# Black hole-neutron star mergers from triples II: the role of metallicity and spin-orbit misalignment

## TL;DR

This paper investigates how metallicity and initial conditions influence black hole-neutron star mergers in triple systems, providing insights into their properties, merger rates, and spin-orbit misalignments relevant for gravitational wave observations.

## Contribution

It offers a detailed analysis of the impact of metallicity and orbital configurations on BH-NS merger characteristics and spin orientations in triple systems.

## Key findings

- Merger rate range: 1.9×10^{-4} to 22 Gpc^{-3} yr^{-1}
- Typical eccentricity in LIGO band: 10^{-2} to 10^{-1}
- Effective spin distribution peaks at zero with tails

## Abstract

Observations of black hole-neutron star (BH-NS) mergers via gravitational waves (GWs) are of great interest for their electromagnetic counterparts, such as short gamma-ray bursts, and could provide crucial information on the nature of BHs and the NS crust and magnetosphere. While no event has been confirmed, a recent possible detection of a BH-NS merger event by the LIGO-Virgo collaboration has attracted a lot of attention to these sources. In this second paper of the series, we follow-up our study of the dynamical evolution of triples comprised of an inner BH-NS binary. In particular, we examine how the progenitor metallicity affects the characteristics of the BH-NS mergers in triples. We determine the distributions of masses, orbital parameters and merger times, as a function of the progenitor metallicity and initial triple orbital distributions, and show that the typical eccentricity in the LIGO band is $\sim 10^{-2}-10^{-1}$. We derive a merger rate range of $\Gamma_\mathrm{BH-NS}=1.9\times 10^{-4}-22 \ \mathrm{Gpc}^{-3}\ \mathrm{yr}^{-1}$, consistent the LIGO-Virgo upper limit. Finally, we study the expected spin-orbit misalignments of merging BH-NS binaries from this channel, and find that typically the effective spin distribution is peaked at $\chi_{\rm eff}\sim 0$ with significant tails.