Hypervelocity Stars: From the Galactic Center to the Halo

Scott J. Kenyon(1), Benjamin C. Bromley(2), Margaret J. Geller(1), Warren R. Brown(1)


(1) Smithsonian Astrophysical Observatory,, 60 Garden St., Cambridge, MA 02138
(2) Department of Physics, University of Utah,, 115 S 1400 E, Rm 201, Salt Lake City, UT 84112

Paper: ApJ, submitted

EPrint Server: 0801.3479


Abstract:

Hypervelocity stars (HVS) traverse the Galaxy from the central black hole to the outer halo. We show that the Galactic potential within 200 pc acts as a high pass filter preventing low velocity HVS from reaching the halo. To trace the orbits of HVS throughout the Galaxy, we construct two forms of the potential which reasonably represent the observations in the range 5-105 pc, a simple spherically symmetric model and a bulge-disk-halo model. We use the Hills mechanism (disruption of binaries by the tidal field of the central black hole) to inject HVS into the Galaxy and compute the observable spatial and velocity distributions of HVS with masses in the range 0.6-4 Mo. These distributions reflect the mass function in the GC, properties of binaries in the GC, and aspects of stellar evolution and the injection mechanism. For 0.6-4 \msun main sequence stars, the fraction of unbound HVS and the asymmetry of the velocity distribution for their bound counterparts increases with stellar mass. The density profiles for unbound HVS decline with distance from the GC approximately as r-2 (but are steeper for the most massive stars which evolve off the main sequence during their travel time from the GC); the density profiles for the bound ejecta decline with distance approximately as r-3. In a survey with limiting magnitude V <= 23, the detectability of HVS (unbound or bound) increases with stellar mass.


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

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