First and second-generation black hole and neutron star mergers in 2+2 quadruples: population statistics

TL;DR

This study investigates how hierarchical quadruple systems can lead to successive neutron star and black hole mergers, explaining some gravitational wave observations through secular evolution and scattering effects.

Contribution

It models the evolution of 2+2 quadruple systems to understand the formation of first and second-generation mergers, highlighting the role of secular dynamics and scattering.

Findings

Most first-generation mergers resemble isolated binary outcomes.

Second-generation mergers are about 10 million times less common.

Scattering can produce low-mass gap mergers, eccentric sources, and negative spins.

Abstract

Recent detections of gravitational waves from mergers of neutron stars (NSs) and black holes (BHs) in the low and high-end mass gap regimes pose a puzzle to standard stellar and binary evolution theory. Mass-gap mergers may originate from successive mergers in hierarchical systems such as quadruples. Here, we consider repeated mergers of NSs and BHs in stellar 2+2 quadruple systems, in which secular evolution can accelerate the merger of one of the inner binaries. Subsequently, the merger remnant may interact with the companion binary, yielding a second-generation merger. We model the initial stellar and binary evolution of the inner binaries as isolated systems. In the case of successful compact object formation, we subsequently follow the secular dynamical evolution of the quadruple system. When a merger occurs, we take into account merger recoil, and model subsequent evolution using direct N-body integration. With different assumptions on the initial properties, we find that the majority of first-generation mergers are not much affected by secular evolution, with their observational properties mostly consistent with isolated binaries. A small subset shows imprints of secular evolution through residual eccentricity in the LIGO band, and retrograde spin-orbit orientations. Second-generation mergers are ~10^7 times less common than first-generation mergers, and can be strongly affected by scattering (i.e., three-body interactions) induced by the first-generation merger. In particular, scattering can account for mergers within the low-end mass gap, although not the high-end mass gap. Also, in a few cases, scattering could explain highly eccentric LIGO sources and negative effective spin parameters.