First stars XII. Abundances in extremely metal-poor turnoff stars,and
  comparison with the giants
  CONTEXT:The detailed chemical abundances of extremely metal-poor (EMP) stars
are key guides to understanding the early chemical evolution of the Galaxy.
Most existing data are, however, for giant stars which may have experienced
internal mixing later. AIMS: We aim to compare the results for giants with new,
accurate abundances for all observable elements in 18 EMP turnoff stars.
METHODS:VLT/UVES spectra at R ~45,000 and S/N~ 130 per pixel (330-1000 nm) are
analysed with OSMARCS model atmospheres and the TURBOSPECTRUM code to derive
abundances for C, Mg, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, and Ba. RESULTS:
For Ca, Ni, Sr, and Ba, we find excellent consistency with our earlier sample
of EMP giants, at all metallicities. However, our abundances of C, Sc, Ti, Cr,
Mn and Co are ~0.2 dex larger than in giants of similar metallicity. Mg and Si
abundances are ~0.2 dex lower (the giant [Mg/Fe] values are slightly revised),
while Zn is again ~0.4 dex higher than in giants of similar [Fe/H] (6 stars
only). CONCLUSIONS:For C, the dwarf/giant discrepancy could possibly have an
astrophysical cause, but for the other elements it must arise from shortcomings
in the analysis. Approximate computations of granulation (3D) effects yield
smaller corrections for giants than for dwarfs, but suggest that this is an
unlikely explanation, except perhaps for C, Cr, and Mn. NLTE computations for
Na and Al provide consistent abundances between dwarfs and giants, unlike the
LTE results, and would be highly desirable for the other discrepant elements as
well. Meanwhile, we recommend using the giant abundances as reference data for
Galactic chemical evolution models.
