ar X iv : a st ro - p h / 05 10 37 9 v 1 1 3 O ct 2 00 5 Evolution and Merging of Binaries with Compact Objects ⋆

In the light of recent observations in which short γ-ray bursts are interpreted as arising from black-hole, neutron-star or neutron-star, neutron-star mergings we would like to review our research on the evolution of compact binaries, especially those containing neutron stars. 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 double neutron star binaries always ends up with a low-mass black hole, neutron star binary. This is where the factor ∼ 20 in the ratio of these to double neutron star binaries comes from. In fact, the double neutron star binaries can only be evolved if their two giant progenitors burn helium at the same time. This requires the two progenitors to be within 4% of each other in mass and it is this special requirement that results in the double neutron star binaries being relatively scarce compared with the low-mass black hole neutron star binaries. Our scenario of double neutron star binaries as having been proceeded by a double He-star binary is collecting observational support in terms of the nearly equal neutron star 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 neutron stars that will merge gravitationally, giving signals to LIGO. This discovery brings in the low ZAMS mass main sequence progenitors that can evolve into a neutron star binary, adding importantly to the “visible” binaries that can merge. However it does not affect the factor ∼ 20 increase found by Bethe and Brown coming from the much greater number of low-mass black-hole, neutron-star binaries, which have only a small probability of being observed before they merge. Preprint submitted to Elsevier Science 22 March 2008 We develop the phenomenology which suggests that neutron stars evolve from ZAMS mass ∼ 10 − 18M⊙ star, low-mass black holes from 18 − 20M⊙, and highmass black holes from 20−30M⊙. These brackets follow fromWoosley’s C(α, γ)O rate of 170 MeV barns at 300 keV. We discuss the observed violation of our maximum neutron star mass M NS = 1.5M⊙, and comment on how our scenario would change if the maximum neutron star mass is greater than 1.5M⊙.