Improved constraints on Galactic Centre ejection of hypervelocity stars based on novel search method

TL;DR

This paper introduces a novel Gaia-based method to identify hypervelocity star candidates, significantly improves constraints on their ejection rate from the Galactic Centre, and predicts the number of unbound HVSs in the galaxy.

Contribution

The study develops a new, efficient trajectory-based search method for HVSs using Gaia data and refines the ejection rate constraints through detailed simulations.

Findings

Identified 600 HVS candidates with Gaia data, including known stars.

No new HVSs were confirmed through radial velocity measurements.

Ejection rate of massive HVSs is constrained to be less than 10^{-5} per year.

Abstract

Hypervelocity stars (HVSs) are stars which have been ejected from the Galactic Centre (GC) at velocities of up to a few thousand km/s. They are tracers of the Galactic potential and can be used to infer properties of the GC, such as the initial-mass function and assembly history. HVSs are rare, however, with only about a dozen promising candidates discovered so far. In this work we make use of a novel, highly efficient method to identify new HVS candidates in Gaia. This method uses the nearly radial trajectories of HVSs to infer their distances and velocities based on their position and Gaia proper motion alone. Through comparison of inferred distances with Gaia parallaxes and photometry we identified 600 HVS candidates with G<20 including the previously discovered S5-HVS1, out of which we obtained ground-based follow-up observations for 196 stars. As we found no new HVSs based on their radial velocity, we used detailed HVS ejection simulations to significantly improve previous HVS ejection rate constraints. In particular, the ejection rate of HVSs more massive than 1 $\mathrm{M_\odot}$ cannot be higher than $10^{-5}$ yr$^{-1}$ at $2\sigma$ significance. Additionally, we predict that there are 5-45 unbound HVSs in the complete Gaia catalogue ($1\sigma$ interval), most of which will be main-sequence stars of a few M$_\odot$ at heliocentric distances of tens to hundreds of kpc. By comparing our results to literature HVS candidates, we find an indication of either a time-dependent ejection rate of HVSs or a non-GC origin of previously identified HVS candidates.