We describe two aspects of the physics of hybrid stars that have a sharp
interface between a core of quark matter and a mantle of nuclear matter.
Firstly, we analyze the mass-radius relation. We describe a generic "Constant
Speed of Sound" (CSS) parameterization of the quark matter equation of state
(EoS), in which the speed of sound is independent of density. In terms of the
three parameters of the CSS EoS we obtain the phase diagram of possible forms
of the hybrid star mass-radius relation, and we show how observational
constraints on the maximum mass and typical radius of neutron stars can be
expressed as constraints on the CSS parameters.
Secondly, we propose a mechanism for the damping of density oscillations,
including r-modes, in hybrid stars with a sharp interface. The dissipation
arises from the periodic conversion between quark matter and nuclear matter
induced by the pressure oscillations in the star. We find the damping grows
nonlinearly with the amplitude of the oscillation and is powerful enough to
saturate an r-mode at very low saturation amplitude, of order 10−10, which
is compatible with currently-available observations of neutron star spin
frequencies and temperatures.Comment: 15 pages, 12 figures. Published versio
