Core-collapse supernovae in binaries as the origin of galactic hyper-runaway stars

Abstract

ABSTRACT Several stars detected moving at velocities near to or exceeding the Galactic escape speed likely originated in the Milky Way disc. We quantitatively explore the ‘binary supernova scenario’ hypothesis, wherein these ‘hyper-runaway’ stars are ejected at large peculiar velocities when their close, massive binary companions undergo a core-collapse supernova and the binary is disrupted. We perform an extensive suite of binary population synthesis simulations evolving massive systems to determine the assumptions and parameters that most impact the ejection rate of fast stars. In a simulation tailored to eject fast stars, we find the most likely hyper-runaway star progenitor binary is composed of a massive (${\sim}30\, \mathrm{ M}_{\odot }$) primary and an ${\sim}3\!-\!4\, \mathrm{ M}_{\odot }$ companion on an orbital period that shrinks to ≲1 d prior to the core collapse following a common-envelope phase. The black hole remnant formed from the primary must receive a natal kick ≳1000 km s−1 to disrupt the binary and eject the companion at a large velocity. We compare the fast stars produced in these simulations to a contemporary census of early-type Milky Way hyper-runaway star candidates. We find that these rare objects may be produced in sufficient number only when poorly constrained binary evolution parameters related to the strength of post-core-collapse remnant natal kicks and common-envelope efficiency are adjusted to values currently unsupported – but not excluded – by the literature. We discuss observational implications that may constrain the existence of these putative progenitor systems.