Double Neutron Star Mergers from Hierarchical Triple-star Systems

Abstract

The isolated binary evolution model for merging neutron stars (NSs) involves processes such as mass transfer, common-envelope evolution, and natal kicks, all of which are poorly understood. Also, the predicted NS-NS merger rates are typically lower than the rates inferred from the LIGO GW170817 event. Here, we investigate merger rates of NS and black hole (BH)-NS binaries in hierarchical triple-star systems. In such systems, the tertiary can induce Lidov-Kozai (LK) oscillations in the inner binary, accelerating its coalescence, and potentially enhancing compact object merger rates. However, since compact objects originate from massive stars, the prior evolution should also be taken into account. Natal kicks, in particular, could significantly reduce the rates by unbinding the tertiary before it can affect the inner binary through LK evolution. We carry out simulations of massive triples taking into account stellar evolution starting from the main sequence, secular and tidal evolution, and the effects of supernovae. For large NS birth kicks (σ k = 265 km s −1 ), we find that the triple NS-NS merger rate (several hundred Gpc −3 yr −1 ) is lower by a factor of ∼ 2 − 3 than the binary rate, but for no kicks (σ k = 0 km s −1 ), the triple rate (several thousand Gpc −3 yr −1 ) is comparable to the binary rate. Our results indicate that a significant fraction of NS-NS mergers could originate from triples if a substantial portion of the NS population is born with low kick velocities, as indicated by other work. However, uncertainties and open questions remain because of our simplifying assumption of dynamical decoupling after inner binary interaction has been triggered.

Detecting double neutron stars with LISA

“…These chirp mass and eccentricity measurements will make it possible to distinguish at least a fraction of the better-measured eccentric DNSs from the much larger Galactic DWD population. They can also elucidate the origin of the DNS systems: although the isolated binary channel is generally assumed to dominate DNS formation, with globular clusters expected to contribute less than 10 per cent of all merging DNSs (Phinney 1991; Grindlay et al 2006;Ivanova et al 2008;Kremer et al 2018), recent work has suggested that dynamical or three-body formation channels may be relevant (Hamers & Thompson 2019;Andrews & Mandel 2019). Moreover, LISA's measurement of the eccentricity distribution in the early DNS evolutionary history could shed light on uncertainties in models of isolated binary evolution, such as the stability of case BB mass transfer.…”

“…Significant orbital eccentricities may be imparted to DNSs by supernova kicks (e.g., Tauris et al 2017) or Blaauw kicks (Blaauw 1961). Short-period DNSs may also be formed through dynamical hardening interactions in globular clusters until the binary is ejected into the field, presents too small of a cross-section for further interactions, or merges through GW emission (Kulkarni et al 1990;Phinney & Sigurdsson 1991), or in hierarchical triple-star systems (Hamers & Thompson 2019). The typical separation of the ejected DNSs depends on the globular cluster properties, but may fall in the range of a few solar radii, or orbital frequencies of a few times 10 −5 Hz (Andrews & Mandel 2019).…”

Modelling double neutron stars: radio and gravitational waves

“…However, it is still possible that some of the DNSs in the observed sample were formed through an alternative evolutionary channel (e.g. Hamers & Thompson 2019;Andrews & Mandel 2019).…”

Common envelope episodes that lead to double neutron star formation

“…This could take the form of dynamically induced mergers in dense stellar environments, such as globular clusters (Andrews & Mandel 2019 but see, e.g., Ye et al 2020). Meanwhile, Kozai–Lidov oscillations (Kozai 1962; Lidov 1962) in hierarchical triple systems can drive up the eccentricity of the inner binary (see Naoz et al 2016 for a review) and contribute to the formation of merging DNSs (Hamers & Thompson 2019). Both types of dynamical encounters can change the binary orbital evolution, including the eccentricity.…”