Spin misalignment of black hole binaries from young star clusters: implications for the origin of gravitational waves events

TL;DR

This study explores how spin misalignments in black hole binaries formed in young star clusters affect gravitational wave signals, highlighting the role of dynamical interactions and common envelope evolution.

Contribution

It combines binary population synthesis and few-body simulations to estimate spin properties and merger rates of black hole binaries from young star clusters, considering dynamical effects.

Findings

Post-common envelope BH binaries typically undergo only one encounter.

Estimated local BH merger rate density is approximately 6.6 per Gpc^3 per year.

The model reproduces observed spin distribution trends from GWTC-2 data.

Abstract

Recent studies indicate that the progenitors of merging black hole (BH) binaries from young star clusters can undergo a common envelope phase just like isolated binaries. If the stars emerge from the common envelope as naked cores, tidal interactions can efficiently synchronize their spins before they collapse into BHs. Contrary to the isolated case, these binary BHs can also undergo dynamical interactions with other BHs in the cluster before merging. The interactions can tilt the binary orbital plane, leading to spin-orbit misalignment. We estimate the spin properties of merging binary BHs undergoing this scenario by combining up-to-date binary population synthesis and accurate few-body simulations. We show that post-common envelope binary BHs are likely to undergo only a single encounter, due to the high binary recoil velocity and short coalescence times. Adopting conservative limits on the binary-single encounter rates, we obtain a local BH merger rate density of ~6.6 yr^-1 Gpc^-3. Assuming low (<0.2) natal BH spins, this scenario reproduces the trends in the distributions of effective spin Xeff and precession parameters Xp inferred from GWTC-2, including the peaks at (Xeff, Xp) ~ (0.1, 0.2) and the tail at negative Xeff values.