Color superconductivity and the strange quark

At ultra-high density, matter is expected to form a degenerate Fermi gas of quarks in which there is a condensate of Cooper pairs of quarks near the Fermi surface: color superconductivity. In these proceedings I review some of the underlying physics, and discuss outstanding questions about the phase structure of ultra-dense quark matter.Comment: 11 pages, proceedings of QCD@Work 2005 and Johns Hopkins Workshop 200

What the Timing of Millisecond Pulsars Can Teach us about Their Interior

The cores of compact stars reach the highest densities in nature and therefore could consist of novel phases of matter. We demonstrate via a detailed analysis of pulsar evolution that precise pulsar timing data can constrain the star's composition, through unstable global oscillations (r-modes) whose damping is determined by microscopic properties of the interior. If not efficiently damped, these modes emit gravitational waves that quickly spin down a millisecond pulsar. As a first application of this general method, we find that ungapped interacting quark matter is consistent with both the observed radio and x-ray data, whereas for ordinary nuclear matter some additional enhanced damping mechanism is required.Comment: 6 pages, 5 figures, version to be published in PR

Thickness of the strangelet-crystal crust of a strange star

It has recently been pointed out that if the surface tension of quark matter is low enough, the surface of a strange star will be a crust consisting of a crystal of charged strangelets in a neutralizing background of electrons. This affects the behavior of the surface, and must be taken into account in efforts to observationally rule out strange stars. We calculate the thickness of this ``mixed phase'' crust, taking into account the effects of surface tension and Debye screening of electric charge. Our calculation uses a generic parametrization of the equation of state of quark matter. For a reasonable range of quark matter equations of state, and surface tension of order a few MeV/fm^2, we find that the preferred crystal structure always involves spherical strangelets, not rods or slabs of quark matter. We find that for a star of radius 10 km and mass 1.5 Msolar, the strangelet-crystal crust can be from zero to hundreds of meters thick, the thickness being greater when the strange quark is heavier, and the surface tension is smaller. For smaller quark stars the crust will be even thicker.Comment: 10 pages, LaTe

Impact of r-modes on the cooling of neutron stars

Studying the frequency and temperature evolution of a compact star can give us valuable information about the microscopic properties of the matter inside the star. In this paper we study the effect of dissipative reheating of a neutron star due to r-mode oscillations on its temperature evolution. We find that there is still an impact of an r-mode phase on the temperature long after the star has left the instability region and the r-mode is damped completely. With accurate temperature measurements it may be possible to detect this trace of a previous r-mode phase in observed pulsars.Comment: 7 pages, 5 figures, Proceedings of QCD@work 2012 International Workshop on QCD Theory and Experimen

Large amplitude behavior of the bulk viscosity of dense matter

We study the bulk viscosity of dense matter, taking into account non-linear effects which arise in the large amplitude "supra-thermal" region where the deviation $\mu_\Delta$ of the chemical potentials from chemical equilibrium fulfills $\mu_\Delta>T$. This regime is relevant to unstable modes such as r-modes, which grow in amplitude until saturated by non-linear effects. We study the damping due to direct and modified Urca processes in hadronic matter, and due to nonleptonic weak interactions in strange quark matter. We give general results valid for an arbitrary equation of state of dense matter and find that the viscosity can be strongly enhanced by supra-thermal effects. Our study confirms previous results on quark matter and shows that the non-linear enhancement is even stronger in the case of hadronic matter. Our results can be applied to calculations of the r-mode-induced spin-down of fast-rotating neutron stars, where the spin-down time will depend on the saturation amplitude of the r-modeComment: 15 pages, 11 figure