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

Abstract

Hypervelocity stars (HVSs) were first predicted by theory to be theresult of the tidal disruption of a binary system by a supermassiveblack hole (SMBH) that accelerates one component to beyond the Galacticescape velocity (the Hills mechanism). Because the Galactic center hostssuch a SMBH it is the suggested place of origin for HVSs. However, theSMBH paradigm has been challenged recently by the young HVS HD 271791because its kinematics point to a birthplace in the metal-poor rim ofthe Galactic disk. Here we report the atmosphere of HD 271791 to indeedshow a subsolar iron abundance along with an enhancement of the{α}-elements, indicating capture of nucleosynthesis products from asupernova or a more energetic hypernova. This implies that HD 271791 isthe surviving secondary of a massive binary system disrupted in asupernova explosion. No such runaway star has ever been found to exceedthe Galactic escape velocity; hence HD 271791 is the first hyperrunawaystar. Such a runaway scenario is an alternative to the Hills mechanismfor the acceleration of some HVSs with moderate velocities. The observedchemical composition of HD 271791 puts invaluable observationalconstraints on nucleosynthesis in a supernova from the core collapse ofa very massive star (M_{ZAMS}{\gt}~55 M_{solar}), whichmay be observed as a gamma-ray burst of the long-duration/soft-spectrumtype.Based on observations obtained at the European Southern Observatory,proposal 073.D-0495(A).