The structure and thermal evolution of strange dwarf stars

We study the structure and evolution of strange dwarf stars: stellar objects composed of a compact core made up of strange matter surrounded by a normal matter envelope, which at its bottom has a density lower than or equal to that of the neutron drip. We restrict our analysis to the case of low-central pressure objects that have a mass-radius relation very similar to the corresponding to white dwarf stars. We show by means of a simplified analysis that strange dwarfs resembling white dwarfs correspond to a very narrow range of central pressures. The almost discontinuous behavior of these structures with respect to changes of the central pressure is studied by means of a polytropic-like analysis, which shows that the envelope of all these objects is well described by the Lane-Emden equation with n = 3 but with boundary conditions different from the ordinary ones. In contrast to earlier expectations, we show that strange dwarf stars are stable only if the density at the bottom of the normal matter envelope is lower than that of the neutron drip. We have computed the evolution of strange dwarf stars of 0.4, 0.55, and 0.8 M(.) in the range of luminosities usually attributed to white dwarf stars. Because of the lack of computations of the previous evolution for such objects, two types of chemical composition were assumed: carbon-oxygen up to a density rho of rho = 10(9) g cm(-3) (type A models) and up to rho = 10(7) g cm(-3) (type B models), respectively. For higher densities, we assumed nuclear statistical equilibrium. We show the central and maximum temperature, neutrino emission, crystallization profile, ages, and the luminosity function versus the stellar luminosity for each type of model and each stellar mass. We found that if the density at the base of the normal matter envelope is slightly lower than that of neutron drip, these objects have a luminosity function observationally indistinguishable from the corresponding luminosity function in white dwarf stars. This is independent of the chemical composition of the normal matter, high-density layers. Thus, the observational data on the cooling of white dwarfs are not in contradiction with the strange matter hypothesis. However, strange dwarfs should behave very differently from white dwarfs in mass-exchanging close binary systems.