http://arxiv.org/abs/1710.05504
There are important but unresolved processes in the standard formation scenarios of double compact star binaries (DCBs; BH-BH, BH-NS, NS-NS systems), such as the mass transfer and common envelope (CE). We analyze the effects of different assumptions on key physical processes of the massive star binary evolution toward DCBs, which have not been tested in previous studies. We find that different prescriptions for the mass transfer and the binding energy parameter in the CE phase have moderate influence on the merger rates and properties of BH-BH systems, but significantly affect those of DCBs containing a NS. The small initial separation enhances the merger rate of BH-BH systems. The small initial separation enhances the BH-BH merger rate. We confirm that a low metallicity (Population II) is a key for the GW150914-like BH-BH binaries, and we find the predicted rate can be compatible to the inferred rate if an optimized model is adopted. The GW151226-like systems are naturally expected at the solar metallicity (Population I) within the standard CE evolution. If these two systems are to be explained this way through the CE evolution as results of Pop II and Pop I systems, respectively, then the transition in the mode of the evolution must occur at $Z \sim 0.002 – 0.005$ $(0.13-0.25Z_{\odot})$ to compare to the observed relative frequency of these two types of events. On the other hand, the indirectly indicated NS-NS merger rate from observed NS-NS systems (Kim et al. 2015) can be reproduced by the BPS simulations only by the conditions inconsistent with those to explain the BH-BH merger rate through the isolated binary evolution scenario. Therefore, we predict that the NS-NS merger rate is indeed higher than so far inferred by the indirect method, or otherwise the BH-BH GW systems should be formed through a different evolutionarily pathway than NS-NS systems.
I. Ablimit and K. Maeda
Tue, 17 Oct 17
60/163
Comments: N/A