Massive runaways and walkaway stars

TL;DR

This study uses numerical simulations to predict the velocities of stars after binary system disruptions, finding most are walkaways rather than runaways, and highlights the impact of physical processes and model assumptions on these outcomes.

Contribution

It provides robust predictions of star velocities post-supernova, analyzes the effects of physical processes, and offers publicly available models to compare with observations.

Findings

Most unbound stars are walkaways, not runaways.

The fraction of runaway stars is lower than observed, suggesting missing physics.

High-mass star velocities are sensitive to black hole natal kicks.

Abstract

Anticipating the kinematic constraints from the Gaia mission, we perform an extensive numerical study of the evolution of massive binary systems to predict the peculiar velocities that stars obtain when their companion collapses and disrupts the system. Our aim is to (1) identify which predictions are robust against model uncertainties and assess their implications, (2) investigate which physical processes leave a clear imprint and may therefore be constrained observationally and (3) provide a suite of publicly available model predictions. We find that $22_{-8}^{+26}$% of all massive binary systems merge prior to the first core collapse in the system. Of the remainder, $86_{-9}^{+11}$% become unbound because of the core-collapse. Remarkably, this rarely produce runaway stars (i.e., stars with velocities above 30 km/s). These are outnumbered by more than an order of magnitude by slower unbound companions, or "walkaway stars". This is a robust outcome of our simulations and is due to the reversal of the mass ratio prior to the explosion and widening of the orbit, as we show analytically and numerically. We estimate a $10^{+5}_{-8}$% of massive stars to be walkaways and only $0.5^{+1.0}_{-0.4}$% to be runaways, nearly all of which have accreted mass from their companion. Our findings are consistent with earlier studies, however the low runaway fraction we find is in tension with observed fractions 10%. If Gaia confirms these high fractions of massive runaway stars resulting from binaries, it would imply that we are currently missing physics in the binary models. Finally, we show that high end of the mass distributions of runaway stars is very sensitive to the assumed black hole natal kicks and propose this as a potentially stringent test for the explosion mechanism. We discuss companions remaining bound which can evolve into X-ray and gravitational wave sources.