STAR CLUSTERS IN THE GALACTIC ANTICENTER STELLAR STRUCTURE AND THE ORIGIN OF OUTER OLD OPEN CLUSTERS

摘要

The Galactic anticenter stellar structure (GASS) has been identified with excess surface densities of field stars in several large-area sky surveys and with an unusual, stringlike grouping of five globular clusters. At least two of these are diffuse, young "transitional" clusters between open and globular types. Here we call attention to the fact that four younger open or transitional clusters extend the previously identified, stringlike cluster grouping, with at least one having a radial velocity consistent with the previously found GASS velocity-longitude trend. All nine clusters lie close to a plane tipped 17° to the Galactic plane. This planar orientation is used to forage for additional potential cluster members in the inner Galaxy, and a number are found along the same plane and stringlike sequence, including almost all 15 known outer, old open clusters. Tidal accretion of a dwarf satellite galaxy on a low-inclination orbit―perhaps the GASS system―appears to be a plausible explanation for the origin of the outer, old open and transitional clusters of the Milky Way. We use these clusters to explore the age-metallicity relation of the putative accreted GASS progenitor. Finally, we provide the first radial velocity of a star in the cluster BH 176 and discuss its implications.