Abstract

Contributed Talk - Splinter GWAstronomy

Thursday, 18 September 2025, 16:40

Gravitational-wave mergers from star clusters and isolated binaries

Sambaran Banerjee, Aleksandra Olejak, Koushik Sen
HISKP/AIfA, University of Bonn

Dynamical interactions among stellar-remnant black holes (BH) in dense star clusters is widely conceived as a formation mechanism of merging binary black holes (BBH) that the LIGO-Virgo-KAGRA (LVK) gravitational-wave (GW) interferometer network detects in numbers. In this presentation, I shall present results from an in-preparation grid of evolutionary models of star clusters, focussing on BBH mergers occurring in them. The homogeneous grid comprises long-term-evolved (up to at least a few Gyr age) star clusters with initial conditions as follows: mass range over 10^4 Msun - 10^5 Msun, size over 1 - 3 pc, 100% primordial binary fraction for massive stars, 10% primordial binary fraction for low-mass stars. The simulations incorporate stellar and binary evolution, that include recent prescriptions for stellar remnant formation, and post-Newtonian treatment of compact binaries, and are performed with a customised version of the direct N-body code NBODY7. I shall focus on (a) BBH merger mass spectrum, (b) redshift evolution of BBH merger rate, (c) properties of dynamically assembled versus primordially assembled BBH mergers, as obtained from these star cluster models. In the second part, I shall focus on BBH-merger formation from isolated massive binary evolution - another widely debated formation channel of GW events. In particular, I shall present results from a comprehensive suite of binary population synthesis models that demonstrate the occurrence of an anti-correlation between the merging BBHs’ spin and mass ratio. Existence of such an "Xeff-q" anti-correlation is hinted by the observed GW-event population. I shall discuss the origin of this anti-correlation in these binary-evolution models.