Double-core evolution and the formation of neutron star binaries with compact companions

We present the results of a systematic exploration of an alternative evolutionary scenario to form double neutron star (DNS) binaries, first proposed by Brown (1995) , which does not involve a neutron star passing through a common envelope. In this scenario, the initial binary components have very similar masses, and both components have left the main sequence before they evolve into contact; preferably the primary has already developed a CO core. We have performed population synthesis simulations to study the formation of DNS binaries via this channel and to predict the orbital properties and system velocities of such systems. We obtain a merger rate for DNSs in this channel in the range of 0.1–12 Myr⁻¹. These rates are still subject to substantial uncertainties such as the modelling of the contact phase.