Abundance patterns of old open clusters as tracers of galactic chemical evolution

Abstract

We have initiated a program to explore the presence of chemical inhomogeneities in the Galactic disk using the open clusters (OCs) as ideal probes. We have obtained high S/N - ratio and high- dispersion echelle spectra (R ≥ 55,000) of red giant members for eighteen OCs using the 2.7-m Harlan J. Smith telescope at the McDonald observatory and measured abundances for many elements representing different production mechanisms (α- and r- process, Fe- peak, and s-process) and sites (i.e. Type II SN, Type Ia SN, and AGB star environments). The membership to the cluster has been confirmed through their radial velocities and proper motions. The spread in temperatures and gravities being very small among the red giants, nearly the same stellar lines were employed thereby reducing the random errors. The errors of average abundance for the cluster were generally in 0.02 to 0.08 dex range. Synthetic spectra were computed for species affected by hyperfine and isotopic splitting or affected by blends. Our sample of eighteen OCs supports the view that both the field and OCs giants of near-solar metallicity have very similar, if not identical, compositions (within the errors of measurements) for alpha, Fe-peak and r-process elements. We have noticed a small but significant enrichment in [s-process/Fe] abundance ratios among young OCs, suggesting that the Galaxy has received significant contribution from low mass AGB stars. We find intracluster abundance variations for some s-process elements, for example Zr and Ba. We merged our sample of OCs with the available high-quality results in the literature and a suitable normalization has been done with extreme care to place all the results on a common abundance scale. We recalculated the Rgc value for each of these clusters to bring them to a common distance scale to study the metallicity gradient(s) in the Galactic disk. We derived membership probabilities and assigned all these OCs to either the thin disk, thick disk or halo stellar populations to know their kinematic origin. We also studied the dynamics of these OCs using a multicomponent galactic gravitational potential model and derived birthplaces and other orbital parameters. The connection between the observed gradients in the Galactic disk and the spiral density waves is explored. The modulation of smooth metallicity distribution with Rgc and the spread in metallicity near 8-9 kpc and 11-12 kpc is well explained by the resonance interaction of disk material with spiral density waves and hence the subsequent exchange of metal rich gas and OCs near corotation. We argue that orbital migration of old OCs born in the inner regions is responsible for the flat abundance gradient in the outer disk. The ratio of alpha-elements to Fe of the sample does not vary appreciably with the Rgc, which reveals an homogeneous history of star formation. Future studies of significantly extended sample of OCs allowing the study of radial as well as azimuthal variations of metallicity for a range of elements analysed identically and on homogeneous scale is required to test these results and to enhance our knowledge on the chemical evolution of the Galactic disk. We emphasize that all these studies eventually would provide insight on the evolution of disks in galaxies in time and space.