SDSS J013655.91+242546.0 – an A-type hyper-velocity star from the outskirts of the Galaxy

Abstract

Hyper-velocity stars (HVS) are moving so fast that they are unbound to the Galaxy. Dynamical ejection by a supermassive black hole is favoured to explain their origin. Locating the place of birth of an individual HVS is of utmost importance to understanding the ejection mechanism. SDSS J013655.91+242546.0 (J0136+2425 for short) was found amongst three high-velocity stars (drawn from a sample of more than 10000 blue stars), for which proper motions were measured. A kinematical as well as a quantitative NLTE spectral analysis was performed. When combined with the radial velocity (RV) and the spectroscopic distance, the trajectory of the star in the Galactic potential was reconstructed. J0136+2425 is found to be an A-type main-sequence star travelling at $\approx$590 \kms, possibly unbound to the Galaxy and originating in the outer Galactic rim nowhere near the Galactic centre. J0136+2425 is the second HVS candidate with measured proper motion, besides the massive B star HD 271791, and also the second for which its proper motion excludes a Galactic centre origin and, hence, the SMBH slingshot mechanism. Most known HVS are late B-type stars of about 3 M$_\odot$. With a mass of 2.45 M$_\odot$, J0136+2425 resembles a typical HVS far more than HD 271791 does. Hence, this is the first time that a typical HVS is found not to originate in the Galactic centre. Its ejection velocity from the disk is so high (550 \kms) that the extreme supernova binary scenario proposed for HD 271791 is very unlikely.