The Galactic potential and the asymmetric distribution of hypervelocity stars

TL;DR

This paper investigates the asymmetry in hypervelocity star kinematics to constrain the Galactic potential, identify stellar types, and estimate the number of ejected stars, revealing insights into the Galaxy's structure and star origins.

Contribution

It introduces a novel method using kinematic asymmetry to probe the Galactic potential and determine stellar origins, enhancing understanding of hypervelocity stars and Galactic structure.

Findings

Observed asymmetry favors specific Galactic potential models.

Estimated at least 54±8 main sequence HVSs in the survey.

Other stars are likely halo-born blue horizontal branch stars.

Abstract

In recent years several hypervelocity stars (HVSs) have been observed in the halo of our Galaxy. Such HVSs have possibly been ejected from the Galactic center and then propagated in the Galactic potential up to their current position. The recent survey for candidate HVSs show an asymmetry in the kinematics of candidate HVSs (position and velocity vectors), where more outgoing stars than ingoing stars (i.e. positive Galactocentric velocities vs. negative ones) are observed. We show that such kinematic asymmetry, which is likely due to the finite lifetime of the stars and Galactic potential structure, could be used in a novel method to probe and constrain the Galactic potential, identify the stellar type of the stars in the survey and estimate the number of HVSs. Kinematics-independent identification of the stellar types of the stars in such surveys (e.g. spectroscopic identification) could further improve these results. We find that the observed asymmetry between ingoing and outgoing stars favors specific Galactic potential models. It also implies a lower limit of ~54+-8 main sequence HVSs in the survey sample (>=648+-96 in the Galaxy), assuming that all of the main sequence stars in the survey originate from the Galactic center. The other stars in the survey are likely to be hot blue horizontal branch stars born in the halo rather than stars ejected from the Galactic center.