Structure and stability of strange and charm stars at finite temperatures

This paper consists of four parts. Part one deals with an investigation of the properties of \ensuremath{\beta}-equilibrated, electrically charged neutral quark-star matter at zero and finite temperatures, and the determination of its equation of state. In part two, the properties of sequences of quark stars, divided into strange- and charm-quark stars, depending on quark-flavor content, are investigated. The strange stars are constructed for absolutely stable strange-quark matter, whose energy per baryon number lies below the one in $^{56}\mathrm{Fe}$. In part three, the electrostatic potential of electrons inside and in the close vicinity outside of strange stars, which is of decisive importance for the possible existence of nuclear crusts on the surfaces of such stars, is computed. It is found that finite temperatures lead to a considerable reduction of the electrostatic electron potential at the surface of a strange star, which is accompanied by a strong reduction of the Coulomb barrier asssociated with the difference of the electrostatic potential at the surface of the star's strange-matter core and the base of the crust. This finding is of great importance for the stable existence of crusts on strange stars, since the Coulomb barrier plays the important role of preventing atomic nuclei bound in the nuclear crust from coming into contact with the star's strange-matter core, where atomic matter by hypothesis would be converted into strange matter. The structure and stability of quark stars against radial oscillations is discussed in part four, where it is found that charm-quark stars are unstable against radial oscillations. Thus no charm-quark stars (and, as is demonstrated too, no quark-matter stars possessing still higher central mass densities) can exist in nature.