"The Hercules Stream: resonant stars visiting the Solar vicinity"
The origin of the stars with coherent velocities discovered in the solar neighborhood remains uncertain, but their dynamics and evolution are key to understanding the dynamics of the Galactic stellar disk and ultimately the history of the Milky Way. The Gaia satellite is currently mapping the phase-space of a few million stars in the solar neighborhood. About 3.5 million stars within ~1.5 kpc from the Sun are identified in streams, bundles of stars that move together in the same direction with a velocity that is distinct from neighboring stars. The most prominent among them is the Hercules stream revealed with accurate details by Gaia data. This study performs an angle-action-space analysis and investigates the origin of this stream. Contrary to previous theories, our analysis, based on a fully consistent high-resolution simulation of the Milky Way, indicates that the Hercules Stream does not originate at the outer Lindbland resonance, or at corotation with the stellar bar or spiral arms. Instead we show that the Hercules stream consists of visiting stars with eccentric orbits that arise in the inner part of the Galaxy, at the inner-ultra harmonics resonances located at the edge of the bar. We anticipate that stars with the same kinematic features of the Hercules stream should be observed at different azimuthal angle at the Solar radius, a prediction that can be tested by the upcoming releases of Gaia and SDSS-V.