http://arxiv.org/abs/1807.05909
Young massive stars in the halo are supposed to be runaway stars from the Galactic disk. Possible ejection scenarios are binary supernova ejections (BSE) or dynamical ejections from star clusters (DE). Hypervelocity stars (HVSs) are extreme runaway stars that are potentially unbound to the Galaxy. Powerful acceleration mechanisms like the tidal disruption of a binary by a supermassive black hole are required. Hence, HVSs are believed to originate in the Galactic center (GC). Gaia DR2 offers the opportunity to study HVSs in an unprecedented manner. We revisit the most interesting HVSs, i.e., 15 stars for which proper motions with the Hubble Space Telescope were obtained in the pre-Gaia era, to unravel their origin. By carrying out kinematic analyses in three different Galactic mass models, kinematic properties are obtained that help to constrain the spatial origins of the HVSs. While HVSs previously considered unbound remain unbound in two Galactic potentials, most stars become bound in the most massive Galactic model. For nine stars (including five unbound candidates), the GC can be ruled out as spatial origin at $2\sigma$ confidence. Using optical and infrared photometry to determine its spectrophotometric distance, we confirm that HVS3 originates in the Large Magellanic Cloud. Our results suggest that a large fraction of the HVSs are actually disk runaway stars launched close to or beyond Galactic escape velocities. Population synthesis models predict that only a small fraction of the HVSs stems from BSE. Furthermore, a maximum ejection velocity of 540\,km/s is predicted for BSE and a similar limit has been found for DE. The ejection velocities of five of our non-GC HVSs are close to or above this limit, calling for the existence of another dynamical ejection mechanism (e.g., massive perturbers such as intermediate mass black holes) besides the classical scenarios mentioned above.
A. Irrgang, S. Kreuzer and U. Heber
Tue, 17 Jul 18
1/79
Comments: Submitted to A&A (Astronomy and Astrophysics)