Local merger rates of double neutron star systems and related puzzle

Martyna Chruslinska

Nicolaus Copernicus Astronomical Center, Warsaw, Poland

The first detection of gravitational waves from a merging double neutron star (DNS) binary challenged our understanding of evolution of its potential progenitor systems, implying a much higher rate of DNS coalescences in the local Universe than predicted on theoretical grounds. Those theoretical estimates for the isolated binary evolution scenario are usually based on results from population synthesis calculations.
The wide range of values for the calculated merger rates often quoted in population synthesis studies reflects our limited knowledge of the details of evolution and interactions of massive stars in binaries, which one can hope to improve by confrontation with the observational limits. In the era of gravitational wave observations this becomes a promising tool to learn about the formation of merging systems. However, this comparison is far from being straightforward. Binaries that merge within the local Universe originate from progenitor systems that formed at different redshifts and in various environments. The efficiency of formation of double compact objects is highly sensitive to metallicity of the star formation. Therefore, to confront the theoretical estimates with observational limits resulting from gravitational waves observations one has to account for the formation and evolution of progenitor stars in chemically evolving Universe. This introduces another layer of uncertainty to those calculations that needs to be better understood.