Interface Effects of Quark Matter: Light-quark Nuggets and Compact Stars

The interface effects of quark matter play important roles in the properties of compact stars and small nuggets, such as strangelets and nonstrange quark matter (udQM) nuggets. By introducing a density derivative term to the Lagrangian density and adopting Thomas-Fermi approximation, we find it is possible to reproduce the results obtained by solving Dirac equations. Adopting certain parameter sets, the energy per baryon of udQM nuggets decreases with baryon number A and become more stable than nuclei at A greater than or similar to 300. The effects of quark matter symmetry energy are examined, where udQM nuggets at A approximate to 1000 can be more stable than others if large symmetry energy is adopted. In such cases, larger udQM nuggets will decay via fission and the surface of a udQM star will fragment into a crust made of udQM nuggets and electrons, which resembles the cases of a strange star's crust. The corresponding microscopic structures are then investigated adopting spherical and cylindrical approximations for the Wigner-Seitz cells, where the droplet phase is found to be the most stable configuration with udQM stars' crusts and udQM dwarfs made of udQM nuggets (A approximate to 1000) and electrons. For the cases considered here, the crust thickness of udQM stars is typically similar to 200 m, which reaches a few kilometers if we neglect the interface effects and adopt Gibbs construction. The masses and radii of udQM dwarfs are smaller than typical white dwarfs, which would increase if the interface effects are neglected.