Speaker
Description
The general behavior of the nuclear equation of state (EOS), relevant for the description of neutron stars and possible signatures of the presence of hyperons inside neutron stars are studied within a Bayesian approach applied to a set of models based on a density-dependent relativistic mean-field description of nuclear matter. The EOS is subjected to a minimal number of constraints based on nuclear saturation properties and the low-density pure neutron matter EOS obtained from a precise next-to-next-to-next-to-leading order calculation in chiral effective field theory.
General properties of neutron stars such as the maximum mass, radius, tidal deformability, proton fraction, hyperon fraction and speed of sound are discussed. We found that a 90% confidence interval for the allowed NS mass–radius relationship and tidal deformabilities for nucleonic stars is compatible with GW170817 and recent NICER observations, without invoking the exotic degrees of freedom. A central speed of sound of the order of c is obtained. The maximum NS mass allowed by the model is 2.5 M_Sun.
It is shown that the two solar mass constraint imposes that neutron stars described by equations of state that include hyperons have in average a larger radius, and a larger tidal deformability than the stars determined from a nucleonic equation of state, while the speed of sound at the center of the star is more than 25% smaller.
If a 1.4 M_Sun star with a radius smaller than 12.5 km is measured it is quite improbable that a massive star described by the same model contains hyperons. A similar conclusion is drawn if a two solar mass star with a radius ≲11.5km or a neutron star with a mass above 2.2 M_Sun is observed: the possible hyperon content of these stars is ruled out or very reduced. The hyperon presence inside neutron stars is compatible with the present NICER mass-radius observations of the pulsars PSR J0030+0451 and PSR J0740+6620 and the gravitational wave detection GW170817.
It is discussed how the onset of the nucleonic direct Urca process may be used to determine the high density behavior of the smmetry energy.