Abundance Ratios in Open Clusters and Field Giants of the Outer Galactic Disk

Abstract

We present elemental abundance ratios for bright red giants in the old, distant open clusters Berkeley 20, Berkeley 21, NGC 2141, Berkeley 29, and Berkeley 31 as well as for distant field red giants in the direction of the southern Galactic warp. At Galactocentric distances beyond 10 to 12 kpc, the metallicity gradient disappears. The open cluster and field red giants of the outer disk exhibit an almost constant iron abundance [Fe/H] = −0.5. We also measured the abundances for 15 other elements and found very similar compositions in the field and cluster stars. Specifically, they all show enhancements of the α elements O, Mg, Si, Ca, and Ti as well as the r-process element Eu. These abundance patterns reveal a rapid star formation history for the outer Galactic disk. A possible explanation for the measured compositions is that the outer disk formed as a result of star formation triggered by a series of merger or accretion events.