7 - Hercules-stream stars and the metal-rich thick disk

Summary

Using the MIKE spectrograph, mounted on the 6.5-m Magellan/Clay telescope at the Las Campanas observatory in Chile, we have obtained highresolution spectra for 60 F and G dwarf stars, all likely members of a density enhancement in the local velocity distribution, referred to as the Hercules stream. By comparing with an existing sample of 102 thin- and thick-disk stars we have used space velocities, detailed elemental abundances, and stellar ages to trace the origin of the Hercules stream. We find that the Hercules-stream stars exhibit a wide spread in stellar ages, metallicities, and element abundances. However, the spreads are not random but separate the Hercules stream into the abundance and age trends outlined by either the thin disk or the thick disk. We hence claim that the major constituents of the Hercules stream actually are thin- and thick-disk stars. These diverse properties of the Hercules stream indicate a dynamical origin, probably caused by the Galactic bar. However, we can at the moment not entirely rule out the possibility that the Hercules stream could be the remnants of a relatively recent merger event.