Are rotating strange quark stars good sources of gravitational waves?

Abstract

We study the viscosity driven bar mode (Jacobi-like) instability of rapidly rotating quark matter stars (strange stars) in general relativity. A triaxial, "bar shaped" compact star could be an efficient source of continuous wave gravitational radiation in the frequency range of the forthcoming interferometric detectors. We locate the secular instability point along several constant baryon mass sequences of uniformly rotating strange stars described by the MIT bag model. Contrary to neutron stars, strange stars with T/|W| (the ratio of the rotational kinetic energy to the absolute value of the gravitational potential energy) much lower than the corresponding value for the mass-shed limit can be secularly unstable to bar mode formation if shear viscosity is high enough to damp out any deviation from uniform rotation. The instability develops for a broad range of gravitational masses and rotational frequencies of strange quark stars. It imposes strong constraints on the lower limit of the frequency at the innermost stable circular orbit around rapidly rotating strange stars. The above results are robust for all linear self-bound equations of state assuming the growth time of the instability is faster than the damping timescale. Whether the instability can grow or not depends on many different physical quantities (e.g. value of viscosities (rather uncertain)). We discuss astrophysical scenarios where triaxial instabilities (r-mode and viscosity driven instability) could be relevant (a new born star, an old star spinning up by accretion) in strange stars described by the standard MIT bag model of normal quark matter. The spin evolution of a strange star strongly depends on the strange quark mass. Taking into account actual values of viscosities in strange quark matter and neglecting the magnetic field we show that Jacobi-like instability cannot develop in any astrophysicaly relevant temperature windows. The main result is that strange quark stars described by the MIT bag model can be accelerated to very high frequency in Low Mass X-ray binaries if the strange quark mass is consistent with values based on particle data tables.