The late stages of evolution of helium star-neutron star binaries and the formation of double neutron star systems

Abstract

With a view to understanding the formation of double neutron star binaries, we investigate the late stages of evolution of helium stars with masses of 2.8–6.4 M⊙ in binary systems with a 1.4-M⊙ neutron star companion. We found that mass transfer from 2.8- to 3.3-M⊙ helium stars (originating from main-sequence stars with masses of 10–12 M⊙ that underwent case B evolution, or 9–10 M⊙ that experienced case C mass transfer) as well as from 3.3 to 3.8 M⊙ in very close orbits (Porb≲ 0.25 d) will end up in a common-envelope and spiral-in phase due to the development of a convective helium envelope at the end of the calculation. If the neutron star has sufficient time to complete the spiralling-in process in the envelope of the helium star before the core collapses, the system will produce very tight double neutron star binaries (Porb ∼ 0.01 d) with a very short merger time-scale, i.e. of the order of 1 Myr or less. These systems would have important consequences for the detection rate of gravitational-wave radiation and for the understanding of γ-ray burst progenitors. On the other hand, if the time left until the explosion is shorter than the orbital-decay time-scale, the system will undergo a supernova (SN) explosion during the common-envelope phase. Helium stars with masses 3.3–3.8 M⊙ in wider orbits (Porb≳ 0.25 d) and those more massive than 3.8 M⊙ do not develop a convective envelope and therefore are not expected to go through common-envelope evolution. The remnants of these massive helium stars are double neutron star pulsars with periods in the range of 0.1–1 d. This suggests that this range of mass (originating from main-sequence stars more massive than 12 M⊙ that underwent case B evolution, or more massive than 10 M⊙ that experienced case C mass transfer) includes the progenitors of the galactic double neutron star pulsars with close orbits (B1913+16 and B1534+12). A minimum kick velocity of 70 and 0 km s−1 (for B1913+16 and B1534+12, respectively) must have been imparted at the birth of the companion to the pulsar. The double neutron stars with wider orbits (J1518+4904 and probably J1811−1736) are produced from helium star-neutron star binaries that avoid Roche lobe overflow, with the helium star being more massive than 2.5 M⊙, i.e. the remnants of main-sequence stars more massive than 10 M⊙ in relatively wide orbits. For these systems, the minimum kick velocities are 50 and 10 km s−1 (for J1518+4904 and J1811−1736, respectively).