Twin stars as probes of the nuclear equation of state: Effects of rotation through the PSR J0952-0607 pulsar and constraints via the tidal deformability from the GW170817 event

In agreement with the constantly increasing gravitational wave events, new aspects of the internal structure of compact stars can be considered. A scenario in which a first-order transition takes place inside these stars is of particular interest, as it can lead, under certain conditions, to a third gravitationally stable branch (besides white dwarfs and neutron stars), the twin stars. The new branch yields stars with the same mass as normal compact stars but quite different radii. We focus on hybrid stars undergoing a hadron-toquark phase transition near their core and how this new stable configuration arises. Emphasis is given on the aspects of the phase transition and its parametrization in two different ways-namely, with the Maxwell and Gibbs constructions. We systematically study the gravitational mass, the radius, and the tidal deformability, and we compare them with the predictions of the recent observation by the LIGO/VIRGO Collaboration, the GW170817 event, and the mass and radius limits, suggesting possible robust constraints. Moreover, we extend the study to include rotation effects on the twin star configurations. The recent discovery of the fast rotating supermassive pulsar PSR J0952-0607 triggered the efforts to constrain the equation of state and, moreover, to examine possible predictions related to the phase transition in dense nuclear matter. We pay special attention to relate the PSR J0952-0607 pulsar properties to the twin star predictions, and mainly to explore the possibility that the existence of such a massive object would rule out the existence of twin stars. Finally, we discuss the constraints on the radius and mass of the recently observed compact object within the supernova remnant HESS J1731-347. The estimations imply that this object is either the lightest known neutron star or a star with a more exotic equation of state.

Automated summary

In this study, the internal structure of compact stars and the possibility of a first-order transition resulting in the creation of twin stars are explored. The phase transition is analyzed using the Maxwell and Gibbs constructions, and the gravitational mass, radius, and tidal deformability of hybrid stars are compared with observational data from LIGO/VIRGO Collaboration's GW170817 event. The study also includes considerations of the effects of rotation on twin star configurations and the constraints imposed by the discovery of the supermassive pulsar PSR J0952-0607 and the compact object within the supernova remnant HESS J1731-347.