Phase Conversions in Neutron Stars: Implications for Stellar Stability and Gravitational Wave Astrophysics

Germán Lugones,Ana Gabriela Grunfeld

doi:10.3390/universe7120493

Abstract

We review the properties of hybrid stars with a quark matter core and a hadronic mantle, focusing on the role of key micro-physical properties such as the quark/hadron surface and curvature tensions and the conversion speed at the interface between both phases. We summarize the results of works that have determined the surface and curvature tensions from microscopic calculations. If these quantities are large enough, mixed phases are energetically suppressed and the quark core would be separated from the hadronic mantle by a sharp interface. If the conversion speed at the interface is slow, a new class of dynamically stable hybrid objects is possible. Densities tens of times larger than the nuclear saturation density can be attained at the center of these objects. We discuss possible formation mechanisms for the new class of hybrid stars and smoking guns for their observational identification.

Highlights

A longstanding unsolved problem in neutron star (NS) physics is whether deconfined quark matter can be present at their cores, which would imply the existence of the so-called hybrid stars, composed by a quark matter core and a hadronic mantle
In this review we have explored some micro-physical properties of the quark–hadron interface in hybrid stars, their consequences for stellar structure and stability, and possible implications for gravitational wave astrophysics
Microscopic calculations of the surface tension span a wide range of values depending the equations of state and the method used for the calculation, which makes it uncertain whether the quark–hadron interface is mixed or sharp

Summary

Introduction

A longstanding unsolved problem in neutron star (NS) physics is whether deconfined quark matter can be present at their cores, which would imply the existence of the so-called hybrid stars, composed by a quark matter core and a hadronic mantle. The LIGO Livingston detector observed the event GW190425, a compact binary coalescence with total mass ∼3.4 M [16] This was the first confirmed GW detection based on data from a single observatory and no electromagnetic counterpart was found. The LIGO/ Virgo Collaboration announced the discovery of a gravitational wave binary, GW190814 [19] from which no electromagnetic counterpart was identified. ∼1 − 2n0 still faces many difficulties that are bypassed by using phenomenological models that match experimental data at low densities and perturbative QCD at asymptotically large ones Within such an approach, many internal compositions are possible that agree with all present theoretical and empirical requirements.

The Nature of the Quark Hadron Interface

Extended Dynamical Stability

Interface Conversions

Astrophysical Scenarios for the Formation of Slow Stable Hybrid Stars

Implications for Gravitational Wave Astrophysics

Summary and Conclusions