Hyperon threshold and stellar radii

We study how the $\Lambda$ hyperon threshold influences the radius of the canonical $1.4 ~M_\odot$ neutron star in the light of the measurements found in the recent literature. We show that the onset of a new degree of freedom not only causes the well known reduction of the maximum mass, but also compacts the neutron stars with high central density. With the help of the strange mesons $\phi$ and $\sigma^*$, we show that it is possible to simulate very compact neutron stars keeping realistic hyperon potentials, $U_\Lambda(n_0)= -28$ MeV and $ U_\Lambda^\Lambda(n_0/5)$ in agreement with recents measurements. In the end we generalize these results showing that the onset of a yet not known dark matter particle with mass of 1.04 GeV is able to produce simultaneously a 2 $M_\odot$ neutron star and a canonical one with a radius of only 11.62 km.