Hypervelocity stars from star clusters hosting Intermediate-Mass Black Holes

TL;DR

This paper investigates hypervelocity stars ejected from star clusters with intermediate-mass black holes, showing their distinct observable properties and potential to reveal IMBH presence, contrasting with Galactic Centre ejections.

Contribution

It models the ejection of hypervelocity stars from IMBH-hosting clusters, incorporating the cluster's orbit and Galactic potential effects, providing new observational signatures for IMBH detection.

Findings

HVSs from IMBHs do not follow radial Galactic centre orbits.

Ejected stars show distinctive velocity and positional distributions.

These signatures can differentiate IMBH ejections from other origins.

Abstract

Hypervelocity stars (HVSs) represent a unique population of stars in the Galaxy reflecting properties of the whole Galactic potential. Determining their origin is of fundamental importance to constrain the shape and mass of the dark halo. The leading scenario for the ejection of HVSs is an encounter with the supermassive black hole in the Galactic Centre. However, new proper motions from the \textit{Gaia} mission indicate that only the fastest HVSs can be traced back to the Galactic centre and the remaining stars originate in the disc or halo. In this paper, we study HVSs generated by encounters of stellar binaries with an intermediate-mass black hole (IMBH) in the core of a star cluster. For the first time, we model the effect of the cluster orbit in the Galactic potential on the observable properties of the ejected population. HVSs generated by this mechanism do not travel on radial orbits consistent with a Galactic centre origin, but rather point back to their parent cluster, thus providing observational evidence for the presence of an IMBH. We also model the ejection of high-velocity stars from the Galactic population of globular clusters, assuming that they all contain an IMBH, including the effects of the cluster's orbit and propagation of the star in the Galactic potential up to detection. We find that high-velocity stars ejected by IMBHs have distinctive distributions in velocity, Galactocentric distance and Galactic latitude, which can be used to distinguish them from runaway stars and stars ejected from the Galactic Centre.