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

Hypervelocity stars (HVSs) are stars which have been ejected from the Galactic Centre (GC) at velocities of up to a few thousand km s -1 . 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, ho we ver, with only about a dozen promising candidates disco v ered so far. In this work, we make use of a no v el, 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 disco v ered 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 impro v e previous HVS ejection rate constraints. In particular, the ejection rate of HVSs more massive than 1 M⊙ cannot be higher than 10 -5 yr -1 at 2 σsignificance. Additionally, we predict that there are 5-45 unbound HVSs in the complete Gaia catalogue (1 σinterval), most of which will be main-sequence stars of a few M⊙ 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.