The Gaia-ESO Survey: the present-day radial metallicity distribution of
  the Galactic disc probed by pre-main-sequence clusters
  The radial metallicity distribution in the Galactic thin disc represents a
crucial constraint for modelling disc formation and evolution. Open clusters
allow us to derive both the radial metallicity distribution and its evolution
over time. In this paper we perform the first investigation of the present-day
radial metallicity distribution based on [Fe/H] determinations in late type
members of pre-main-sequence clusters. Because of their youth, these clusters
are therefore essential for tracing the current inter-stellar medium
metallicity. We used the products of the Gaia-ESO Survey analysis of 12 young
regions (age<100 Myr), covering Galactocentric distances from 6.67 to 8.70 kpc.
For the first time, we derived the metal content of star forming regions
farther than 500 pc from the Sun. Median metallicities were determined through
samples of reliable cluster members. For ten clusters the membership analysis
is discussed in the present paper, while for other two clusters (Chamaeleon I
and Gamma Velorum) we adopted the members identified in our previous works. All
the pre-main-sequence clusters considered in this paper have close-to-solar or
slightly sub-solar metallicities. The radial metallicity distribution traced by
these clusters is almost flat, with the innermost star forming regions having
[Fe/H] values that are 0.10-0.15 dex lower than the majority of the older
clusters located at similar Galactocentric radii. This homogeneous study of the
present-day radial metallicity distribution in the Galactic thin disc favours
models that predict a flattening of the radial gradient over time. On the other
hand, the decrease of the average [Fe/H] at young ages is not easily explained
by the models. Our results reveal a complex interplay of several processes
(e.g. star formation activity, initial mass function, supernova yields, gas
flows) that controlled the recent evolution of the Milky Way.
