A new formation channel for double neutron stars without recycling: Implications for gravitational wave detection

We report on a new evolutionary path leading to the formation of close double neutron stars (NSs), with the unique characteristic that none of the two NSs ever had the chance to be recycled by accretion. The existence of this channel stems from the evolution of helium-rich stars (cores of massive NS progenitors), which has been neglected in most previous studies of double compact object formation. We find that these nonrecycled NS-NS binaries are formed from bare carbon-oxygen cores in tight orbits, with formation rates comparable to or maybe even higher than those of recycled NS-NS binaries. On the other hand, their detection probability as binary pulsars is greatly reduced (by ∼10³) relative to recycled pulsars because of their short lifetimes. We conclude that, in the context of gravitational wave detection of NS-NS in-spiral events, this new type of binaries calls for an increase of the rate estimates derived from the observed NS-NS systems with recycled pulsars, typically by factors of 1.5-3 or even higher.