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
Summary
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
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