Double neutron stars: merger rates revisited

The gravitational waves detections from astrophysical sources provide a new promising tool to constrain the evolution of the progenitors of compact binaries. We revisit the topic of the uncertainties of the still undetected double neutron stars (DNS) merger rate estimates. The most direct observational constraint available: Galactic DNS merger rate ($R_{\rm MW}=21^{+28}_{-14}$ Myr$^{-1}$) based on 3 Galactic DNS systems fully supports our standard input physics ($R_{\rm MW} =24$ Myr$^{-1}$). Our estimate for the Milky Way translates in non-trivial way (cosmological evolution of progenitor stars in chemically evolving Universe) into local ($z\approx0$) DNS merger rate density ($R_{\rm local}=48$ Gpc$^{-3}$yr$^{-1}$), consistent with short GRB rates and is well within current LIGO upper limits. DNS merger rates, apart from being particularly sensitive to the common envelope treatment, are rather robust against variations of several key factors probed in our study (mass transfer, angular momentum loss, natal kicks). In fact, we are not able to increase our standard DNS local merger rate density by more than a factor of 6 even for rather notable changes in DNS formation physics. Our standard model rate implies LIGO O3 (170 Mpc) DNS detection rate $0.5$ yr$^{-1}$, while for the full advanced LIGO sensitivity (215 Mpc) this rate is $1$ yr$^{-1}$. We do not predict more than a few DNS detections per year (LIGO duty cycle: $\sim 40\%$) even for our most optimistic models. On the other hand, some of our models allow for the local DNS merger rate density decrease by a factor $\gtrsim 10$. This is still marginally consistent with observational constraints, but it does not guarantee DNS detection even in the entire year of LIGO observations at the full design sensitivity.