The B-type giant HD 271791 in the Galactic halo

Abstract

Context: Young, massive stars have been found in the distant Galactichalo. Dynamical ejection from the Galactic disc has been suggested asthe origin of these {\ldquo}run-away stars{\rdquo}. The so-calledhyper-velocity stars have been found to travel so fast that they areunbound to the Galaxy. Only a supermassive black hole (SMBH) appears tobe able to accelerate the stars to such high velocities, which suggeststhat the Galactic centre is their place of origin. Aims: Werevisit the run-away B star HD 271791 to determine its nature andorigin. Methods: High-resolution optical echelle spectra areanalysed using LTE model atmospheres. Based on proper-motionmeasurements, the 3D kinematic of the star is investigated by means ofnumerical experiments. Results: HD 271791 is found to be amassive (11 M$_{&sun;}$), rapidly-rotating B-type star. Itschemical composition is found to be consistent with that of the sun. ItsGalactic rest-frame velocity exceeds the Galactic escape velocity. Conclusions: According to its space velocity, HD 271791 qualifies as ahyper-velocity star. Its kinematic constrains the place of birth to theouter Galactic disc and rules out the Galactic centre. HD 271791 is alsotoo young ({\lt}30 Myr) to have originated in the Galactic centre. Thischallenges the SMBH paradigm for the origin of hyper-velocity stars.Based onobservations collected at the European Southern Observatory at La Silla,Chile,ESO proposal No. 073.D-0495(A).