Hyperons in neutron stars

Neutron stars (NSs) are compact objects with the density of the order of the nuclear density and, accordingly, methods of nuclear physics are effective for describing the NS matter. At the same time, the central density is several times higher than the nuclear density, which leads to the hyperon appearance and allows testing models of both nucleonic and hyperonic interactions under extreme conditions. The appearance of hyperons softens the equation of state (EOS) of the NS matter and decreases the maximum mass, which makes it problematic to achieve the maximum NS mass [Formula: see text] in calculations, as required by astrophysical observations. We consider the NS matter in the Skyrme force approximation taking into account [Formula: see text] and [Formula: see text] hyperons. It is shown that the density of the hyperon appearance is of great importance for the maximum NS mass problem. Correlations between the NS characteristics and the properties of baryonic interactions are demonstrated and it is shown that the most important factors are the hyperonic interaction contracting power, as well as the nuclear matter incompressibility and the derivatives of the symmetry energy characterizing the properties of nuclear matter at high densities. A complex combination of nucleonic and hyperonic interaction properties can lead to different orders of appearance of [Formula: see text] and [Formula: see text] hyperons or their possible absence, depending on the interactions used. The absence of [Formula: see text] hyperons does not solve the hyperon puzzle, since [Formula: see text] hyperons also soften the EOS.