Runaway Stars, Hypervelocity Stars, and Radial Velocity Surveys

TL;DR

This paper models the distribution and properties of runaway stars ejected from the Galactic disk, comparing them with hypervelocity stars, and assesses their detectability in radial velocity surveys.

Contribution

It provides a simulation-based analysis of runaway star trajectories, spatial distributions, and their observational signatures, highlighting differences from hypervelocity stars.

Findings

Runaways have a flattened spatial distribution with higher velocities near the Galactic plane.

Massive runaways are more concentrated towards the disk due to shorter lifetimes.

Radial velocity surveys are unlikely to confuse runaways with hypervelocity stars, with minimal contamination.

Abstract

Runaway stars ejected from the Galactic disk populate the halo of the Milky Way. To predict the spatial and kinematic properties of runaways, we inject stars into a Galactic potential, compute their trajectories through the Galaxy, and derive simulated catalogs for comparison with observations. Runaways have a flattened spatial distribution, with higher velocity stars at Galactic latitudes less than 30 degrees. Due to their shorter stellar lifetimes, massive runaway stars are more concentrated towards the disk than low mass runaways. Bound (unbound) runaways that reach the halo probably originate from distances of 6--12 kpc (10--15 kpc) from the Galactic center, close to the estimated origin of the unbound runaway star HD 271791. Because runaways are brighter and have smaller velocities than hypervelocity stars (HVSs), radial velocity surveys are unlikely to confuse runaway stars with HVSs. We estimate that at most 1 runaway star contaminates the current sample. We place an upper limit of 2% on the fraction of A-type main sequence stars ejected as runaways.