The B-type giant HD 271791 in the galactic halo - Linking run-away stars to hyper-velocity stars

Context. Young, massive stars have been found in the distant Galactic halo. Dynamical ejection from the Galactic disc has been suggested as the origin of these "run-away stars". The so-called hyper-velocity stars have been found to travel so fast that they are unbound to the Galaxy. Only a supermassive black hole ( SMBH) appears to be able to accelerate the stars to such high velocities, which suggests that the Galactic centre is their place of origin. Aims. We revisit the run-away B star HD 271791 to determine its nature and origin. Methods. High-resolution optical echelle spectra are analysed using LTE model atmospheres. Based on proper-motion measurements, the 3D kinematic of the star is investigated by means of numerical experiments. Results. HD 271791 is found to be a massive (11 M circle dot), rapidly-rotating B-type star. Its chemical composition is found to be consistent with that of the sun. Its Galactic rest-frame velocity exceeds the Galactic escape velocity. Conclusions. According to its space velocity, HD 271791 qualifies as a hyper-velocity star. Its kinematic constrains the place of birth to the outer Galactic disc and rules out the Galactic centre. HD 271791 is also too young (< 30 Myr) to have originated in the Galactic centre. This challenges the SMBH paradigm for the origin of hyper-velocity stars.