Evolution and merging of binaries with compact objects

Abstract

In the light of recent observations in which short γ -ray bursts are interpreted as arising from black-hole(BH), neutron-star(NS) or NS–NS mergings we would like to review our research on the evolution of compact binaries, especially those containing NS's. These were carried out with predictions for LIGO in mind, but are directly applicable to short γ -ray bursts in the interpretation above. Most important in our review is that we show that the standard scenario for evolving NS–NS binaries always ends up with a low-mass BH (LMBH), NS binary. Bethe and Brown [1998, Astrophys. J. 506, 780] showed that this fate could be avoided if the two giants in the progenitor binary burned He at the same time, and that in this way the binary could avoid the common envelope evolution of the NS with red giant companion which sends the first born NS into a BH in the standard scenario. The burning of He at the same time requires, for the more massive giants such as the progenitors of the Hulse–Taylor binary NS that the two giants be within 4% of each other in zero age main sequence (ZAMS) mass. Applying this criterion to all binaries results in a factor ∼ 5 of LMBH–NS binaries as compared with NS–NS binaries. Although this factor is substantially less than the originally claimed factor of 20 which Bethe and Brown (1998) estimated, largely because a careful evolution has been carried through here, our factor 5 is augmented by a factor of ∼ 8 arising from the higher rate of star formation in the earlier Galaxy from which the BH–NS binaries came from. Furthermore, here we calculate the mergers for short-hard gamma-ray bursts, whereas Bethe and Brown's factor 20 included a factor of 2 for the higher chirp masses in a BH–NS binary as compared with NS–NS one. In short, we end up with an estimate of factor ∼ 40 over that calculated with NS–NS binary mergers in our Galaxy alone. Our total rate is estimated to be about one merging of compact objects per year. Our scenario of NS–NS binaries as having been preceded by a double He-star binary is collecting observational support in terms of the nearly equal NS masses within a given close binary. We review our work on population synthesis of compact binaries, pointing out that it is in excellent agreement with the much more detailed synthesis carried out by Portegies Zwart. This is currently of interest because the recent discovery of the double pulsar has substantially increased the number of binary NS's that will merge gravitationally, giving signals to LIGO. This discovery brings in the low ZAMS mass main sequence progenitors that can evolve into a NS binary, adding importantly to the “visible” binaries that can merge. However it does not affect the factor ≳ 40 increase, mostly from the much greater number of LMBH–NS binaries, which have only a small probability of being observed before they merge. We develop the phenomenology which suggests that NS's evolve from ZAMS mass ∼ 10 – 18 M ⊙ star, LMBH's from 18 – 20 M ⊙ , and high-mass BH's from 20 – 30 M ⊙ . These brackets follow from Woosley's 12 C ( α , γ ) 16 O rate of 170 MeV barns at 300 keV. We discuss the observed violation of our previous maximum NS mass M NS max = 1.5 M ⊙ , raising our M NS max to 1.7 M ⊙ and comment on how our scenario would change if the maximum NS mass is greater than 1.7 M ⊙ .