Merger rates of double neutron stars and stellar origin black holes: The Impact of Initial Conditions on Binary Evolution Predictions

TL;DR

This study assesses how uncertainties in initial stellar conditions influence predictions of neutron star and black hole merger rates, finding that most assumptions have minimal impact except for the initial mass function.

Contribution

The paper demonstrates that recent observational constraints on initial binary properties do not significantly alter merger rate predictions, except for the initial mass function.

Findings

Merger rate predictions increase by less than a factor of 2 with new initial conditions.

Uncertainties in initial binary parameters have minimal effect on mass and delay time distributions.

Variations in the initial mass function can change merger rate predictions by a factor of 6.

Abstract

The initial mass function (IMF), binary fraction and distributions of binary parameters (mass ratios, separations and eccentricities) are indispensable input for simulations of stellar populations. It is often claimed that these are poorly constrained significantly affecting evolutionary predictions. Recently, dedicated observing campaigns provided new constraints on the initial conditions for massive stars. Findings include a larger close binary fraction and a stronger preference for very tight systems. We investigate the impact on the predicted merger rates of neutron stars and black holes. Despite the changes with previous assumptions, we only find an increase of less than a factor 2 (insignificant compared with evolutionary uncertainties of typically a factor 10-100). We further show that the uncertainties in the new initial binary properties do not significantly affect (within a factor of 2) our predictions of double compact object merger rates. An exception is the uncertainty in IMF (variations by a factor of 6 up and down). No significant changes in the distributions of final component masses, mass ratios, chirp masses and delay times are found. We conclude that the predictions are, for practical purposes, robust against uncertainties in the initial conditions concerning binary parameters with exception of the IMF. This eliminates an important layer of the many uncertain assumptions affecting the predictions of merger detection rates with the gravitational wave detectors aLIGO/aVirgo.