We perform cooling simulations for isolated neutron stars using recently developed equations of state for their core .
The equations of state are obtained from new parametrizations of the FSU2 relativistic mean-field functional that reproduce the properties of nuclear matter and finite nuclei , while fulfilling the restrictions on high-density matter deduced from heavy-ion collisions , measurements of massive 2 M _ { \odot } neutron stars , and neutron star radii below 13 km .
We find that two of the models studied , FSU2R ( with nucleons ) and in particular FSU2H ( with nucleons and hyperons ) , show very good agreement with cooling observations , even without including extensive nucleon pairing .
This suggests that the cooling observations are more compatible with an equation of state that produces a soft nuclear symmetry energy and , hence , generates small neutron star radii .
However , both models favor large stellar masses , above 1.8 M _ { \odot } , to explain the colder isolated neutron stars that have been observed , even if nucleon pairing is present .