Bibcode
Asplund, M.; Norris, J. E.; Placco, V. M.; García Pérez, A. E.; Frebel, A.; Yong, D.; Bessell, M. S.; Beers, T. C.; Christlieb, N.; Hansen, C. J.; Hansen, T.
Bibliographical reference
The Astrophysical Journal, Volume 807, Issue 2, article id. 173, 17 pp. (2015).
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2015
Journal
Citations
121
Refereed citations
110
Description
We present a high-resolution elemental-abundance analysis for a sample
of 23 very metal-poor ([Fe/H] <‑2.0) stars, 12 of which are
extremely metal-poor ([Fe/H] <‑3.0), and 4 of which are
ultra-metal-poor ([Fe/H] <‑4.0). These stars were targeted to
explore differences in the abundance ratios for elements that constrain
the possible astrophysical sites of element production, including Li, C,
N, O, the α-elements, the iron-peak elements, and a number of
neutron-capture elements. This sample substantially increases the number
of known carbon-enhanced metal-poor (CEMP) and nitrogen-enhanced
metal-poor (NEMP) stars—our program stars include eight that are
considered “normal” metal-poor stars, six CEMP-no stars,
five CEMP-s stars, two CEMP-r stars, and two CEMP-r/s stars. One of the
CEMP-r stars and one of the CEMP-r/s stars are possible NEMP stars. We
detect lithium for three of the six CEMP-no stars, all of which are Li
depleted with respect to the Spite plateau. The majority of the CEMP
stars have [C/N] > 0. The stars with [C/N] < 0 suggest a larger
degree of mixing; the few CEMP-no stars that exhibit this signature are
only found at [Fe/H] <‑3.4, a metallicity below which we also
find the CEMP-no stars with large enhancements in Na, Mg, and Al. We
confirm the existence of two plateaus in the absolute carbon abundances
of CEMP stars, as suggested by Spite et al. We also present evidence for
a “floor” in the absolute Ba abundances of CEMP-no stars at
A(Ba) ∼ ‑2.0.
Based on observations made with the European Southern Observatory
telescopes.