We analyze Keck ESI spectroscopy of HVS17 , a B-type star traveling with a Galactic rest frame radial velocity of +445 km s ^ { -1 } 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 \log { g } with stellar evolution tracks implies that HVS17 is a 3.91 \pm 0.09 M _ { \odot } , 153 \pm 9 Myr old star at a Galactocentric distance of r = 48.5 \pm 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 .