Abstract

Context. Carbon-enhanced metal-poor (CEMP) stars are common objects in the metal-poor regime. The lower the metallicity we look at, the larger the fraction of CEMP stars with respect to metal-poor stars with no enhancement in carbon. The chemical pattern of CEMP stars is diversified, strongly suggesting a different origin of the C enhancement in the different types of CEMP stars. Aims. We selected a CEMP star, SDSS J0222–0313, with a known high carbon abundance and, from a low-resolution analysis, a strong enhancement in neutron-capture elements of the first peak (Sr and Y) and of the second peak (Ba). The peculiarity of this object is a greater overabundance (with respect to iron) of the first s-process peak than the second s-process peak. Methods. We analysed a high-resolution spectrum obtained with the Mike spectrograph at the Clay Magellan 6.5 m telescope in order to derive the detailed chemical composition of this star. Results. We confirmed the chemical pattern we expected; we derived abundances for a total of 18 elements and significant upper limits. Conclusions. We conclude that this star is a carbon-enhanced metal-poor star enriched in elements produced by s-process (CEMP-s), whose enhancement in heavy elements is due to mass transfer from the more evolved companion in its asymptotic giant branch (AGB) phase. The abundances imply that the evolved companion had a low main sequence mass and it suggests that it experienced a proton ingestion episode at the beginning of its AGB phase.

Highlights

  • Carbon-enhanced metal-poor (CEMP) stars are very common objects in the metal-poor regime ([Fe/H] < −2.0)

  • We conclude that this star is a carbon-enhanced metal-poor star enriched in elements produced by s-process (CEMP-s), whose enhancement in heavy elements is due to mass transfer from the more evolved companion in its asymptotic giant branch (AGB) phase

  • These stars are expected to be enriched in heavy elements produced in the slow n-capture process (s-process); Based on observations collected with Magellan Inamori Kyocera Echelle (Mike) at the Magellan–II

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Summary

Introduction

Carbon-enhanced metal-poor (CEMP) stars are very common objects in the metal-poor regime ([Fe/H] < −2.0). Beers & Christlieb (2005) divide CEMP stars into the following sub-classes according to the abundance ratios (implying, besides C, Ba, and Eu): (i) CEMP-r when [C/Fe] > 1.0 and [Eu/Fe] > 1.0 These stars are supposed to be enhanced in heavy elements produced in the rapid n-capture process (r-process); (ii) CEMP-s when [C/Fe] > 1.0, [Ba/Fe] > 1.0, and [Ba/Eu] > 0.5. We confirm here the abundances derived from a low-resolution FORS spectrum and we increase the number of elements for which we derive the abundance This is a CEMP star, rich in heavy elements that we classify as a CEMP-s star, according to the scheme of Beers & Christlieb (2005). Normal stars are in the upper left part of the diagram, sharing the surface with CEMP-no stars; CEMP-s stars are in the lower right part of the diagram

Selection
Observations
Radial velocity and kinematics
Stellar parameters
G-band
Abundances
NLTE computations
Uncertainties
Discussion
Findings
At low metallicities an enrichment of α isotopes is expected
Conclusions
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