Robustness of third family solutions for hybrid stars against mixed phase effects

TL;DR

This paper examines the stability of high-mass twin hybrid stars with quark cores against phase transition softening, showing that such solutions remain robust under mixed phase effects and could explain observations like GW170817.

Contribution

It introduces a construction mimicking pasta structures in the mixed phase and demonstrates the robustness of third family solutions for hybrid stars against phase transition softening.

Findings

High-mass twin star solutions are robust against mixed phase effects.

The maximum pressure broadening compatible with twin stars is approximately equal to the critical pressure.

At least one neutron star in GW170817 could belong to the third family of hybrid stars.

Abstract

We investigate the robustness of third family solutions for hybrid compact stars with a quark matter core that correspond to the occurrence of high-mass twin stars against a softening of the phase transition by means of a construction that mimics the effects of pasta structures in the mixed phase. We consider a class of hybrid equations of state that exploits a relativistic mean-field model for the hadronic as well as for the quark matter phase. We present parametrizations that correspond to branches of high-mass twin star pairs with maximum masses between $2.05~M_\odot$ and $1.48~M_\odot$ having radius differences between 3.2 km and 1.5 km, respectively. When compared to a Maxwell construction with a fixed value of critical pressure $P_c$ the effect of the mixed phase construction consists in the occurrence of a region of pressures around $P_c$ belonging to the coexistence of hadronic and quark matter phases between the onset pressure at $P_H$ and the end of the transition at $P_Q$. The maximum broadening which would still allow mass twin compact stars is found to be $(P_Q-P_H)_{\rm max} \approx P_c$ for all parametrizations within the present class of models. At least the heavier of the neutron stars of the binary merger GW170817 could have been a member of the third family of hybrid stars.