LAMOST J2217+2104: a new member of carbon-enhanced extremely metal-poor stars with excesses of Mg and Si

TL;DR

This paper reports the discovery and detailed elemental abundance analysis of a unique carbon-enhanced extremely metal-poor star, J2217+2104, revealing insights into early nucleosynthesis and stellar progenitors.

Contribution

It presents the first detailed abundance pattern of J2217+2104, a new CEMP star with excesses of Mg and Si, and compares it with supernova models to constrain progenitor mass.

Findings

J2217+2104 has [Fe/H]=-4.0 with high C, N, O, Mg, Si enhancements.

The star shows similar abundance patterns to other CEMP stars up to Si, but with differences in neutron-capture elements.

Progenitor mass estimated at about 25 solar masses based on nucleosynthesis models.

Abstract

We report on the elemental abundances of the carbon-enhanced metal-poor (CEMP) star J2217+2104 discovered by our metal-poor star survey with LAMOST and Subaru. This object is a red giant having extremely low Fe abundance ([Fe/H]=-4.0) and very large enhancement of C, N, and O with excesses of Na, Mg, Al, and Si. This star is a new example of a small group of such CEMP stars identified by previous studies. We find a very similar abundance pattern for O-Zn in this class of objects that shows enhancement of elements up to Si and normal abundance of Ca and Fe-group elements. Whereas the C/N ratio is different among these stars, the (C+N)/O ratio is similar. This suggests that C was also yielded with similar abundance ratios relative to O-Zn in progenitors, and was later affected by the CN-cycle. By contrast, the heavy neutron-capture elements Sr and Ba are deficient in J2217+2104, compared to the four objects in this class previously studied. This indicates that the neutron-capture process in the early Galaxy, presumably the r-process, has no direct connection to the phenomenon that has formed such CEMP stars. Comparisons of the abundance pattern well determined for such CEMP stars with those of supernova nucleosynthesis models constrain the progenitor mass to be about 25Msun, which is not particularly different from typical mass of progenitors expected for extremely metal-poor stars in general.