Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center

Warren R. Brown, Margaret J. Geller, Scott J. Kenyon, Michael J. Kurtz Benjamin C. Bromley

(1) Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge, MA 02138
(2) Department of Physics, University of Utah, 115 S 1400 E, Salt Lake City, UT 84112

Paper: ApJ, 2007, accepted

EPrint Server: 0709.1471


We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars traveling with such extreme velocities that dynamical ejection from a massive black hole (MBH) is their only suggested origin. We also detect a population of possibly bound HVSs. The significant asymmetry we observe in the velocity distribution - we find 26 stars with v_rf>275 km s-1 and 1 star with v_rf<-275 km s-1 - shows that the HVSs must be short-lived, probably 3 - 4 M_\sun main sequence stars. Any population of hypervelocity post-main sequence stars should contain stars falling back onto the Galaxy, contrary to the observations. The spatial distribution of HVSs also supports the main sequence interpretation: longer-lived 3 M_\sun HVSs fill our survey volume; shorter-lived 4 M_\sun HVSs are missing at faint magnitudes. We infer that there are 96+/-10 HVSs of mass 3 - 4 M_\sun within R<100 kpc, possibly enough HVSs to constrain ejection mechanisms and potential models. Depending on the mass function of HVSs, we predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs favor a continuous ejection process, although a 120 Myr-old burst of HVSs is also allowed.

Preprints available from the authors at wbrown@cfa.harvard.edu , or the raw TeX (no figures) if you click here.

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