12 Conclusions

The current understanding of the evolution of close binaries is firmly based on observations of many types of binary systems, from wide non-interacting pairs to very close compact binaries consisting of stellar remnants – white dwarfs, neutrons stars, and black holes. The largest uncertainties in the specific parameters of the compact binary formed at the end of the evolution of a massive binary system are related to the physical properties of the pre-supernovae: masses, magnetic fields, equations of state (for NSs), spins, possible kick velocities, etc. This situation is due to our limited understanding of both the late stages of stellar evolution and especially of the supernovae explosion mechanisms and physics of NS/BH formation.

The understanding of the origin and evolution of compact white dwarf binaries also suffers from incompleteness of our knowledge of white dwarf formation and, in particular, of the common envelope treatment. The progress in these fields, both observational and theoretical, will have a major effect on the understanding of the formation and evolution of compact binary systems. On the other hand, the phenomenological approach used to describe these uncertainties proves to be successful in explaining many observed properties of double stars of different types, so the constraints derived from the studies of binaries should be taken into account in modeling stellar evolution and supernovae explosions.

Of course, specifying and checking the initial distributions of orbital parameters of binaries and parameters of binary evolution (such as evolution in the common envelopes), as well as the modeling of accretion and merger processes stay in the short-list of important work to be done. Here an essential role belongs to detailed numerical simulations.

Further observations of compact binaries.

Clearly, discoveries of new types of compact binary systems have provided the largest impetus for studies of stellar binary evolution. Well-known examples include the discovery of X-ray binaries, relativistic binary pulsars, millisecond recycled pulsars and accreting millisecond X-ray pulsars, and close WD binaries. In the near future we expect the discovery of the NS + BH binaries that are predicted by the massive binary evolution scenario in the form of radio pulsar binaries with BH companions [431, 424, 581]. Their immediate possible progenitors are observed as well-known Galactic system Cyg X-3 and extra-Galactic objects like IC10 X-1, NGC300 X-1 harboring Wolf–Rayet stars and NS or BH. It is very likely that we have already observed the coalescence of NS/BH binary systems as short gamma-ray bursts in other galaxies [226, 500], and the recent discovery of the IR afterglow after short/hard GRB 130603B [43, 751] provided a beautiful confirmation of the expected possible electromagnetic phenomenon (“kilonova” or “macronova”) – the radioactively-powered transient, predicted by Li & Paczyński in 1998 [416].

It is very likely that NS + BH or BH + BH binaries will be found first in GW data analysis [789, 252, 157]. The efforts of the LIGO collaboration to put constraints on compact binary coalescences from the analysis of existing GW observations are very important [3], as well as the hard work on modeling expected signal waveforms [56].

The formation and evolution of compact binaries is a very interdisciplinary field of modern astrophysics, ranging from studies of the equation of state for super-dense matter inside neutron stars and testing effects of strong gravity in relativistic compact binaries to hydrodynamical simulations of stellar winds, the formation and evolution of common envelopes, and stellar explosions. Therefore, further progress in this rapidly flourishing field of ‘multi-messenger astronomy’, which will be made by means of traditional astronomical observations and new tools, like gravitational wave and neutrino detectors [13], will undoubtedly have a strong impact on astronomy and astrophysics as a whole.