Advanced LIGO Constraints on Neutron Star Mergers and R-Process Sites

TL;DR

This study combines stellar abundances, chemical evolution models, population synthesis, and gravitational wave data to assess whether neutron star mergers are the primary source of r-process elements, revealing discrepancies and future observational constraints.

Contribution

It integrates multiple modeling approaches with gravitational wave data to evaluate the role of neutron star mergers in r-process element production, highlighting the need for optimistic assumptions.

Findings

NS-NS mergers need ~10 times more events than standard models predict.

Current LIGO upper limits are consistent with models assuming high merger rates.

Future LIGO measurements will refine the role of mergers in r-process nucleosynthesis.

Abstract

The role of compact binary mergers as the main production site of r-process elements is investigated by combining stellar abundances of Eu observed in the Milky Way, galactic chemical evolution (GCE) simulations, binary population synthesis models, and Advanced LIGO gravitational wave measurements. We compiled and reviewed seven recent GCE studies to extract the frequency of neutron star - neutron star (NS-NS) mergers that is needed in order to reproduce the observed [Eu/Fe] vs [Fe/H] relationship. We used our simple chemical evolution code to explore the impact of different analytical delay-time distribution (DTD) functions for NS-NS mergers. We then combined our metallicity-dependent population synthesis models with our chemical evolution code to bring their predictions, for both NS-NS mergers and black hole - neutron star mergers, into a GCE context. Finally, we convolved our results with the cosmic star formation history to provide a direct comparison with current and upcoming Advanced LIGO measurements. When assuming that NS-NS mergers are the exclusive r-process sites, and that the ejected r-process mass per merger event is 0.01 Msun, the number of NS-NS mergers needed in GCE studies is about 10 times larger than what is predicted by standard population synthesis models. These two distinct fields can only be consistent with each other when assuming optimistic rates, massive NS-NS merger ejecta, and low Fe yields for massive stars. Population synthesis models and GCE simulations are in agreement with the current upper limit (O1) established by Advanced LIGO during their first run of observations. Upcoming measurements will provide important constraints on the actual local NS-NS merger rate, will provide insights on the plausibility of the GCE requirement, and will help to define whether or not compact binary mergers can be the dominant source of r-process elements in the Universe.