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SkyMapper Protected Science Project

Hypervelocity Stars

Warren R. Brown (CfA), Brian Schmidt (RSAA), Stefan Keller (RSAA), Margaret Geller (CfA), Scott Kenyon (CfA)

Hypervelocity stars travel with such extreme velocities that dynamical ejection involving a massive black hole is their most likely origin. Observers have discovered about two dozen unbound main sequence stars since the first in 2005, and their anisotropic spatial distribution is puzzling. Interestingly, different predicted ejection mechanisms yield different spatial and flight time distributions of unbound stars. SkyMapper is crucial for understanding what is going on. SkyMapper uvgri-band photometry will enable us to efficiently identify short-lived main sequence stars at faint magnitudes, stars that should not exist in the outer halo unless they were ejected there. Spectroscopy of these targets will yield a large and complete sample of unbound stars over the southern sky, and thereby provide a strong constraint on hypervelocity star origin. Hypervelocity stars are important because their properties probe the nature and environment of the Milky Way's massive black hole, and, with future Gaia proper motions measurements, their trajectories may provide unique tracers of the dark matter halo that surrounds the Milky Way.