Volume 571, November 2014
|Number of page(s)||9|
|Published online||05 November 2014|
GEPI Observatoire de Paris, CNRS, Université Paris
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
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.
Key words: stars: abundances / stars: Population II / Galaxy: evolution / Galaxy: halo
Based on observations obtained with the ESO Very Large Telescope at Paranal Observatory, Chile (ID 077.D-0299(A) PI Bonifacio, and ID 078.B-0238(A) PI Spite), and on archive data ID 076.D-0451(A) PI Johnson.
The line list and the abundances line by line are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/571/A40
© ESO, 2014
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