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� ), 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.