Elemental Abundance Ratios in Stars of the Outer Galactic Disk. I. Open Clusters

We summarize radial velocity studies of selected stars in the old, distantclusters Berkeley 20, Berkeley 21, NGC 2141, Berkeley 29, and Berkeley 31.Three members of M67 were observed similarly, and those velocities compareextremely well with prior measures. We measured the chemical compositions ofall six clusters for [Fe/H] as well as abundances of alpha elements, iron-peakelements, and those synthesized in either the s-process or the r-process. For the outer disk, the abundance gradient for [Fe/H] deviates from the trenddefined near the solar neighborhood. Rather than declining with increasingGalactocentric distance, [Fe/H] appears to reach a "basement" at [Fe/H]=-0.5beyond R_GC = 10 to 12kpc. We find enhanced [O/Fe], [alpha/Fe], and [Eu/Fe] inthe outer disk revealing a rapid star formation history. An intriguing, but tentative, conclusion is that the outer disk open clusterabundance ratios are consistent with the outer disk being formed via a mergerevent. The basement in [Fe/H] and enhanced [alpha/Fe] suggest that the outerdisk formed from a reservoir of gas with a star formation history distinct fromthe solar neighborhood. The open clusters may have formed as a result of starformation triggered by a merger event in the outer disk. The ages of the outerdisk open clusters would then be a measure of when the merger occurred.[Abridged]Comment: Accepted for publication in A