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

arXiv (Cornell University)

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 $2σ$ significance. Additionally, we predict that there are 5-45 unbound HVSs in the complete Gaia catalogue ($1σ$ 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.

Authors 9

1. Sill Verberne Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory , Leiden University , P.O. Box 9513 , 2300 RA Leiden , the Netherlands

2. Elena Maria Rossi Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory , Leiden University , P.O. Box 9513 , 2300 RA Leiden , the Netherlands

3. Sergey E. Koposov
4. Tommaso Marchetti

   European Southern Observatory

   Affiliation as printed

   European Southern Observatory , Karl-Schwarzschild-Strasse 2 , 85748 Garching bei Munchen , Germany

5. Konrad H. Kuijken Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory , Leiden University , P.O. Box 9513 , 2300 RA Leiden , the Netherlands

6. Zephyr Penoyre Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory , Leiden University , P.O. Box 9513 , 2300 RA Leiden , the Netherlands

7. Fraser A. Evans

   University of Toronto

   Affiliation as printed

   David A. Dunlap Department of Astronomy and Astrophysics , University of Toronto , 50 St. George Street , Toronto , ON M5S 3H4 , Canada

   Dunlap Institute for Astronomy and Astrophysics , University of Toronto , 50 St. George Street , Toronto , ON M5S 3H4 , Canada

8. Dimitris Souropanis

   FORTH Institute of Astrophysics · Isaac Newton Group · Foundation for Research and Technology Hellas

   Affiliation as printed

   Institute of Astrophysics FORTH , 71110 Heraklion , Greece

   Isaac Newton Group of Telescopes , Apartado 321 , E-38700 Santa Cruz de La Palma , Canary Islands , Spain

9. Clár-Bríd Tohill

   University of Nottingham · Isaac Newton Group

   Affiliation as printed

   Isaac Newton Group of Telescopes , Apartado 321 , E-38700 Santa Cruz de La Palma , Canary Islands , Spain

   University of Nottingham , School of Physics & Astronomy , Nottingham , NG7 2RD , UK