HD 271791: An Extreme Supernova Runaway B Star Escaping from the Galaxy^*

Hypervelocity stars (HVSs) were first predicted by theory to be the result of the tidal disruption of a binary system by a supermassive black hole (SMBH) that accelerates one component to beyond the Galactic escape velocity (the Hills mechanism). Because the Galactic center hosts such a SMBH it is the suggested place of origin for HVSs. However, the SMBH paradigm has been challenged recently by the young HVS HD 271791 because its kinematics point to a birthplace in the metal-poor rim of the Galactic disk. Here we report the atmosphere of HD 271791 to indeed show a subsolar iron abundance along with an enhancement of the α-elements, indicating capture of nucleosynthesis products from a supernova or a more energetic hypernova. This implies that HD 271791 is the surviving secondary of a massive binary system disrupted in a supernova explosion. No such runaway star has ever been found to exceed the Galactic escape velocity; hence HD 271791 is the first hyperrunaway star. Such a runaway scenario is an alternative to the Hills mechanism for the acceleration of some HVSs with moderate velocities. The observed chemical composition of HD 271791 puts invaluable observational constraints on nucleosynthesis in a supernova from the core collapse of a very massive star (M_ZAMS≳ 55 M_☉), which may be observed as a gamma-ray burst of the long-duration/soft-spectrum type.