The Gaia-ESO Survey: Sodium and aluminium abundances in giants and
  dwarfs - Implications for stellar and Galactic chemical evolution
  Stellar evolution models predict that internal mixing should cause some
sodium overabundance at the surface of red giants more massive than ~ 1.5--2.0
Msun. The surface aluminium abundance should not be affected. Nevertheless,
observational results disagree about the presence and/or the degree of the Na
and Al overabundances. In addition, Galactic chemical evolution models adopting
different stellar yields lead to quite different predictions for the behavior
of [Na/Fe] and [Al/Fe] versus [Fe/H]. Overall, the observed trends of these
abundances with metallicity are not well reproduced. We readdress both issues,
using new Na and Al abundances determined within the Gaia-ESO Survey, using two
samples: i) more than 600 dwarfs of the solar neighborhood and of open clusters
and ii) low- and intermediate-mass clump giants in six open clusters.
Abundances of Na in giants with mass below ~2.0 Msun, and of Al in giants below
~3.0 Msun, seem to be unaffected by internal mixing processes. For more massive
giants, the Na overabundance increases with stellar mass. This trend agrees
well with predictions of stellar evolutionary models. Chemical evolution models
that are able to fit well the observed [Na/Fe] vs. [Fe/H] trend in solar
neighborhood dwarfs can not simultaneously explain the run of [Al/Fe] with
[Fe/H], and viceversa. The comparison with stellar ages is hampered by severe
uncertainties. Indeed, reliable age estimates are available for only a half of
the stars of the sample. We conclude that Al is underproduced by the models,
except for stellar ages younger than about 7 Gyr. In addition, some significant
source of late Na production seems to be missing in the models. Either current
Na and Al yields are affected by large uncertainties, and/or some important
Galactic source(s) of these elements has not been taken into account up to now.
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