The Origin of HVS17, an Unbound Main Sequence B Star at 50 kpc

TL;DR

HVS17 is a chemically peculiar main sequence B star traveling at high velocity in the Milky Way's outer halo, likely ejected from the Galactic center, and can be used to probe the galaxy's dark matter halo shape.

Contribution

This study provides the first detailed analysis of HVS17's properties, confirming its Galactic center origin and demonstrating its potential as a dark matter halo probe.

Findings

HVS17 is a 3.91 solar mass, 153 Myr old star at 48.5 kpc from the Galactic center.

Its velocity and age challenge disk runaway ejection scenarios.

Gaia proper motions can distinguish its origin, aiding dark matter studies.

Abstract

We analyze Keck ESI spectroscopy of HVS17, a B-type star traveling with a Galactic rest frame radial velocity of +445 km/s in the outer halo of the Milky Way. HVS17 has the projected rotation of a main sequence B star and is chemically peculiar, with solar iron abundance and sub-solar alpha abundance. Comparing measured T_eff and logg with stellar evolution tracks implies that HVS17 is a 3.91 +-0.09 Msun, 153 +-9 Myr old star at a Galactocentric distance of r=48.5 +-4.6 kpc. The time between its formation and ejection significantly exceeds 10 Myr and thus is difficult to reconcile with any Galactic disk runaway scenario involving massive stars. The observations are consistent, on the other hand, with a hypervelocity star ejection from the Galactic center. We show that Gaia proper motion measurements will easily discriminate between a disk and Galactic center origin, thus allowing us to use HVS17 as a test particle to probe the shape of the Milky Way's dark matter halo.