Hypervelocity Stars: From the Galactic Center to the Halo

TL;DR

This paper models the trajectories and distributions of hypervelocity stars ejected from the Galactic Center, analyzing how the Galactic potential influences their ability to reach the halo and their observable properties based on mass and evolution.

Contribution

It introduces detailed potential models and simulations of hypervelocity star distributions, linking their properties to the Galactic Center's stellar population and ejection mechanisms.

Findings

Unbound HVS density declines as r^{-2} from the Galactic Center.

Velocity distribution asymmetry increases with stellar mass.

Detectability of HVS depends on stellar mass and survey magnitude limit.

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--100,000 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 Msun. These distributions reflect the mass function in the Galactic Center, properties of binaries in the Galactic Center, 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 Galactic Center approximately as r^{-2} (but are steeper for the most massive stars which evolve off the main sequence during their travel time from the Galactic Center); the density profiles for the bound ejecta decline with distance approximately as r^{-3}. In a survey with a limiting visual magnitude V of 23, the detectability of HVS (unbound or bound) increases with stellar mass.