The evolution of the Milky Way: New insights from open clusters

Abstract

We have collected high-dispersion echelle spectra of red giant members in the twelve open clusters (OCs) and derived stellar parameters and chemical abundances for 26 species by either line equivalent widths or synthetic spectrum analyses. We confirm the lack of an age-metallicity relation for OCs but argue that such a lack of trend for OCs arise from the limited coverage in metallicity compared to that of field stars which span a wide range in metallicity and age. We confirm that the radial metallicity gradient of OCs is steeper (flatter) for Rgc < 12 kpc (> 12 kpc). We demonstrate that the sample of clusters constituting a steep radial metallicity gradient of slope $-$0.052$\pm$0.011 dex kpc$^{-1}$ at Rgc < 12 kpc are younger than 1.5 Gyr and located close to the Galactic midplane (|z| < 0.5 kpc) with kinematics typical of the thin disc. Whereas the clusters describing a shallow slope of $-$0.015$\pm$0.007 dex kpc$^{-1}$ at Rgc > 12 kpc are relatively old, thick disc members with a striking spread in age and height above the midplane (0.5 < |z| < 2.5 kpc). Our investigation reveals that the OCs and field stars yield consistent radial metallicity gradients if the comparison is limited to samples drawn from the similar vertical heights. We argue via the computation of Galactic orbits that all the outer disc clusters were actually born inward of 12 kpc but the orbital eccentricity has taken them to present locations very far from their birthplaces.