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Media type:
E-Article
Title:
Order in the chaos : Eccentric black hole binary mergers in triples formed via strong binary–binary scatterings
:
Eccentric black hole binary mergers in triples formed via strong binary–binary scatterings
Published in:
Astronomy & Astrophysics, 650 (2021), Seite A189
Language:
Not determined
DOI:
10.1051/0004-6361/202038795
ISSN:
0004-6361;
1432-0746
Origination:
Footnote:
Description:
Black hole (BH) triples represent one of the astrophysical pathways for BH mergers in the Universe detectable by LIGO and VIRGO. We study the formation of BH triples via binary–binary encounters in dense clusters, showing that up to two-thirds of the triples formed through this channel are hierarchical, whereas the remaining one-third are in a non-hierarchical, unstable configuration. We built a database of 32 000 N-body simulations to investigate the evolution of BH triples focusing on mildly hierarchical and non-hierarchical unstable configurations. Varying the mutual orbital inclination, the three BH masses and the inner and outer eccentricities, we show that retrograde, nearly planar configurations lead to a significant shrinkage of the inner binary. We find a universal trend of triple systems, namely that they tend to evolve toward prograde configurations and that the orbital flip, driven by the torque exerted on the inner BH binary (BHB) by the outer BH, leads in general to tighter inner orbits. In some cases, the resulting BHB undergoes coalescence within a Hubble time, releasing gravitational waves. A large fraction of merging BHBs with an initial separation of 1 AU enter the 10−3 − 10−1 Hz frequency band with large eccentricities, thus representing potential eccentric LISA sources. Mergers originating from an initially tighter BHB (a ∼ 0.01 AU), instead often have eccentricities above 0.7 in the 1 Hz band. We find that the mass distribution of the mergers in this astrophysical channel maps the original BH binary spectrum. This might have interesting consequences in light of the growing population of BH mergers detected by LIGO and VIRGO, namely that eccentric sources detected in high-frequency detectors are most likely connected with a high-velocity dispersion stellar environment, whereas eccentric sources detected in low-frequency detectors are likely to develop in low-density clusters.