Interface effects of quark matter: Light-quark nuggets and compact stars

TL;DR

This paper investigates the interface effects of quark matter on compact stars and nuggets, introducing a density derivative term to model stability, structure, and properties of $ud$QM nuggets and related stellar objects.

Contribution

It develops a simplified model incorporating interface effects to study the stability and structure of $ud$QM nuggets and their role in compact star crusts.

Findings

$ud$QM nuggets become more stable than nuclei at baryon number $A oughly 300.

$ud$QM star crusts are typically about 200 meters thick.

$ud$QM dwarfs have smaller masses and radii than white dwarfs.

Abstract

The interface effects of quark matter play important roles in the properties of compact stars and small nuggets such as strangelets and $ud$QM 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 $ud$QM nuggets decreases with baryon number $A$ and become more stable than nuclei at $A\gtrsim 300$. The effects of quark matter symmetry energy are examined, where $ud$QM nuggets at $A\approx 1000$ can be more stable than others if large symmetry energy is adopted. In such cases, larger $ud$QM nuggets will decay via fission and the surface of an $ud$QM star will fragment into a crust made of $ud$QM 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 $ud$QM stars' crusts and $ud$QM dwarfs made of $ud$QM nuggets ($A\approx 1000$) and electrons. For the cases considered here, the crust thickness of $ud$QM stars is typically $\sim$200 m, which reaches a few kilometers if we neglect the interface effects and adopt Gibbs construction. The masses and radii of $ud$QM dwarfs are smaller than typical white dwarfs, which would increase if the interface effects are neglected.