Massive runaway and walkaway stars

Abstract

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 (i) identify which predictions are robust against model uncertainties and assess their implications, (ii) investigate which physical processes leave a clear imprint and may therefore be constrained observationally, and (iii) provide a suite of publicly available model predictions to allow for the use of kinematic constraints from the Gaia mission. We find that 22+26−8% of all massive binary systems merge prior to the first core-collapse in the system. Of the remainder, 86+11−9% become unbound because of the core-collapse. Remarkably, this rarely produces runaway stars (observationally defined as stars with velocities above 30 km s−1). 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. For stars more massive than 15 M⊙, we estimate that 10+5−8% are walkaways and only 0.5+1.0−0.4% are 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 of about 10%. Thus, astrometric data on presently single massive stars can potentially constrain the physics of massive binary evolution. Finally, we show that the high end of the mass distributions of runaway stars is very sensitive to the assumed black hole natal kicks, and we propose this as a potentially stringent test for the explosion mechanism. We also discuss companions remaining bound that can evolve into X-ray and gravitational wave sources.

A nearby recent supernova that ejected the runaway star ζ Oph, the pulsar PSR B1706−16, and 60Fe found on Earth

“…If the pulsar beaming fraction is ∼ 1/8 (Kolonk et al 2004) and if ∼ 1/3 of massive stars release runaway stars in core-collapse SNe (Renzo et al 2019, Zapartas et al 2017b), then, among ∼ 16 expected SNe (B16), we would expect about one detectable pulsar-runaway-pair -and we did find one case (Sect. 4).…”

“…That the binary SN ejection mechanism for producing runaway stars works, can also be shown inside SN remnants: E.g., ∼ 30 kyr ago, both a pulsar and a runaway star were near the center of the SN remnant S147 (Dinçel et al 2015); the distance of SN and SNR was estimated from the spectrophotometric distance towards the runaway star HD 37424 (1.3 ± 0.1 kpc), confirmed by Gaia DR2 (1.47 ± 0.15 kpc). Most recently numerical simulations of the evolution of binary stars with one supernova progenitor were performed by Zapartas et al (2017b) and Renzo et al (2019), who could show that about one third of original binaries should produce at least one ejected runaway star.…”

Analysis of absorption lines in the high-resolution spectra of five hot post-AGB candidates

“…The combination of a very high rotational velocity, high Galactic latitude, slightly large radial velocity and mild CNO enhancement is suggestive of a history of binary interaction. LSE 163 then would be the result of interaction in a binary system where it gained mass and angular momentum from a massive companion that exploded (see Renzo et al 2019, and comments in the LSE 45 paragraph), although in this scenario it is difficult to explain the high temperature at the surface. Alternatively, we may consider LSE 163 being a mildly stripped star, following the recent work by Götberg et al (2018).…”

Properties of OB star−black hole systems derived from detailed binary evolution models

“…Here we neglect the metallicity difference between both systems, which, indeed, for stars below 40 M , is not expected to cause big differences (e.g., Brott et al 2011), at the level of the accuracy of our consideration. Shao & Li exploit the advantage of rapid binary calculations by producing four population models for Galatic MS+BH binaries, which differ in the assumptions made for the BH kick distribution (see also Renzo et al 2019). They find that essentially no low-mass BH-binaries are produced if efficient BH kicks are assumed.…”