Nucleosynthesis constraints on the neutron star-black hole merger rate

TL;DR

This paper uses nucleosynthesis data and simulations to set an upper limit on neutron star-black hole merger rates, impacting gravitational wave detection expectations.

Contribution

It introduces a novel method linking r-process element abundances to merger rate constraints, incorporating simulation data and population models.

Findings

Upper limit of NS-BH merger rate ~6×10^-5 per year.

Constraints challenge optimistic gravitational wave detection predictions.

Uncertainties mainly due to black hole spin and neutron star matter properties.

Abstract

We derive constraints on the time-averaged event rate of neutron star-black hole (NS-BH) mergers by using estimates of the population-integrated production of heavy rapid neutron-capture (r-process) elements with nuclear mass numbers A > 140 by such events in comparison to the Galactic repository of these chemical species. Our estimates are based on relativistic hydrodynamical simulations convolved with theoretical predictions of the binary population. This allows us to determine a strict upper limit of the average NS-BH merger rate of ~6*10^-5 per year. We quantify the uncertainties of this estimate to be within factors of a few mostly because of the unknown BH spin distribution of such systems, the uncertain equation of state of NS matter, and possible errors in the Galactic content of r-process material. Our approach implies a correlation between the merger rates of NS-BH binaries and of double NS systems. Predictions of the detection rate of gravitational-wave signals from such compact-object binaries by Advanced LIGO and Advanced Virgo on the optimistic side are incompatible with the constraints set by our analysis.