Core-Collapse Supernovae in Binaries as the Origin of Galactic Hyper-Runaway Stars

TL;DR

This paper investigates the binary supernova scenario as a mechanism for producing hyper-runaway stars with velocities exceeding the Galactic escape speed, through population synthesis simulations of massive binary systems.

Contribution

It identifies key binary system parameters and conditions that can produce hyper-runaway stars, highlighting the importance of natal kicks and common envelope evolution.

Findings

Most likely progenitors are massive binaries with short orbital periods.

High natal kicks (>1000 km/s) are necessary to disrupt binaries and eject hyper-runaway stars.

Production of hyper-runaway stars depends on poorly constrained binary evolution parameters.

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 which 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$$30\,\mathrm{M_{\odot}}$) primary and a $\sim$$3-4\,\mathrm{M_{\odot}}$ companion on an orbital period that shrinks to $\lesssim$1 day prior to the core collapse following a common envelope phase. The black hole remnant formed from the primary must receive a natal kick $\gtrsim$1000 $\mathrm{km\ s^{-1}}$ 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.