The role of strangeness and isospin in low density expansions of hadronic matter

We compare relativistic mean field models with their low density expansion counterparts used to mimic non-relativistic models by consistently expanding the baryonic scalar density in powers of the baryonic number density up to ${\cal O}(13/3)$, which goes two orders beyond the order considered in previous works. We show that, due to the non-trivial density dependence of the Dirac mass, the convergence of the expansion is very slow, and the validity of the non-relativistic approximation {\bf is} questionable even at subsaturation densities. In order to analyze the roles played by strangeness and isospin we consider $n-\Lambda$ and $n-p$ matter separately. Our results indicate that these degrees of freedom play quite different roles in the expansion mechanism and $n-\Lambda$ matter can be better described by low density expansions than $n-p$ matter in general.