Relativistic mergers of compact binaries in clusters: The fingerprint of the spin

TL;DR

This paper introduces a novel implementation of spin effects in relativistic stellar cluster simulations, enabling more accurate modeling of gravitational wave sources and revealing spin evolution patterns.

Contribution

The first implementation of spin effects in mergers within a direct-summation code, including relativistic corrections and spin precession, for dense stellar cluster simulations.

Findings

Formation of a runaway star with spin decay proportional to its mass.

Correlation between initial spin distribution and final spin states of compact objects.

Robustness of the algorithm across numerous merger scenarios.

Abstract

Dense stellar systems such as globular clusters and dense nuclear clusters are the breeding ground of sources of gravitational waves for the advanced detectors LIGO and Virgo. These systems deserve a close study to estimate rates and parameter distribution. This is not an easy task, since the evolution of a dense stellar cluster involves the integration of $N$ bodies with high resolution in time and space and including hard binaries and their encounters and, in the case of gravitational waves (GWs), one needs to take into account important relativistic corrections. In this work we present the first implementation of the effect of spin in mergers in a direct-summation code, NBODY6. We employ non-spinning post-Newtonian corrections to the Newtonian accelerations up to 3.5 post-Newtonian (PN) order as well as the spin-orbit coupling up to next-to-lowest order and the lowest order spin-spin coupling. We integrate spin precession and add a consistent treatment of mergers. We analyse the implementation by running a set of two-body experiments and then we run a set of 500 simulations of a relativistic stellar cluster. In spite of the large number of mergers in our tests, the application of the algorithm is robust. We find in particular the formation of a runaway star whose spin decays with the mass it wins, independently of the initial value of the spins of the stars. More remarkably, the subset of compact objects that do not undergo many mergers, and hence represent a more realistic system, has a correlation between the final absolute spin and the initial choice for the initial distribution, which could provide us with information about the evolution of spins in dense clusters once the first detections have started.