tokens: younger objects suggests a much shallower gradient: we derive slopes equal to and dex kpc for the SFRs and all the associations younger than 100 Myr (i.e. SFRs + YOCs), respectively, through a linear fit based on the orthogonal distance regression method and taking into account the error bars on both the axes. In other words, comparison with the old clusters analysed by Jacobson et al. suggests a flattening of the gradient at very young ages. The results on the evolution of the Galactic metallicity distribution is still based on a small number of metallicity determinations and limited to clusters covering a range of 2 kpc in around the Solar location, thus they cannot be taken as conclusive. However, it is in agreement with and, crucially, extends to younger ages, previous estimates of the evolution of the radial metallicity gradient based on the study of open clusters ( Carraro et al. 1998 ; Friel et al. 2002 ; Magrini et al. 2009 ; Andreuzzi et al. 2011 ; Cunha et al. 2016 ). On the other hand, this result is at variance with other studies based on observations of field stars (e.g. Nordström et al. 2004 ; Casagrande et al. 2011 ; Anders et al. 2017 ). As mentioned already, however, the main difficulty of using field stars to trace the chemical pattern of the Galaxy and its evolution over time is that their ages and distances are not as well constrained as for open clusters, and that field star samples are normally older than 1 Gyr, thus not allowing the study of the latest phases of disc evolution. Whilst the Gaia mission ( Perryman et al. 2001 ; Gaia Collaboration 2016 ) and asteroseismology are allowing and will continue to allow a big step forward in the determination of the distances and ages of field stars, the second problem still remains a major and intrinsic limitation. Fig. 5 Radial metallicity distribution of all the clusters included in the iDR4 catalogue. Their Galactocentric radii and metallicities are reported in Tables 2 and 4 . Red circles indicate star forming regions, blue circles denote young clusters in the age range between 10 and 100 Myr, while intermediate age clusters are shown as black symbols. 5.1. Comparison with literature results and the metallicity distribution at young ages from other tracers In a recent paper Netopil et al. (2016) investigated the metallicity distribution of a sample of 172 clusters, using a homogenised compilation from the literature. They suggested that young clusters (defined in that paper as those younger than 500 Myr) may be characterised by lower metallicities than the older ones, at least in the region between 7 and 9 kpc from the Galactic centre; at the same time, they confirmed the presence of a negative gradient for these clusters. We mention, however, that their sample did not include any clusters younger than Myr located in the inner Galaxy. Fig. 6 Comparison between the metallicity distributions of SFRs and YOCs by our analysis (red and blue dots, respectively) and those of
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