The Carina Project. VIII. The {\alpha}-element abundances
  We have performed a new abundance analysis of Carina Red Giant (RG) stars
from spectroscopic data collected with UVES (high resolution) and
FLAMES/GIRAFFE (high and medium resolution) at ESO/VLT. The former sample
includes 44 RGs, while the latter consists of 65 (high) and ~800 (medium
resolution) RGs, covering a significant fraction of the galaxy's RG branch
(RGB), and red clump stars. To improve the abundance analysis at the faint
magnitude limit, the FLAMES/GIRAFFE data were divided into ten surface gravity
and effective temperature bins. The spectra of the stars belonging to the same
gravity/temperature bin were stacked. This approach allowed us to increase by
at least a factor of five the signal-to-noise ratio in the faint limit
(V>20.5mag). We took advantage of the new photometry index cU,B,I introduced by
Monelli et al. (2014), as an age and probably a metallicity indicator, to split
stars along the RGB. These two stellar populations display distinct [Fe/H] and
[Mg/H] distributions: their mean Fe abundances are -2.15$\pm$0.06dex
(sig=0.28), and -1.75$\pm$0.03dex (sig=0.21), respectively. The two iron
distributions differ at the 75% level. This supports preliminary results by
Lemasle et al. (2012) and by Monelli et al. (2014). Moreover, we found that the
old and intermediate-age stellar populations have mean [Mg/H] abundances of
-1.91$\pm$0.05dex (sig=0.22) and -1.35$\pm$0.03dex (sig=0.22); these differ at
the 83% level. Carina's {\alpha}-element abundances agree, within 1sigma, with
similar abundances for field Halo stars and for cluster (Galactic, Magellanic)
stars. The same outcome applies to nearby dwarf spheroidals and ultra-faint
dwarf galaxies, in the iron range covered by Carina stars. Finally, we found
evidence of a clear correlation between Na and O abundances, thus suggesting
that Carina's chemical enrichment history is quite different than in the
globular clusters.
