Red Runaways II: Low mass Hills stars in SDSS Stripe 82

TL;DR

This study searches for nearby stars ejected from the Galactic center using SDSS Stripe 82 data, identifying candidates with velocities consistent with ejection but not exceeding escape speed, especially among metal-rich stars.

Contribution

It introduces a new method combining proper motions and radial velocities to efficiently identify potential Galactic center ejecta candidates.

Findings

226 stars on orbits consistent with Galactic center ejection

29 metal-rich candidates with [Fe/H] > -0.8 dex

Improved candidate selection efficiency by 20 times

Abstract

Stars ejected from the Galactic centre can be used to place important constraints on the Milky Way potential. Since existing hypervelocity stars are too distant to accurately determine orbits, we have conducted a search for nearby candidates using full three-dimensional velocities. Since the efficacy of such studies are often hampered by deficiencies in proper motion catalogs, we have chosen to utilize the reliable, high-precision SDSS Stripe 82 proper motion catalog. Although we do not find any candidates which have velocities in excess of the escape speed, we identify 226 stars on orbits that are consistent with Galactic centre ejection. This number is significantly larger than what we would expect for halo stars on radial orbits and cannot be explained by disk or bulge contamination. If we restrict ourselves to metal-rich stars, we find 29 candidates with [Fe/H] > -0.8 dex and 10 with [Fe/H] > -0.6 dex. Their metallicities are more consistent with what we expect for bulge ejecta, and so we believe these candidates are especially deserving of further study. We have supplemented this sample using our own radial velocities, developing an algorithm to use proper motions for optimizing candidate selection. This technique provides considerable improvement on the blind spectroscopic sample of SDSS, being able to identify candidates with an efficiency around 20 times better than a blind search.