We revisit double neutron star ( DNS ) formation in the classical binary evolution scenario in light of the recent LIGO/Virgo DNS detection ( GW170817 ) .
The observationally estimated Galactic DNS merger rate of R _ { MW } = 21 ^ { +28 } _ { -14 } Myr ^ { -1 } , based on 3 Galactic DNS systems , fully supports our standard input physics model with R _ { MW } = 24 Myr ^ { -1 } .
This estimate for the Galaxy translates in a non-trivial way ( due to cosmological evolution of progenitor stars in chemically evolving Universe ) into a local ( z \approx 0 ) DNS merger rate density of R _ { local } = 48 Gpc ^ { -3 } yr ^ { -1 } , which is not consistent with the current LIGO/Virgo DNS merger rate estimate ( 1540 ^ { +3200 } _ { -1220 } Gpc ^ { -3 } yr ^ { -1 } ) .
Within our study of the parameter space we find solutions that allow for DNS merger rates as high as R _ { local } \approx 600 ^ { +600 } _ { -300 } Gpc ^ { -3 } yr ^ { -1 } which are thus consistent with the LIGO/Virgo estimate .
However , our corresponding BH-BH merger rates for the models with high DNS merger rates exceed the current LIGO/Virgo estimate of local BH-BH merger rate ( 12 – 213 { ~ { } Gpc } ^ { -3 } { ~ { } yr } ^ { -1 } ) .
Apart from being particularly sensitive to the common envelope treatment , DNS merger rates are rather robust against variations of several of the key factors probed in our study ( e.g .
mass transfer , angular momentum loss , natal kicks ) .
This might suggest that either common envelope development/survival works differently for DNS ( \sim 10 - 20 { ~ { } M } _ { \odot } stars ) than for BH-BH ( \sim 40 - 100 { ~ { } M } _ { \odot } stars ) progenitors , or high BH natal kicks are needed to meet observational constraints for both types of binaries .
Note that our conclusion is based on a limited number of ( 21 ) evolutionary models and is valid only within this particular DNS and BH-BH isolated binary formation scenario .