# Double Neutron Star Populations and Formation Channels

## TL;DR

This paper classifies double neutron star populations based on orbital properties, challenges existing formation models with new constraints, and suggests some DNSs originate from globular clusters, impacting our understanding of their formation and merger rates.

## Contribution

It identifies three distinct DNS sub-populations, derives new formation constraints for the high-eccentricity group, and proposes a globular cluster origin for some DNSs, challenging previous binary evolution models.

## Key findings

- Three DNS sub-populations with distinct orbital and spin properties.
- Second-born NSs in the high-eccentricity group likely formed with small natal kicks.
- Some DNSs may originate from globular clusters, explaining mergers in old stellar populations.

## Abstract

In the past five years, the number of known double neutron stars (DNS) in the Milky Way has roughly doubled. We argue that the observed sample can be split into three distinct sub-populations based on their orbital characteristics: (i) short-period, low-eccentricity binaries; (ii) wide binaries; and (iii) short-period, high-eccentricity binaries. These sub-populations also exhibit distinct spin period and spindown rate properties. We focus on sub-population (iii), which contains the Hulse-Taylor binary. Contrary to previous analysis, we demonstrate that, if they are the product of primordial binary evolution, the $P_{\rm orb}$ and $e$ distribution of these systems requires that the second-born NSs must have been formed with small natal kicks ($\lesssim$25 km s$^{-1}$) and have pre-SN masses narrowly distributed around 3.2 M$_{\odot}$. These constraints challenge binary evolution theory and further predict closely aligned spin and orbital axes, inconsistent with the Hulse-Taylor binary's measured spin-orbit misalignment angle of $\approx$20$^{\circ}$. Motivated by the similarity of these DNSs to B2127+11C, a DNS residing in the globular cluster M15, we argue that this sub-population is consistent with being formed in, and then ejected from, globular clusters. This scenario provides a pathway for the formation and merger of DNSs in stellar environments without recent star formation, as observed in the host galaxy population of short gamma ray bursts and the recent detection by LIGO of a merging DNS in an old stellar population.