DOUBLE NEUTRON STAR SYSTEMS

Image: Artistic representation of Doulbe Neutron Stars.
Credit: NASA/Goddard Space Flight Center

Double Neutron Stars (DNS) have to survive two supernovae and still remain bound. For this reason these systems are a unique and rare population of neutron stars and sets strong limits on the nature of the second collapse. Moreover, DNS emit gravitational radiation and consequently their orbit decays and they merge. This make DNS systems prime candidates for detection of gravitational radiation.

The image shows the NS merger and the gravity waves
it produce. Credit: NASA/Goddard Space Flight Center

A major question is how do the two neutron stars remain bound after the second supernova. If more than half of the mass of the system is lost the system will become unbound, unless the supernova results also in a significant kick velocity to the newborn neutron star. Assuming that the system was on a circular orbit before the second collapse and given the orbital parameters of the DNS system one can estimate the mass ejection and the kick velocity during the second collapse.

In a recent paper (Beniamini & Piras 2016), the authors show that there is strong evidence for two distinct types of supernovae in these systems, where the second collapse in the majority of the observed systems involved small mass ejection (ΔM≤0.5 M_☉ - solar masses) and a corresponding low-kick velocity  (v_k≤ 30km/s). This formation scenario is  compatible, for example, with an electron capture supernova (see appendix below).
Only a minority of the systems have formed via the standard SN scenario involving larger mass ejection of ~2.2 M_☉ and kick velocities of up to 400 km/s. The authors predict that most of these systems reside close to the galactic disc. This implies that more NS-NS mergers occur close to the galactic plane.

The paper (Beniamini & Piras 2016) is available online and is published in the MNRAS >>
http://arxiv.org/pdf/1510.03111v2.pdf
http://mnras.oxfordjournals.org/content/456/4/4089.abstract

APPENDIX - Electron capture supernova

A massive star with a main-sequence mass M>8 M_☉ ends up as a core-collapse supernova. Core collapse is inaugurated by electron capture for a star with an O+Ne+Mg core (M≤10 M_☉) or Fe photodisintegration for a star with an Fe core (M > 10M_☉ ).
The fate of the less-massive star with the O+Ne+Mg core is different from that of the star with an Fe core. The O+Ne+Mg core is supported  by electron  degenerate pressure. The mass and density of the O+Ne+Mg core increase through phases of shell burning of He and H. As the O+Ne+Mg core grows, an envelope undergoes mass loss to reduce the H mass and He dredge-up to enhance He abundance. When the central density exceeds a critical value (4 × 10¹² kg m^-3), electrons begin to be captured by magnesium, the degenerate pressure decreases, and thus the O+Ne+Mg core collapses gravitationally. Ensuing core bounce and neutrino heating can eject the envelope and part of the O+Ne+Mg core. This  explosion is called an electron-capture supernova.