TY - JOUR

T1 - The mass-ratio distribution of tertiary-induced binary black hole mergers

AU - Su, Yubo

AU - Liu, Bin

AU - Lai, Dong

PY - 2021/6/7

Y1 - 2021/6/7

N2 - Many proposed scenarios for black hole (BH) mergers involve a tertiary companion that induces von Zeipel-Lidov-Kozai (ZLK) eccentricity cycles in the inner binary. An attractive feature of such mechanisms is the enhanced merger probability when the octupole-order effects, also known as the eccentric Kozai mechanism, are important. This can be the case when the tertiary is of comparable mass to the binary components. Since the octupole strength [proportional to(1 - q)/(1 + q)] increases with decreasing binary mass ratio q, such ZLK-induced mergers favour binaries with smaller mass ratios. We use a combination of numerical and analytical approaches to fully characterize the octupole-enhanced binary BH mergers and provide semi-analytical criteria for efficiently calculating the strength of this enhancement. We show that for hierarchical triples with semimajor axial ratio a/a(out) greater than or similar to 0.01-0.02, the binary merger fraction can increase by a large factor (up to similar to 20) as q decreases from unity to 0.2. The resulting mass-ratio distribution for merging binary BHs produced in this scenario is in tension with the observed distribution obtained by the LIGO/VIRGO collaboration, although significant uncertainties remain about the initial distribution of binary BH masses and mass ratios.

AB - Many proposed scenarios for black hole (BH) mergers involve a tertiary companion that induces von Zeipel-Lidov-Kozai (ZLK) eccentricity cycles in the inner binary. An attractive feature of such mechanisms is the enhanced merger probability when the octupole-order effects, also known as the eccentric Kozai mechanism, are important. This can be the case when the tertiary is of comparable mass to the binary components. Since the octupole strength [proportional to(1 - q)/(1 + q)] increases with decreasing binary mass ratio q, such ZLK-induced mergers favour binaries with smaller mass ratios. We use a combination of numerical and analytical approaches to fully characterize the octupole-enhanced binary BH mergers and provide semi-analytical criteria for efficiently calculating the strength of this enhancement. We show that for hierarchical triples with semimajor axial ratio a/a(out) greater than or similar to 0.01-0.02, the binary merger fraction can increase by a large factor (up to similar to 20) as q decreases from unity to 0.2. The resulting mass-ratio distribution for merging binary BHs produced in this scenario is in tension with the observed distribution obtained by the LIGO/VIRGO collaboration, although significant uncertainties remain about the initial distribution of binary BH masses and mass ratios.

KW - binaries: close

KW - stars: black holes

KW - black hole mergers

KW - SPIN-ORBIT MISALIGNMENT

KW - STAR-CLUSTERS

KW - HOT JUPITERS

KW - KOZAI MECHANISM

KW - EVOLUTION

KW - ECCENTRICITY

KW - DYNAMICS

KW - SYSTEMS

KW - PERTURBATIONS

KW - OSCILLATIONS

U2 - 10.1093/mnras/stab1617

DO - 10.1093/mnras/stab1617

M3 - Journal article

VL - 505

SP - 3681

EP - 3697

JO - Royal Astronomical Society. Monthly Notices

JF - Royal Astronomical Society. Monthly Notices

SN - 0035-8711

IS - 3

ER -