RSAA Colloquia / Seminars / Feast-of-Facts: Thursday, 08 November 2018, 11:00-12:00; Duffield Lecture Theatre

Elena D'Onghia

"The Hercules Stream: resonant stars visiting the Solar vicinity from the inner Galaxy"

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 ~5,000 light-years 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. This study performs action and phase-space analysis to reveal the origin of this stream. We show that Hercules stars originate in the inner part of the Galaxy with stars that have the eccentricity to reach the solar neighborhood. Contrary to previous theories, our analysis, based on a fully consistent simulation of the Milky Way, indicates that the Hercules Stream does not originate in resonances, such as the Lindblad resonances, or at co-rotation with spiral arms. We anticipate the existence of a twin of the Hercules Stream being located in a patch with 180 degrees symmetry from the solar vicinity. Furthermore, our model predicts that Hercules extends to stars moving faster than the Local Standard Rest, a prediction that can be tested by the upcoming releases of Gaia, Galah and SDSS-V.