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Two Phase Model for Warm Stellar Matter

Authors: André Duarte

Supervisors: Constança Providência

MSc thesis, Mestrado em Física, Universidade de Coimbra (2017)

Abstract: In this work a two-phase model for warm dense stellar matter is presented and used to study the properties and structure of neutron stars. In this approach two effective models of dense matter, the NL3ωρ model of hadronic matter, and the NJL model for quark matter, are used to describe matter at low (around nuclear saturation density) and high (much higher than saturation density) densities, respectively. These are unified in a single model by treating each as a model of a separate phase (the confined and deconfined phases) of dense matter, with the two phases coexisting in thermodynamic equilibrium. This two-phase approach predicts an interval of densities at which both hadronic and deconfined matter coexist, the so called “mixed phase”, as well as purely hadronic matter at low densities and pure quark matter at high densities. At very low densities matter no longer exists as uniform matter in the ground state; it exists instead as a lattice of nuclei: the neutron star crust. This is accounted for with an approximation scheme that yields very good results for stars with masses of astrophysical interest. The parametrization of the models in terms of experimental quantities is discussed. The properties and structure of neutron stars made of this matter are calculated, and the effect of varying the parameters of the models is studied. It is shown that the model predicts neutron stars of mass and radius compatible with the latest astronomical observations, provided that the NJL model vector coupling constant lies in a certain interval. It is concluded that it is possible for the heavier stars predicted by this model to have a hybrid or even pure quark core. Furthermore, proto-neutron stars are idealized as constant-entropy stars and their properties are studied. This enables conclusions to be drawn about the formation and evolution of neutron stars via a purely stationary analysis. Proto-neutron stars at various stages of formation are thus modelled, and their properties discussed.