Chemical analysis of very metal-poor turn-off stars from SDSS-DR12^⋆

¹ GEPI, Observatoire de Paris, PSL Research University, CNRS, 61 Avenue de l’Observatoire, 75014 Paris, France
e-mail patrick.francois@obspm.fr
² UPJV, Université de Picardie Jules Verne, 33 rue St Leu, 80080 Amiens, France
³ GEPI, Observatoire de Paris, PSL Research University, CNRS, Place Jules Janssen, 92190 Meudon, France
⁴ Department of Engineering and Applied Sciences, Sophia University, Chiyoda-ku, Tokyo 102-8554, Japan
⁵ iTHEMS Research Group, RIKEN, Wako, Saitama 351-0198, Japan
⁶ Instituto de Astrofìsica de Canarias, Vía Láctea, 38205 La Laguna, Tenerife, Spain
⁷ Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁸ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Muenchen, Germany
⁹ National Observatory of Japan, Mitaka, Tokyo, Japan
¹⁰ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹¹ Dipartimento di Fisica e Astronomia, Universitá di Firenze, Via G. Sansone 1, Sesto Fiorentino, Italy
¹² INAF/Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, Firenze, Italy

Received: 11 July 2018
Accepted: 13 August 2018

Abstract

Context. The most metal-poor stars are the relics of the early chemical evolution of the Galaxy. Their chemical composition is an important tool to constrain the nucleosynthesis in the first generation of stars. The aim is to observe a sample of extremely metal-poor star (EMP stars) candidates selected from the Sloan Digital Sky Survey Data Release 12 (SDSS DR12) and determine their chemical composition.

Aims. We obtain medium resolution spectra of a sample of six stars using the X-shooter spectrograph at the Very Large Telescope (VLT) and we used ATLAS models to compute the abundances.

Methods. Five stars of the sample have a metallicity [Fe/H] between −2.5 dex and −3.2 dex. We confirm the recent discovery of SDSS J002314.00+030758.0 as a star with an extremely low [Fe/H] ratio. Assuming the α-enhancement [Ca/Fe] = +0.4 dex, we obtain [Fe/H] = −6.1 dex.

Results. We could also determine its magnesium abundance and found that this star exhibits a very high ratio [Mg/Fe]≤ +3.60 dex assuming [Fe/H] = −6.13 dex. We determined the carbon abundance and found A(C) = 6.4 dex. From this carbon abundance, this stars belongs to the lower band of the A(C)–[Fe/H] diagram.