Two novel approaches to the hadron-quark mixed phase in compact stars

Abstract

First-order phase transitions, like the liquid-gas transition, proceed via formation of structures such as bubbles and droplets. In strongly interacting compact star matter, at the crust-core transition, but also at the hadron-quark transition in the core, these structures form different shapes dubbed "pasta phases". We describe two methods to obtain one-parameter families of hybrid equations of state (EoS) which mimic the thermodynamic behavior of pasta phases in between a low-density hadron and a high-density quark matter phase, thus generalizing the Maxwell construction. The first method replaces the behavior of pressure vs. chemical potential in a finite region around the critical %chemical potential pressure of the Maxwell construction by a polynomial interpolation. The second method uses extrapolations of the hadronic and quark matter EoS beyond the Maxwell point to define a mixing of both with weight functions bounded by finite limits around the Maxwell point. We apply both methods to the case of a hybrid EoS with a strong first order transition that entails the formation of a third family of compact stars and the corresponding mass twin phenomenon. We investigate for both models the robustness of this phenomenon against variation of the single parameter, the pressure increment at the critical chemical potential which quantifies the deviation from the Maxwell construction. We also show sets of results for other compact star observables than mass and radius, namely the moment of inertia and the baryon mass.