  The symmetry energy of nucleonic matter is usually assumed to be quadratic in
the isospin density. While this may be justified at sub-saturation densities,
there is no need to enforce this restriction at super-saturation densities. The
presence of a quartic term can strongly modify the critical density for the
direct Urca process which leads to faster cooling of neutron stars. Neutron
star cooling predictions which lie below the observational data can, for some
equations of state, be repaired with a quartic term which effectively turns off
the direct Urca process.
