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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 463; no. 4; p. 4366
• Main Authors: Reddy, Arumalla BS, Lambert, David L, Giridhar, Sunetra
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 21.12.2016
• Subjects: Accretion disks; Astronomy; Clusters; Disks; Galactic clusters; Kinematics; Metallicity; Milky Way; Open clusters; Slopes; Spectrum analysis; Star & galaxy formation; Stars
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stw2287

We have collected high-dispersion echelle spectra of red giant members in the 12 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 R sub( gc) < 12 kpc (>12 kpc). We demonstrate that the sample of clusters constituting a steep radial metallicity gradient of slope -0.052 plus or minus 0.011 dex kpc super( -1) at R sub( gc) < 12 kpc are younger than 1.5 Gyr and located close to the Galactic mid-plane (|...z| <...0.5 kpc) with kinematics typical of the thin disc. Whereas the clusters describing a shallow slope of -0.015 plus or minus 0.007 dex kpc super( -1) at R sub( gc) > 12 kpc are relatively old, thick disc members with a striking spread in age and height above the mid-plane (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 inwards of 12 kpc but the orbital eccentricity has taken them to present locations very far from their birthplaces. (ProQuest: ... denotes formulae/symbols omitted.)