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) multi-fiber 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 had not been available during previous studies of the GASS candidate cluster system. Similar data for the clusters Berkeley 20 and Berkeley 39 have been obtained for calibration purposes. We provide the first {\it reliable} radial velocity for BH 176 (V_{helio} = 11.2 +/- 5.3 km/s). We also find that V_{helio} = +95.4 +/- 3.6 and +28.4 +/- 3.6 km/s, for Saurer 1(A) and Berkeley 29, respectively. We show that alpha-enhanced isochrones, while spectroscopically motivated, provide a poor fit to Be29 in contrast to previous findings. We find that the clusters Berkeley 29 and Saurer 1 are consistent with the previously reported characteristics for GASS candidate clustersand 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 suggests 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.

arXiv comment15 pages, 14 figures, AJ accepted

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