A CEMP-no star in the ultra-faint dwarf galaxy Pisces II^⋆

¹ Observatoire de Paris, PSL University, CNRS, GEPI, Place Jules Janssen, 92195 Meudon, France
e-mail: monique.spite@obspm.fr
² Université de Picardie Jules Verne, 33 rue St-Leu, 80080 Amiens, France
³ Dipartimento di Fisica e Astronomia, Universitá di Firenze, Via G. Sansone 1, Sesto Fiorentino, Italy
⁴ INAF/Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, Firenze, Italy

Received: 1 June 2018
Accepted: 20 June 2018

Abstract

Aims. A probable carbon enhanced metal-poor (CEMP) star, Pisces II 10694, was discovered recently in the ultra-faint (UFD) galaxy Pisces II. This galaxy is supposed to be very old, suspected to include dark matter, and likely formed the bulk of its stars before the reionisation of the Universe.

Methods. New abundances have been obtained from observations of Pisces II 10694 at the Kueyen ESO VLT telescope, using the high-efficiency spectrograph: X-shooter.

Results. We found that Pisces II 10694 is a CEMP-no star with [Fe/H] = −2.60 dex. Careful measurements of the CH and C₂  bands confirm the enhancement of the C abundance ([C/Fe] = +1.23). This cool giant has very probably undergone extra mixing and thus its original C abundance could be even higher. Nitrogen, O, Na, and Mg are also strongly enhanced, but from Ca to Ni the ratios [X/Fe] are similar to those observed in classical very metal-poor stars. With its low Ba abundance ([Ba/Fe] = −1.10 dex) Pisces II 10694 is a CEMP-no star. No variation in the radial velocity could be detected between 2015 and 2017. The pattern of the elements has a shape similar to the pattern found in galactic CEMP-no stars like CS 22949-037 ([Fe/H] = −4.0) or SDSS J1349+1407 ([Fe/H] = −3.6).

Conclusions. The existence of a CEMP-no star in the UFD galaxy Pisc II suggests that this small galaxy likely hosted zero-metallicity stars. This is consistent with theoretical predictions of cosmological models supporting the idea that UFD galaxies are the living fossils of the first star-forming systems.