The Gaia-ESO Survey: the present-day radial metallicity distribution of the Galactic disc probed by pre-main-sequence clusters

Context. The radial metallicity distribution in the Galactic thin discrepresents a crucial constraint for modelling disc formation andevolution. Open star clusters allow us to derive both the radialmetallicity distribution and its evolution over time.Aims. In this paper we perform the first investigation of thepresent-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 fortracing the current interstellar medium metallicity.Methods. We used the products of the Gaia-ESO Survey analysis of 12young regions (age < 100 Myr), covering Galactocentric distances from6.67 to 8.70 kpc. For the first time, we derived the metal content ofstar forming regions farther than 500 pc from the Sun. Medianmetallicities were determined through samples of reliable clustermembers. For ten clusters the membership analysis is discussed in thepresent paper, while for other two clusters (i.e. Chamaeleon I and GammaVelorum) we adopted the members identified in our previous works.Results. All the pre-main-sequence clusters considered in this paperhave close-to-solar or slightly sub-solar metallicities. The radialmetallicity distribution traced by these clusters is almost flat, withthe innermost star forming regions having [Fe/H] values that are0.10-0.15 dex lower than the majority of the older clusters located atsimilar Galactocentric radii.Conclusions. This homogeneous study of the present-day radialmetallicity distribution in the Galactic thin disc favours models thatpredict a flattening of the radial gradient over time. On the otherhand, the decrease of the average [Fe/H] at young ages is not easilyexplained by the models. Our results reveal a complex interplay ofseveral processes (e.g. star formation activity, initial mass function,supernova yields, gas flows) that controlled the recent evolution of theMilky Way.