A three-layer preon star model from exact piecewise-continuous solutions   of Einstein's equations
  The Einstein equations are solved algebraically to model a hybrid astrophysical compact object consisting of a preon gas core, a mantle of electrically charged hot quark-gluon plasma, and an outer envelope of charged hadronic matter which is matched to an exterior Reissner-Nordstrom vacuum. The piecewise-continuous metric and the pressure and density functions consist of polynomials that are everywhere well-behaved. Boundary conditions at each interface yield estimates for physical parameters applicable to each layer, and to the star as a whole. 
Zoran Pazameta
gr-qc astro-ph.HE astro-ph.SR