The low Sr/Ba ratio on some extremely metal-poor stars^⋆,⋆⋆

¹ GEPI Observatoire de Paris, CNRS, Université Paris Diderot, 92195, Meudon Cedex, France
e-mail: monique.spite@obspm.fr
² Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
³ Université de Picardie Jules Verne, 33 rue St-Leu, 80080 Amiens, France
⁴ Millennium Institute of Astrophysics, Universidad Catolica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile

Received: 17 February 2014
Accepted: 2 September 2014

Abstract

Context. It has been noted that, in classical extremely metal-poor (EMP) stars, the abundance ratio of two well-observed neutron-capture elements, Sr and Ba, is always higher than [Sr/Ba] = –0.5, which is the value of the solar r-only process; however, a handful of EMP stars have recently been found with a very low Sr/Ba ratio.

Aims. We try to understand the origin of this anomaly by comparing the abundance pattern of the elements in these stars and in the classical EMP stars.

Methods. For a rigorous comparison with previous data, four stars with very low Sr/Ba ratios were observed and analyzed in the same way as in the First Stars program: analysis within LTE approximation through 1D (hydrostatic) model atmosphere, providing homogeneous abundances of nine neutron-capture elements.

Results. In CS 22950-173, the only turnoff star of the sample, the Sr/Ba ratio is, in fact, found to be higher than the r-only solar ratio, so the star is discarded. The remaining stars (CS 29493-090, CS 30322-023, HE 305-4520) are cool evolved giants. They do not present a clear carbon enrichment, but in evolved giants C is partly burned into N, and owing to their high N abundance, they could still have initially been carbon-rich EMP stars (CEMP). The abundances of Na to Mg present similar anomalies to those in CEMP stars. The abundance patterns of the neutron-capture elements in the three stars are strikingly similar to a theoretical s-process pattern. This pattern could at first be attributed to pollution by a nearby AGB, but none of the stars presents a clear variation in the radial velocity indicating the presence of a companion. The stellar parameters seem to exclude any internal pollution in a TP-AGB phase for at least two of these stars. The possibility that the stars are early-AGB stars polluted during the core He flash does not seem compatible with the theory.