arXiv subjects
Publications and source records attributed to E. Osnes.
$^{3}P_{2}$ pairing in neutron matter is investigated using the Bonn potential models. We find pairing energy gaps in pure neutron matter comparable to the results of previous investigators when the attractive tensor coupling is included. However, taking into account that in a neutron star we have matter at $β$ equilibrium, we find that the $^{3}P_{2}$-$^{3}F_{2}$ energy gap is reduced considerably.
In this work we calculate neutron and proton energy gaps in neutron star matter, using the Bonn meson--exchange interactions and a model--space approach to the gap equation. This approach allows a consistent calculation of energy gaps and single particle energies with the model--space Brueckner--Hartree--Fock (MBHF) method, without double counting of two--particle correlations. Neutron energy gaps are calculated at zero and finite temperature. Proton energy gaps are calculated at beta equilibrium, and it is shown that the inclusion of muons has a significant effect. The results are compared with those of other works, and the implications for neutron star physics are briefly discussed.
In this work we consider the role of muons in various URCA processes relevant for neutrino emissions in the core region of neutron stars. The calculations are done for $β$--stable nuclear matter with and without muons. We find muons to appear at densities $ρ= 0.15$ fm$^{-3}$, slightly around the saturation density for nuclear matter $ρ_0 =0.16$ fm$^{-3}$. The direct URCA processes for nucleons are forbidden for densities below $ρ= 0.5$ fm$^{-3}$, however the modified URCA processes with muons $(n+N\rightarrow p+N +μ+\overlineν_μ, p+N+μ\rightarrow n+N+ν_μ$), where $N$ is a nucleon, result in neutrino emissivities comparable to those from $(n+N\rightarrow p+N +e +\overlineν_e, p+N+e \rightarrow n+N+ν_e$). This opens up for further possibilities to explain the rapid cooling of neutrons stars. Superconducting protons reduce however these emissivities at densities below $0.4$ fm$^{-3}$.
In this work we study properties of neutron star crusts, where matter is expected to consist of nuclei surrounded by superfluid neutrons and a homogeneous background of relativistic electrons. The nuclei are disposed in a Coulomb lattice, and it is believed that the structure of the lattice influences considerably the specific heat of the neutronic matter inside the crust of a neutron star. Using a modern meson--exchange potential in the framework of a local--density approximation we calculate the neutronic specific heat accounting for various shapes of the Coulomb lattice, from spherical to non--spherical nuclear shapes. We find that a realistic nucleon--nucleon potential leads to a significant increase in the neutronic specific heat with respect to that obtained assuming a uniform neutron distribution. The increase is largest for the non--spherical phase of the crust. These results may have consequences for the thermal history of young neutron stars.
In this work we calculate the total mass, radius, moment of inertia, and surface gravitational redshift for neutron stars using various equations of state (EOS). Modern meson-exchange potential models are used to evaluate the $G$-matrix for asymmetric nuclear matter. We calculate both a non-relativistic and a relativistic EOS. Of importance here is the fact that relativistic Brueckner-Hartree-Fock calculations for symmetric nuclear matter fit the empirical data, which are not reproduced by non-relativistic calculations. Relativistic effects are known to be important at high densities, giving an increased repulsion. This leads to a stiffer EOS compared to the EOS derived with a non-relativistic approach. Both the non-relativistic and the relativistic EOS yield moments of inertia and redshifts in agreement with the accepted values. The relativistic EOS yields, however, too large mass and radius. The implications are discussed.
The convergence of the perturbation expansion for the effective interaction to be used in shell-model calculations is investigated as function of the mass number $A$, from $A=4$ to $A=208$. As the mass number increases, there are more intermediate states to sum over in each higher-order diagram which contributes to the effective interaction. Together with the fact that the energy denominators in each diagram are smaller for larger mass numbers, these two effects could largely enhance higher-order contributions to the effective interaction, thereby deteriorating the order-by-order convergence of the effective interaction. This effect is counterbalanced by the short range of the nucleon-nucleon interaction, which implies that its matrix elements are weaker for valence single-particle states in ``large'' nuclei with large mass number as compared to those in light nuclei. These effects are examined by comparing various mean values of the matrix elements. It turns out that the contributions from higher-order terms remain fairly stable as the mass number increases from $A=4$ to $A=208$. The implications for nuclear structure calculations are discussed.
We calculate properties of neutron stars such as mass and radius using a relativistic Dirac-Brueckner-hartree-Fock apprach for asymmetric nuclear matter. For pure neutron matter we find the maximum mass to be 2.4 solar masses with a radius of 12 km. For a proton fraction of 30% we find a max mass of 2.1 solar masses and a radius of 10.5 km. The implications are discussed.
We comment upon a recent work by Zheng, Barrett, Jaqua, Vary and McCarthy, concerning calculations of spectra of light nuclei with no core. It is demonstrated that the omission of particle-particle ladder diagrams in their calculations, explains the large differences between results obtained with various model spaces.
We calculate the spectra for the neutron deficient Sn isotopes with mass numbers from A=102 to A=108, using standard many-body techniques. The G-matrix used in the perturbative expansion was calculated using the Bonn potentials. A good agreement with the available data is obtained.
We have compared exact numerical results for the Lipkin model at finite temperature with Hartree-Fock theory and with the results of including in addition the ring diagrams. In the simplest version of the Lipkin model the Hartree-Fock approach shows a ``phase transition" which is absent in the exact results. For more realistic cases, Hartree-Fock provides a very good approximation and a modest improvement is obtained by adding the ring diagrams.