Dynamical and evolutionary constraints on the nature and origin of hypervelocity stars

TL;DR

This paper investigates the origins of hypervelocity stars in the Milky Way, using observations and dynamical models to evaluate different ejection scenarios and constrain their likelihood based on stellar populations near the galactic center.

Contribution

It provides strong constraints on hypervelocity star ejection mechanisms by comparing observational data with dynamical and evolutionary models, favoring the binary disruption scenario.

Findings

IMBH inspiral scenario requires too many B stars near the MBH

Scattering by stellar BHs may contribute a small fraction of HVSs

Binary disruption remains consistent with observations

Abstract

In recent years several hypervelocity stars (HVSs) have been observed in the halo of our Galaxy. Such stars are thought to be ejected through dynamical interactions near the massive black hole (MBH) in the Galactic center. Three scenarios have been suggested for their ejection; binary disruption by a MBH, scattering by inspiraling IMBH and scattering by stellar BHs close to MBH. These scenarios involve different stellar populations in the Galactic center. Here we use observations of the Galactic center stellar population together with dynamical and evolutionary arguments to obtain strong constraints on the nature and origin of HVSs. We show that the IMBH inspiral scenario requires too many (O(10^3)) main sequence B stars to exist close to the MBH (<0.01 pc) at the time of inspiral, where current observations show O(10) such stars. Scattering by SBHs also require too many B stars to be observed in the GC, but it may contribute a small fraction of the currently observed HVSs. The binary disruption scenario is still consistent with current observations. In addition it is shown that recently suggested signatures for HVSs origin such as hypervelocity binaries and slow rotating HVSs are much weaker than suggested and require too large statistics. In addition, we show that due to the conditions close to the MBH most binary star systems are not expected to survive for long in this region. Consequently, unique stellar populations that require long evolution in binaries are not expected to be ejected as HVSs in the BHs scattering mechanisms (this may also be related to to the recently observed asymmetry in the velocity distribution of HVSs).