HE 0437–5439: An Unbound Hypervelocity Main-Sequence B-Type Star^*

We report the discovery of a 16th magnitude star, HE 0437-5439, with a heliocentric radial velocity of +723 ± 3 km s^-1. A quantitative spectral analysis of high-resolution optical spectra obtained with the VLT and UVES shows that HE 0437-5439 is a main-sequence B-type star with T_eff = 20,350 K, log g = 3.77, a solar (within a factor of a few) helium abundance and metal content, and rotating at v sin i = 54 km s^-1. Using appropriate evolutionary tracks, we derive a mass of 8 M_☉ and a corresponding distance of 61 kpc. Its Galactic rest-frame velocity is at least 563 km s^-1, almost twice the local Galactic escape velocity, indicating that the star is unbound to the Galaxy. Numerical kinematical experiments are carried out to constrain its place of birth. It has been suggested that such hypervelocity stars can be formed by the tidal disruption of a binary through interaction with the supermassive black hole at the Galactic center (GC). HE 0437-5439 needs about 100 Myr to travel from the GC to its present position, much longer than its main-sequence lifetime of 25 Myr. This can only be reconciled if HE 0437-5439 is a blue straggler star. In this case, the predicted proper motion is so small that it can only be measured by future space missions. Since the star is much closer to the Large Magellanic Cloud (LMC; 18 kpc) than to the GC, it can reach its position from the center of the LMC. The proper motion predicted in this case is about 2 mas yr^-1 (relative to the LMC), large enough to be measurable with conventional techniques from the ground. The LMC origin could also be tested by a high-precision abundance analysis.