Metal abundances in extremely distant Galactic old open clusters. I. Berkeley 29 and Saurer

We report on high-resolution spectroscopy of four giant stars in the Galactic old open clusters Berkeley 29 and Saurer 1 obtained with HIRES at the Keck Telescope. These two clusters possess the largest Galactocentric distances yet known for open star clusters and therefore are crucial objects to probe the chemical pattern and evolution of the outskirts of the Galactic disk. We find that [Fe/H] = -0.38 +/- 0.14 and -0.44 +/- 0.18 for Saurer 1 and Berkeley 29, respectively. On the basis of these data, we first revise the fundamental parameters of the clusters and then discuss them in the context of the Galactic disk radial abundance gradients. Both clusters seem to significantly deviate from the general trend, suggesting that the outer part of the Galactic disk underwent a completely different evolution compared with the inner disk. In particular, Berkeley 29 is clearly associated with the Monoceros stream, whereas Saurer 1 exhibits very different properties. The abundance ratios suggest that the chemical evolution of the outer disk was dominated by the Galactic halo.