# Double Neutron Stars: Evidence For Two Different Neutron-Star Formation Mechanisms

## TL;DR

This paper presents evidence for two distinct neutron-star formation mechanisms, highlighting that low-mass, low-kick neutron stars originate from electron-capture collapse in binary systems, while higher-mass ones form from iron core collapse.

## Contribution

It provides observational evidence supporting two different neutron-star formation pathways based on orbital eccentricities and masses in double neutron star systems.

## Key findings

- Low-eccentricity double neutron stars have low-mass, low-velocity neutron stars.
- Low-mass neutron stars likely formed via electron-capture collapse in binaries.
- Higher-mass neutron stars formed from iron core collapse with higher velocities.

## Abstract

Six of the eight double neutron stars known in the Galactic disk have low orbital eccentricities (< 0.27) indicating that their second-born neutron stars received only very small velocity kicks at birth. This is similar to the case of the B-emission X-ray binaries, where a sizable fraction of the neutron stars received hardly any velocity kick at birth (Pfahl et al. 2002). The masses of the second-born neutron stars in five of the six low-eccentricity double neutron stars are remarkably low (between 1.18 and 1.30 Msun). It is argued that these low-mass, low-kick neutron stars were formed by the electron-capture collapse of the degenerate O-Ne-Mg cores of helium stars less massive than about 3.5 Msun, whereas the higher-mass, higher kick-velocity neutron stars were formed by the collapses of the iron cores of higher initial mass. The absence of low-velocity single young radio pulsars (Hobbs et al. 2005) is consistent with the model proposed by Podsiadlowski et al. (2004), in which the electron-capture collapse of degenerate O-Ne-Mg cores can only occur in binary systems, and not in single stars.