15-19 ([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 radial velocities measured at the two different epochs into consideration, the origin of the excesses of heavy neutron-capture elements in S15-19 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 S15-19 across the binary system. These results are compared with carbon-enhanced metal-poor stars in the field halo.