Hybrid Stars with a Crossover Equation of State

Abstract

The possible occurrence of a phase transition in superdense nuclear matter remains an open problem in nuclear physics. In particular, the transition from purely hadronic matter to the hypothesized deconfined quark matter is a subject of considerable interest. Beyond its fundamental significance, this transition has important implications for the physics of compact objects, including neutron stars, quark stars, and hybrid stars. The conventional approaches to this problem involve the Maxwell and Gibbs constructions, both of which have been extensively studied and applied. In the present study, we focus on a third, alternative approach based on a smooth transition from hadronic to quark matter through the so-called switching function. More specifically, the present work focuses on two main objectives. The first is to systematically investigate the influence of the symmetry energy on the transition densities and chemical potentials associated with the transition to quark matter. Subsequently, we attempt to physically constrain and parametrize the two parameters characterizing the switching function, ensuring that the transition from hadronic to quark matter is not merely continuous but also consistent with relevant physical requirements. The main conclusion is that, with an appropriate choice of the aforementioned parameters, the phenomenological model becomes more physically motivated and can serve as a useful tool for investigating the structure and properties of hybrid compact objects. Moreover, this approach offers the flexibility to predict the properties of compact objects that lie beyond the scope of standard neutron-star theory, such as objects characterized by relatively low masses and small radii, which have recently attracted considerable attention in observational studies.