  Relativistic mean field (RMF) theory of nuclear matter with the isovector
scalar mean field corresponding to the delta-meson [a_0(980)] is studied. While
the delta-meson mean field vanishes in symmetric nuclear matter, it can
influence properties of asymmetric nuclear matter in neutron stars. The RMF
contribution due to delta-field to the nuclear symmetry energy is negative. To
fit the empirical value, E_s=30 MeV, a stronger rho-meson coupling is required
than in the absence of the delta-field. The energy per particle of neutron
matter is then larger at high densities than the one with no delta-field
included. Also, the proton fraction of beta-stable matter increases. Splitting
of proton and neutron effective masses due to the delta-field can affect
transport properties of neutron star matter.
