Photometry and Spectroscopy of Old, Outer Disk Star Clusters: vdB-Hagen 176, Berkeley 29, and Saurer 1

Abstract

It has been previously proposed that some distant open clusters in the Milky Way may have been accreted during a dwarf galaxy merger, perhaps associated with the same event that led to the formation of the Galactic anticenter stellar structure (GASS), also known as the Monoceros Ring. We have obtained VI and Washington + DDO51 photometric and medium-resolution (R ~ 8000) multifiber spectroscopic data for the three distant old open clusters Berkeley 29, Saurer 1, and vdB-Hagen 176 (BH?176). These clusters are spatially coincident with GASS, but radial velocities and spectroscopic metallicities were not available during previous studies of the GASS candidate cluster system. Similar data for the clusters Be 20 and Be 39 have been obtained for calibration purposes. We provide the first reliable radial velocity for BH?176 (Vhelio = 11.2 ? 5.3 km s-1). We also find that Vhelio = +95.4 ? 3.6 and +28.4 ? 3.6 km s-1 for Sa 1(A) and Be 29, respectively. In contrast to previous findings, we show that ?-enhanced isochrones, while spectroscopically motivated, provide a poor fit to the Be 29 color-magnitude diagram. We find that the clusters Be 29 and Sa 1 have properties that are consistent with the previously reported characteristics for GASS candidate clusters and the GASS stellar stream as derived from M giant observations. However, the radial velocity and photometric metallicity ([Fe/H] ~ 0.0 dex) for BH?176 suggest that a connection of this cluster with the putative GASS cluster system is unlikely. We reassess the age-metallicity relation for the most likely members of the GASS clusters system for which spectroscopic metallicities are now available.