The Properties of Dynamically Ejected Runaway and Hyper-Runaway Stars

Abstract

Runaway stars are stars observed to have large peculiar velocities. Two mechanisms are thought to contribute to the ejection of runaway stars, both involve binarity (or higher multiplicity). In the binary supernova scenario a runaway star receives its velocity when its binary massive companion explodes as a supernova (SN). In the alternative dynamical ejection scenario, runaway stars are formed through gravitational interactions between stars and binaries in dense, compact clusters or cluster cores. Here we study the ejection scenario. We make use of extensive N-body simulations of massive clusters, as well as analytic arguments, in order to to characterize the expected ejection velocity distribution of runaways stars. We find the ejection velocity distribution of the fastest runaways (v 80 km s −1 ) depends on the binary distribution in the cluster, consistent with our analytic toy model, whereas the distribution of lower velocity runaways appears independent of the binaries properties. For a realistic log constant distribution of binary separations, we find the velocity distribution to follow a simple power law; Γ(v) ∝ v −8/3 for the high velocity runaways and v −3/2 for the low velocity ones. We calculate the total expected ejection rates of runaway stars from our simulated massive clusters and explore their mass function and their binarity. The mass function of runaway stars is biased towards high masses, and depends strongly on their velocity. The binarity of runaways is a decreasing function of their ejection velocity, with no binaries expected to be ejected with v > 150 km s −1 . We also find that hyper-runaways with velocities of hundreds of km s −1 can be dynamically ejected from stellar clusters, but only at very low rates, which cannot account for a significant fraction of the observed population of hyper-velocity stars in the Galactic halo.

An artificial neural network to discover hypervelocity stars: candidates in Gaia DR1/TGAS

“…The explosion of a supernova in a binary system is a plausible mechanism for having accelerated these stars to such high velocities. It is remarkable that a good fraction of our RS candidates have velocities consistent with being higher than the escape velocity from the Galaxy, since these stars are thought to be extremely rare: approximately 1 for every 100 HVSs (Bromley et al 2009;Perets & Šubr 2012;Kenyon et al 2014;Brown 2015);…”

The Orion fingers: Near-IR adaptive optics imaging of an explosive protostellar outflow

“…In this model, the disruption and ejection of circumstellar disks and envelopes produced the BN/KL outflow. The momentum and kinetic energy of the outflow and ejected stars came from A&A 579, A130 (2015) the release of gravitational binding energy of a compact binary formed by the dynamic interaction of three of more stars (Poveda et al 1967;Gualandris et al 2004;Perets & Šubr 2012;Reipurth et al 2010;Reipurth & Mikkola 2012). Zapata et al (2009), Bally et al (2011), and Goddi et al (2011) found supporting evidence for the dynamical rearrangement model.…”

Constraining the Milky Way mass with hypervelocity stars

“…On the other hand, the ejection rate of HVSs is ≈ 10 −5 −10 −4 yr −1 (Yu & Tremaine 2003). Since the ejection rate of hyper-runaways is 10 − 100 times smaller than HVSs rate, we expect ≈ 1 hyper-runaway each ≈ 10−100 HVSs (Perets & Sǔbr 2012). Since this source of error is ≪ Γ, we conclude that a possible contamination by hyper-runaways does not affect significantly our results.…”