Black hole–neutron star mergers from triples. Issue 3 (29th April 2019)

• Record Type: Journal Article
• Authors: Fragione, Giacomo; Loeb, Abraham
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 486:Issue 3(2019)
• Page Start: 4443
• Page End: 4450
• Publication Date: 2019-04-29
• Subjects: stars: kinematics and dynamics -- stars: neutron -- stars: black holes -- galaxies: kinematics and dynamics; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz1131
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: Mergers of black hole (BH) and neutron star (NS) binaries are expected to be observed by gravitational wave observatories in the coming years. Until now, LIGO has only set an upper limit on this merger rate. BH–NS binaries are expected to merge in isolation, as their formation is suppressed in star clusters by the mass segregation and the strong heating by BHs in the cluster core. Another viable scenario to BH–NS mergers is in triple systems. In this paper, we carry out a systematic statistical study of the dynamical evolution of triples comprised of an inner BH–NS binary by means of high-precision N -body simulations, including Post-Newtonian (PN) terms up to 2.5PN order. We start from the main-sequence massive stars and model the supernovae (SN) events that lead to the formation of BHs and NSs. We adopt different prescriptions for the natal velocity kicks imparted during the SN processes and illustrate that large kicks lead to more compact and massive triples that merge on shorter time-scales. We also show that BH–NS merging in triples have a significant eccentricity in the LIGO band, typically much larger than BH–NS merging in isolated binaries. Finally, we estimate a rate of ΓBH–NS ≈ 1.0 × 10 −3 –3.5 × 10 −2 Gpc −3 yr −1, for non-zero velocity kicks, and ΓBH–NS = 19 Gpc −3 yr −1, for no natal kicks. Our rate estimate overlaps with the expected BH–NS rate in isolated binaries and within the LIGO upper limit.