Mapping the Galactic Metallicity Gradient with Open Clusters: The State-of-the-Art and Future Challenges

In this paper, we make use of data collected for open cluster members by high-resolution spectroscopic surveys and programmes (i.e., APOGEE, Gaia-ESO, GALAH, OCCASO, and SPA). These data have been homogenised and then analysed as a whole. The resulting catalogue contains [Fe/H] and orbital parameters for 251 Galactic open clusters. The slope of the radial metallicity gradient obtained through 175 open clusters with high-quality metallicity determinations is −0.064 ± 0.007 dex kpc⁻¹ . The radial metallicity distribution traced by open clusters flattens beyond R_Gal = 12.1 ± 1.1 kpc. The slope traced by open clusters in the [Fe/H]-L_z diagram is −0.31 ± 0.02 × 10³ dex km⁻¹ kpc⁻¹ s, but it flattens beyond L_z = 2769 ± 177 km kpc s⁻¹ . In this paper, we also review some high-priority practical challenges around the study of open clusters that will significantly push our understanding beyond the state-of-the-art. Finally, we compare the shape of the galactic radial metallicity gradient to those of other spiral galaxies.