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Published July 20, 2012 | Published + Accepted Version
Journal Article Open

The Nature of Hypervelocity Stars and the Time between Their Formation and Ejection


We obtain Keck HIRES spectroscopy of HVS5, one of the fastest unbound stars in the Milky Way halo. We show that HVS5 is a 3.62 ± 0.11 M_☉ main-sequence B star at a distance of 50 ± 5 kpc. The difference between its age and its flight time from the Galactic center is 105 ± 18 (stat) ±30 (sys) Myr; flight times from locations elsewhere in the Galactic disk are similar. This 10^8 yr "arrival time" between formation and ejection is difficult to reconcile with any ejection scenario involving massive stars that live for only 10^7 yr. For comparison, we derive arrival times of 10^7 yr for two unbound runaway B stars, consistent with their disk origin where ejection results from a supernova in a binary system or dynamical interactions between massive stars in a dense star cluster. For HVS5, ejection during the first 10^7 yr of its lifetime is ruled out at the 3σ level. Together with the 10^8 yr arrival times inferred for three other well-studied hypervelocity stars (HVSs), these results are consistent with a Galactic center origin for the HVSs. If the HVSs were indeed ejected by the central black hole, then the Galactic center was forming stars ≃200 Myr ago, and the progenitors of the HVSs took ≃100 Myr to enter the black hole's loss cone.

Additional Information

© 2012 American Astronomical Society. Received 2012 March 27; accepted 2012 June 7; published 2012 June 27. This work was supported in part by the Smithsonian Institution. J. Cohen acknowledges partial support from NSF grant AST-0908139. This research makes use of NASA's Astrophysics Data System Bibliographic Services. We are grateful to the many people who have worked to make the Keck Telescopes and their instruments a reality, and who operate and maintain these observatories. The authors extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible.

Attached Files

Published - Brown2012p19117Astrophys_J_Lett.pdf

Accepted Version - 1206.4057.pdf


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