On the effective spin-mass ratio relation of binary black hole mergers that evolved in isolation

TL;DR

This study shows that the observed spin and mass ratio properties of binary black hole mergers can be naturally explained by isolated binary evolution, especially through the stable mass transfer channel, aligning with gravitational wave observations.

Contribution

It demonstrates that the effective spin-mass ratio relation and anti-correlation observed in BBH mergers can arise from standard isolated binary evolution models, emphasizing the role of the SMT channel.

Findings

Both the $ m ilde{ m extbf{chi}}_{eff}$ asymmetry and anti-correlation with mass ratio are natural outcomes of isolated binary evolution.

The anti-correlation is especially strong in systems with mass-ratio reversal during stable mass transfer.

The characteristic spin-mass ratio trend diminishes with higher metallicity and mixed formation channels.

Abstract

The ground-based measurement of gravitational waves (GW) from merging binary black holes (BBH) uniquely allows determination of spins of stellar-remnant black holes (BH), thereby offering insights into their formation mechanisms. The observed population of BBH mergers exhibits two intriguing peculiarities related to BH spins, namely, a positively biased distribution of effective spin parameter, $\chi_{\rm eff}$, and an apparent anti-correlation between merger mass ratio, $q$, and $\chi_{\rm eff}$. Here we investigate the potential mechanisms for such observed properties, in BBH mergers via isolated binary evolution. We synthesize BBH mergers with the fast binary evolution code BSE. The role of various physical assumptions is explored, including tidal spin-up, compact remnant mass, and mass transfer physics. We compare the properties of WR-BH binaries that formed through stable mass transfer (SMT) and common envelope evolution (CE). We find that both the asymmetry in $\chi_{\rm eff}$ distribution and the $\chi_{\rm eff}-q$ anti-correlation are natural outcomes of isolated-binary BBH formation with the standard assumptions on mass transfer physics. The anti-correlation is particularly pronounced for SMT-channel BBH mergers that experience a mass-ratio-reversal, i.e., those where the second-born BH is the more massive member. The anti-correlation arises from the dependence of orbital shrinking during mass transfer and the Roche lobe size on the system's mass ratio. This characteristic $\chi_{\rm eff}-q$ trend diminishes with increasing metallicity and when the isolated-binary BBH merger population is mixed with a significant contribution of dynamically formed BBH mergers. Our results demonstrate that isolated massive binary evolution via the SMT sub-channel can reproduce the observable BBH merger population, with characteristic signatures in mass, mass ratio, and spin distributions.[abr.]