The role of helium stars in the formation of double neutron stars

We have calculated the evolution of 60 model binary systems consisting of helium stars in the mass range of M_He = 2.5-6 M⊙ with a 1.4 M⊙ neutron star companion to investigate the formation of double neutron star systems. Orbital periods ranging from 0.09 to 2 days are considered, corresponding to Roche lobe overflow starting from the helium main sequence to after the ignition of carbon burning in the core. We have also examined the evolution into a common envelope phase via secular instability, delayed dynamical instability, and the consequence of matter filling the neutron star's Roche lobe. The survival of some close He-star neutron star binaries through the last mass transfer episode (either dynamically stable or unstable mass transfer phase) leads to the formation of extremely short-period double neutron star systems (with P ≲ 0.1 days). In addition, we find that systems throughout the entire calculated mass range can evolve into a common envelope phase, depending on the orbital period at the onset of mass transfer. The critical orbital period below which common envelope evolution occurs generally increases with M_He. In addition, a common envelope phase may occur during a short time for systems characterized by orbital periods of 0.1-0.5 days at low He-star masses (∼2.6-3.3 M⊙). The existence of a short-period population of double neutron stars increases the predicted detection rate of in-spiral events by ground-based gravitational-wave detectors and impacts their merger location in host galaxies and their possible role as γ-ray burst progenitors. We use a set of population synthesis calculations and investigate the implications of the mass transfer results for the orbital properties of DNS populations.