The fate of twin stars on the unstable branch: implications for the formation of twin stars

Abstract

Hybrid hadron-quark equations of state that give rise a third family of stable compact stars have been shown to be compatible with the LIGO-Virgo event GW170817. Stable configurations in the third family are called hybrid hadron-quark stars. The equilibrium stable hybrid hadron-quark star branch is separated by the stable neutron star branch with a branch of unstable hybrid hadronquark stars. The end-state of these unstable configurations has not been studied, yet, and it could have implications for the formation and existence of twin stars -hybrid stars with the same mass as neutron stars but different radii. We modify existing hybrid hadron-quark equations of state with a first-order phase transition in order to guarantee a well-posed initial value problem of the equations of general relativistic hydrodynamics, and study the dynamics of non-rotating or rotating unstable twin stars via 3-dimensional simulations in full general relativity. We find that unstable twin stars naturally migrate toward the hadronic branch. Before settling into the hadronic regime, these stars undergo (quasi)radial oscillations on a dynamical timescale while the core bounces between the two phases. Our study suggests that it may be difficult to form stable twin stars if the phase transition is sustained over a large jump in energy density, and hence it may be more likely that astrophysical hybrid hadron-quark stars have masses above the twin star regime. We also study the minimum-mass instability for hybrid stars, and find that these configurations do not explode, unlike the minimummass instability for neutron stars. Additionally, our results suggest that oscillations between the two Quantum Chromodynamic phases could provide gravitational wave signals associated with such phase transitions in core-collapse supernovae and white dwarf-neutron star mergers.1 Henceforth we refer to EOSs which include both hadron and quark degrees of freedom as "hybrid EOSs", and stars that contain both hadronic and quark phases as "hybrid stars". Hybrid stars with the same mass as neutron stars are referred to as "twin stars."

Properties of hybrid stars with a density-dependent bag model

“…Moreover, large values B as leads to delayed phase transition as in the case of stiffer EoS without hyperons. The delayed phase transition leads to a large jump in energy density which is unfavorable for the formation of twin stars [126,129,130].…”

Properties of Hybrid Stars with Density Dependent Bag Model

“…Moreover, large values B as leads to delayed phase transition as in the case of stiffer EoS without hyperons. The delayed phase transition leads to a large jump in energy density which is unfavorable for the formation of twin stars [126,129,130].…”