From Neutron Stars to Strange Stars

It is generally agreed that the tremendous densities reached in the centers of neutron stars provide a high pressure environment in which numerous particles processes are likely to compete with each other. These processes range from the generation of hyperons to quark deconfinement to the formation of kaon condensates and H-matter [1]. Another striking possibility concerns the formation of absolutely stable strange quark matter, a configuration of matter even more stable than the most stable atomic nucleus, iron. In the latter event all neutron stars would in fact be strange (quark matter) stars [2], objects largely composed of pure strange quark matter, eventually enveloped in a thin nuclear crust made up of ordinary, hadronic matter. There has been much recent progress in our understanding of quark matter, culminating in the discovery that if quark matter exists it will be in a color superconducting state [3, 4, 5, 6]. The phase diagram of such matter appears to be very complex [5, 6]. At asymptotic densities the ground state of QCD with a vanishing strange quark mass is the color-flavor locked (CFL) phase. This phase is electrically neutral in bulk for a significant range of chemical potentials and strange quark masses [7]. If the strange quark mass is heavy enough to be ignored, then up and down quarks may pair in the two-flavor superconducting (2SC) phase. Other possible condensation patters are the recently discovered CFL–K phase [8] and the color-spin locked (2SC+s) phase [9]. The magnitude of the gap energy lies between ∼ 50 and 100 MeV. Color superconductivity thus modifies the equation of state (eos) at the order (∆/μ) level, which is only a few percent. Such small effects can be safely neglected in present determinations of models for the eos of neutron stars and strange quark matter stars. There has been much recent work on how color superconductivity in neutron stars could affect their properties [5, 6, 10, 11, 12]. These studies revealed that possible signatures include the cooling by neutrino emission, the pattern of the arrival times of supernova neutrinos, the evolution of neutron star magnetic fields, rotational (r-mode) instabilities, and glitches in rotation frequencies. In this review I shall complement this