Coalescence of compact binary stars with a quark star component.

摘要

Coalescing compact star binaries are among the most promising sources for the next generation of gravitational detectors. Accurate theoretical waveform templates are required to extract their gravitational wave signal from noisy data and it is expected that about 10 mergers per year (or more) of such binaries will be found by these upcoming detectors. If these binaries tidally lock/synchronize, where synchronization is facilitated by a large viscosity for the stellar fluid, then their inspiral is accelerated. This will result in a phase mismatch between the signal and templates and failure to include these contributions from tidal synchronization can hamper detection. Bildsten and Cutler in 1992 studied tidal synchronization in neutron star-neutron star and black hole-neutron star binaries, and found that the viscosity of the neutron star was too small Here we study the same effect but in binaries with a quark star component. What makes quark stars different from neutron stars is that (a) quark matter can potentially have large viscosities and (b) unlike neutron stars, quark stars do not have a theoretical lower mass limit. These two features imply that quark stars could tidally synchronize under some extreme conditions on certain parameters, unlike neutron stars which never synchronize. Thus, the gravitational wave signal can be used to detect and differentiate quark stars from ordinary neutron stars. One important feature of inspiraling binaries during the final stages of the inspiral is the possibility of mass transfer, which can further alter the expected gravitational wave signal Therefore, we study the conditions necessary for stable mass transfer in the neutron star-quark star binary system and determine that the most likely scenario at this stage is unstable mass transfer from the neutron star to the quark star. Finally, we examine the fate of the binary system after tidal synchronization is achieved. We propose a variational study of tidal synchronization based on the work of Lai et al. in 1994, which accounts for deformations of the component stars of the neutron star-quark star binary. Using a newly devised polytropic equation of state for quark stars, we expect the stars to remain tidally synchronized although more sophisticated numerics will be required to prove this. Our main conclusion is that gravitational waves can discriminate between neutron star-neutron star binaries and neutron star-quark star binaries, but only if one assumes an extreme selection of theoretical parameters. In practice, this distinction may therefore be very difficult to achieve.