Black hole–neutron star mergers from triples – II. The role of metallicity and spin–orbit misalignment

 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 composed 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 ∼10−2–10−1. We derive a merger rate range of ΓBH–NS = 1.9 × 10−4–22 Gpc−3 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 χeff ∼ 0 with significant tails.