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 doubleneutron stars (NS), with the unique characteristic that none of the two NS everhad the chance to be recycled by accretion. The existence of this channel stemsfrom the evolution of helium-rich stars (cores of massive NS progenitors),which has been neglected in most previous studies of double compact objectformation. We find that these non-recycled NS-NS binaries are formed from barecarbon-oxygen cores in tight orbits, with formation rates comparable to ormaybe even higher than those of recycled NS-NS binaries. On the other hand,their detection probability as binary pulsars is greatly reduced (by about1000) relative to recycled pulsars, because of their short lifetimes. Weconclude that, in the context of gravitational-wave detection of NS-NS inspiralevents, this new type of binaries calls for an increase of the rate estimatesderived from the observed NS-NS with recycled pulsars, typically by factors of1.5-3 or even higher.Comment: Accepted for publication in ApJ Letters; 5 pages, 1 figure, 2 tables. Two new paragraphs and one formula adde