The Gaia-ESO Survey: a new approach to chemically characterising young
  open clusters
  Open clusters (OCs) are recognised as excellent tracers of Galactic thin-disc
properties. At variance with intermediate-age and old OCs, for which a
significant number of studies is now available, clusters younger than 150 Myr
have been mostly overlooked in terms of their chemical composition, with few
exceptions. On the other hand, previous investigations seem to indicate an
anomalous behaviour of young clusters, which includes slightly sub-solar iron
(Fe) abundances and extreme, unexpectedly high barium (Ba) enhancements. In a
series of papers, we plan to expand our understanding of this topic and
investigate whether these chemical peculiarities are instead related to
abundance analysis techniques. We present a new determination of the
atmospheric parameters for 23 dwarf stars observed by the Gaia-ESO survey in
five young OCs (younger than 150 Myr) and one star-forming region (NGC 2264).
We exploit a new method based on titanium (Ti) lines to derive the
spectroscopic surface gravity, and most importantly, the microturbulence
parameter. A combination of Ti I and Fe I lines is used to obtain effective
temperatures. We also infer the abundances of Fe II, Ti II, Na I, Mg I, Al I,
Si I, Ca I, Cr I and Ni I. Our findings are in fair agreement with Gaia-ESO
iDR5 results for effective temperatures and surface gravities, but suggest that
for very young stars, the microturbulence parameter is over-estimated when Fe
lines are employed. This affects the derived chemical composition and causes
the metal content of very young clusters to be under-estimated. Our clusters
display a metallicity [Fe/H] between +0.04 and +0.12; they are not more metal
poor than the Sun. Although based on a relatively small sample size, our
explorative study suggests that we may not need to call for ad hoc explanations
to reconcile the chemical composition of young OCs with Galactic chemical
evolution models.
