Neutron Stars in Relativistic Mean Field Theory with Isovector Scalar Meson

We study the equation of state (EOS) of β-stable dense matter and models of neutron stars in the relativistic mean field (RMF) theory with the isovector scalar mean field corresponding to the δ-meson [a0(980)]. A range of values of the δ-meson coupling compatible with the Bonn potentials is explored. Parameters of the model in the isovector sector are constrained to fit the nuclear symmetry energy, Es ≈ 30MeV . We find that the quantity most sensitive to the δ-meson coupling is the proton fraction of neutron star matter. It increases significantly in the presence of the δ-field. The energy per baryon also increases but the effect is smaller. The EOS becomes slightly stiffer and the maximum neutron star mass increases for stronger δ-meson coupling. PACS: 21.65.+f, 97.60.Jd 1. Nucleon matter in the RMF model with the δ-meson The standard RMF model [1] of nuclear matter, frequently used in astrophysical calculations, involves mean fields of σ, ω and ρ mesons. It does not include the contribution of the isovector scalar meson δ [a0(980)] although generally the density to which this field can couple does not vanish, < ψ̄τ3ψ >6= 0. The contribution of the δ-meson field is not expected to be important for finite nuclei of small isospin-asymmetry, as the δ-meson mean field vanishes in symmetric nuclear matter. However, for strongly isospin-asymmetric matter in neutron stars presence of the δ-field can influence the properties of dense matter. In Ref.[2] the RMF model was generalized to include the contribution of the δ-meson. Here we investigate consequences of such a generalized RMF theory for neutron stars. The dynamics of the RMF model is governed by the lagrangian L = L0 + Lint, (1) where L0 = Lψ +Lσ +Lω +Lρ+Lδ is the free-field lagrangian and Lint is the interaction term. The free-field lagrangians for nucleons and meson fields are: