The dragon-II simulations – III. Compact binary mergers in clusters with up to 1 million stars: mass, spin, eccentricity, merger rate, and pair instability supernovae rate

Jul 10, 2023
22 pages
Published in:
  • Mon.Not.Roy.Astron.Soc. 528 (2024) 3, 5140-5159
  • Published: Feb 7, 2024
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Abstract: (Oxford University Press)
Compact binary mergers forming in star clusters may exhibit distinctive features that can be used to identify them among observed gravitational-wave sources. Such features likely depend on the host cluster structure and the physics of massive star evolution. Here, we dissect the population of compact binary mergers in the dragon-II simulation data base, a suite of 19 direct N-body models representing dense star clusters with up to 10^6 stars and |<33  per cent\lt 33~{{\ \rm per\ cent}}| of stars in primordial binaries. We find a substantial population of black hole binary (BBH) mergers, some of them involving an intermediate-mass BH (IMBH), and a handful mergers involving a stellar BH and either a neutron star (NS) or a white dwarf (WD). Primordial binary mergers, |30  per cent\sim 30~{{\ \rm per\ cent}}| of the whole population, dominate ejected mergers. Dynamical mergers, instead, dominate the population of in-cluster mergers and are systematically heavier than primordial ones. Around 20 per cent of dragon-II mergers are eccentric in the Laser Interferometer Space Antenna (LISA) band and 5 per cent in the LIGO band. We infer a mean cosmic merger rate of |R30(4.4)(1.2)\mathcal {R}\sim 30(4.4)(1.2)| yr^−1 Gpc^−3 for BBHs, NS–BH, and WD–BH binary mergers, respectively, and discuss the prospects for multimessenger detection of WD–BH binaries with LISA. We model the rate of pair-instability supernovae (PISNe) in star clusters and find that surveys with a limiting magnitude m_bol = 25 can detect ∼1–15 yr^−1 PISNe. Comparing these estimates with future observations could help to pin down the impact of massive star evolution on the mass spectrum of compact stellar objects in star clusters.
Note:
  • 22 pages, 14 figures, 3 tables. Comments welcome. Submitted to MNRAS
  • methods: numerical
  • stars: general
  • stars: black holes
  • galaxies: star clusters: general