Context. An increasing fraction of carbon-enhanced metal-poor (CEMP)
stars is found as their iron abundance, [Fe/H], decreases below [Fe/H]
=-2.0. The CEMP-s stars have the highest absolute carbon abundances,
[C/H], and are thought to owe their enrichment in carbon and the slow
neutron-capture (s-process) elements to mass transfer from a former
asymptotic giant branch (AGB) binary companion. The most Fe-poor CEMP
stars are normally single, exhibit somewhat lower [C/H] than CEMP-s
stars, but show no s-process element enhancement (CEMP-no stars).
Abundance determinations of CNO offer clues to their formation sites.
Aims: Our aim is to use the medium-resolution spectrograph
X-Shooter/VLT to determine stellar parameters and abundances for C, N,
Sr, and Ba in several classes of CEMP stars in order to further classify
and constrain the astrophysical formation sites of these stars.
Methods: Atmospheric parameters for our programme stars were estimated
from a combination of V-K photometry, model isochrone fits, and
estimates from a modified version of the SDSS/SEGUE spectroscopic
pipeline. We then used X-Shooter spectra in conjunction with the 1D
local thermodynamic equilibrium spectrum synthesis code MOOG, 1D ATLAS9
atmosphere models to derive stellar abundances, and, where possible,
isotopic 12C/13C ratios. Results:
Abundances (or limits) of C, N, Sr, and Ba are derived for a sample of
27 faint metal-poor stars for which the X-Shooter spectra have
sufficient signal-to-noise ratios (S/N). These moderate resolution, low
S/N (~10-40) spectra prove sufficient to perform limited chemical
tagging and enable assignment of these stars into the CEMP subclasses
(CEMP-s and CEMP-no). According to the derived abundances, 17 of our
sample stars are CEMP-s and 3 are CEMP-no, while the remaining 7 are
carbon-normal. For four CEMP stars, the subclassification remains
uncertain, and two of them may be pulsating AGB stars.
Conclusions: The derived stellar abundances trace the formation
processes and sites of our sample stars. The [C/N] abundance ratio is
useful for identifying stars with chemical compositions unaffected by
internal mixing, and the [Sr/Ba] abundance ratio allows us to
distinguish between CEMP-s stars with AGB progenitors and the CEMP-no
stars. Suggested formation sites for the latter include faint supernovae
with mixing and fallback and/or primordial, rapidly-rotating, massive
stars (spinstars). X-Shooter spectra have thus proved to be valuable
tools in the continued search for their origin.
Based on observations obtained at ESO Paranal Observatory, programmes
084.D-0117(A) and 085.D-0041(A).
