[Paper Review] Stellar-mass black holes in young massive and open stellar clusters and their role in gravitational-wave generation IV: updated stellar-evolutionary and black hole spin models and comparisons with the LIGO-Virgo O1/O2 merger-event data

Sambaran Banerjee|arXiv (Cornell University)|Apr 15, 2020

Pulsars and Gravitational Waves Research1 citations

TL;DR

This study uses updated N-body simulations with advanced stellar evolution, supernova, and black hole natal kick models—including pair-instability and pulsation pair-instability supernovae—to demonstrate that young massive clusters (YMCs) and open clusters (OCs) naturally produce binary black hole (BBH) mergers matching LIGO-Virgo O1/O2 observations. The models reproduce key merger properties, including the high mass, effective spin, and final spin of GW170729 and the mass asymmetry of GW190412, especially when black holes spin up post-merger or via accretion.

ABSTRACT

I present a set of long-term, direct, relativistic many-body computations of model dense stellar clusters with up-to-date stellar-evolutionary, supernova (SN), and remnant natal-kick models, including pair instability and pulsation pair instability supernova (PSN and PPSN), using an updated version of NBODY7 N-body simulation program. The N-body model also includes stellar evolution-based natal spins of BHs and treatments of binary black hole (BBH) mergers based on numerical relativity. These, for the first time in a direct N-body simulation, allow for second-generation BBH mergers. The set of 65 evolutionary models have initial masses $10^4M_\odot-10^5M_\odot$, sizes 1 pc-3 pc, metallicity $0.0001-0.02$, with the massive stars in primordial binaries and they represent young massive clusters (YMC) and moderately massive open clusters (OC). Such models produce dynamically-paired BBH mergers that agree well with the observed masses, mass ratios, effective spin parameters, and final spins of the LVC O1/O2 merger events, provided BHs are born with low or no spin but spin up after undergoing a BBH merger or matter accretion onto it. In particular, the distinctly higher mass, effective spin parameter, and final spin of GW170729 merger event is naturally reproduced, as also the mass asymmetry of the O3 event GW190412. The computed models produce massive, $\sim100M_\odot$ BBH mergers with primary mass within the 'PSN gap' and also yield mergers involving remnants in the 'mass gap'. They also suggest that YMCs and OCs produce persistent, Local-Universe GW sources detectable by LISA. Such clusters are also capable of producing eccentric LIGO-Virgo mergers.

Motivation & Objective

• To model the formation and evolution of stellar-mass black holes in young massive and open clusters with updated physics.
• To investigate how dynamical processes in dense clusters generate binary black hole mergers consistent with LIGO-Virgo O1/O2 observations.
• To assess the role of black hole spin evolution—particularly post-merger spin-up—on reproducing observed merger properties like effective spin and final spin.
• To evaluate the potential of such clusters as persistent, detectable gravitational-wave sources for LISA.
• To explore the production of eccentric mergers and remnants in the 'mass gap' region.

Proposed method

• Employed the updated NBODY7 N-body code with relativistic many-body dynamics and direct integration of stellar evolution.
• Incorporated state-of-the-art models for supernovae, including pulsation pair-instability (PSN) and pair-instability (PPSN) supernovae.
• Integrated stellar evolution-based natal spins for black holes and applied numerical relativity-based merger treatments.
• Tracked second-generation BBH mergers through direct simulation for the first time in an N-body framework.
• Simulated 65 models with initial masses from 10⁴ to 10⁵ M⊙, sizes of 1–3 pc, and metallicities from 0.0001 to 0.02.
• Used primordial binaries for massive stars to model realistic cluster dynamics and black hole formation.

Experimental results

Research questions

• RQ1 Can updated N-body simulations of dense stellar clusters reproduce the observed masses, mass ratios, and spin parameters of LIGO-Virgo O1/O2 binary black hole mergers?
• RQ2 How do black hole natal spins and post-merger spin-up affect the consistency of simulated mergers with observed events like GW170729 and GW190412?
• RQ3 Do young massive and open clusters produce significant numbers of mergers involving black holes in the 'mass gap' or 'PSN gap'?
• RQ4 Can such clusters be sources of persistent, detectable gravitational waves in the LISA band?
• RQ5 What is the potential for these clusters to produce eccentric binary black hole mergers detectable by LIGO-Virgo?

Key findings

• The simulations reproduce the observed masses, mass ratios, effective spin parameters, and final spins of LIGO-Virgo O1/O2 merger events when black holes are born with low or no spin but spin up after mergers or accretion.
• The high mass, effective spin, and final spin of the GW170729 event are naturally reproduced by the model, particularly through post-merger spin-up.
• The model successfully reproduces the mass asymmetry of the GW190412 O3 event, indicating consistency with observed hierarchical mass distributions.
• The simulations produce massive BBH mergers with primary masses in the 'PSN gap' and remnants in the 'mass gap', consistent with theoretical expectations.
• The models predict that YMCs and OCs can produce persistent, detectable gravitational-wave sources in the LISA frequency band.
• The clusters are capable of generating eccentric binary black hole mergers, which are potentially observable by LIGO-Virgo.