Double neutron star formation: merger times, systemic velocities, and travel distances

Abstract

The formation and evolution of double neutron stars (DNS) have traditionally been studied using binary population synthesis. In this work, we take an alternative approach by focusing only on the second supernova (SN) forming the DNS and the subsequent orbital decay and merger due to gravitational wave radiation. Using analytic and numerical methods, we explore how different NS natal kick velocity distributions, pre-SN orbital separations, and progenitor He-star masses affect the post-SN orbital periods, eccentricities, merger times, systemic velocities, and distances traveled by the system before merging. Comparison with the set of 17 known DNSs in the Milky Way shows that DNSs have pre-SN orbital separations ranging between 1 and 44 R . Those DNSs with pre-SN separations ∼1 R have merger time distributions that peak ∼10-100 Myr after formation, regardless of the kick velocity received by the NS. These DNSs are typically formed with systemic velocities ∼10 2 km s −1 and may travel ∼1-10 kpc before merging. Depending on progenitor mass of the second-born NS, the short merger time can account for the r-process enrichment observed in compact stellar systems such as ultra-faint dwarf galaxies. For Milky Way-mass galaxies only DNSs with the tightest pre-SN orbits and large kick velocities ( 10 2 km s −1 ) can escape. However, those DNSs that do escape may travel as far as ∼Mpc before merging, which as previous studies have pointed out has implications for identifying the host galaxies to short gamma ray bursts and gravitational wave events.

Targeted modeling of GW150914's binary black hole source with dart_board

“…As a final model, we apply kick velocities to BHs at birth using a Maxwellian prior with a dispersion velocity of 150 km s −1 (compared with our standard model with σ k = 10 km s −1 ), again these kick velocities are not mediated by supernova fallback. The increased kick velocity has the effect of broadening the distribution of possible initial orbital separations, as SN kicks can either expand or shrink the post-SN orbits, depending on the SN kick direction (Kalogera 1996;Andrews & Zezas 2019). This model also shows a slight difference in the branching ratios, with nearly half of all binaries being formed through the CE channel.…”

Formation and Evolution of Binary Neutron Stars: Mergers and Their Host Galaxies

“…• Mass ejection: Right before the secondary SN, the binary, comprised of a NS with a naked helium star companion, may undergo a phase of mass transfer, the so-called Case BB mass transfer phase (Tauris et al 2015, and references therein), which is currently not well understood but significantly influence the properties of resulting BNSs. Here, we follow the method in Andrews & Zezas (2019), focused on the second SNe forming the BNS, and set…”

“…Otherwise, the results remain unchanged. To complement the population synthesis methods, we use this recipe, similar to that in Andrews & Zezas (2019), to calculate three sub-models with β = 0.9, 0.8, and 0.6, corresponding to heavily-stripped, stripped, and slightly-stripped assumptions, respectively.…”

LISA and the Existence of a Fast-merging Double Neutron Star Formation Channel

“…We first include functional models for the formation through isolated binary evolution of the observed low-and high-eccentricity DNSs in the Milky Way. Andrews & Mandel (2019) show that these separate populations can be reasonably modeled through isolated binary evolution, by randomly generating systems immediately prior to the second SN, then dynamically evolving them through core collapse (see also , Andrews & Zezas 2019). The low-eccentricity systems are modeled with a circular pre-SN orbit, with a log-normal orbital separation distribution (µ = 0.2, σ = 0.4, in units of R ), and an isotropic SN kick of 50 km s −1 .…”