The CEMP star SDSS J0222–0313: the first evidence of proton ingestion in very low-metallicity AGB stars?^★

¹ GEPI, Observatoire de Paris, Université PSL, CNRS, 5 place Jules Janssen, 92190 Meudon, France
e-mail: Elisabetta.Caffau@obspm.fr
² Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
³ Crimean Astrophysical Observatory, Nauchny 298409, Republic of Crimea
⁴ Astronomical Observatory, Odessa National University, Shevchenko Park, 65014 Odessa, Ukraine
⁵ INAF – Osservatorio Astronomico d’Abruzzo, Via M. Maggini snc, Teramo, Italy
⁶ INFN – Sezione di Perugia, Via A. Pascoli, Perugia, Italy
⁷ European Southern Observatory, Casilla 19001, Santiago, Chile
⁸ GEPI, Observatoire de Paris, Université PSL, CNRS, 77 Av. Denfert-Rochereau, 75014 Paris, France
⁹ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
¹⁰ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, Sesto Fiorentino, Italy

Received: 12 April 2019
Accepted: 27 June 2019

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.