28 research outputs found
Dense matter equation of state and neutron star properties from nuclear theory and experiment
The equation of state of dense matter determines the structure of neutron
stars, their typical radii, and maximum masses. Recent improvements in
theoretical modeling of nuclear forces from the low-energy effective field
theory of QCD has led to tighter constraints on the equation of state of
neutron-rich matter at and somewhat above the densities of atomic nuclei, while
the equation of state and composition of matter at high densities remains
largely uncertain and open to a multitude of theoretical speculations. In the
present work we review the latest advances in microscopic modeling of the
nuclear equation of state and demonstrate how to consistently include also
empirical nuclear data into a Bayesian posterior probability distribution for
the model parameters. Derived bulk neutron star properties such as radii,
moments of inertia, and tidal deformabilities are computed, and we discuss as
well the limitations of our modeling.Comment: 9 pages, 5 figures. To appear in the AIP Proceedings of the
Xiamen-CUSTIPEN Workshop on the EOS of Dense Neutron-Rich Matter in the Era
of Gravitational Wave Astronomy, Jan. 3-7, Xiamen, Chin
Proton pairing in neutron stars from chiral effective field theory
We study the proton pairing gap in beta-equilibrated neutron star
matter within the framework of chiral effective field theory. We focus on the
role of three-body forces, which strongly modify the effective proton-proton
spin-singlet interaction in dense matter. We find that three-body forces
generically reduce both the size of the pairing gap and the maximum density at
which proton pairing may occur. The pairing gap is computed within BCS theory,
and model uncertainties are estimated by varying the nuclear potential and the
choice of single-particle spectrum in the gap equation. We find that a
second-order perturbative treatment of the single-particle spectrum suppresses
the proton pairing gap relative to the use of a free spectrum. We
estimate the critical temperature for the onset of proton superconductivity to
be K, which is consistent with previous
theoretical results in the literature and marginally within the range deduced
from a recent Bayesian analysis of neutron star cooling observations.Comment: 8 pages, 9 figure
Improved determination of the oscillator parameters in nuclei
The oscillator parameter in nuclei is refitted to reproduce the available
charge radius data. As an important improvement, we include the Coulomb term
evaluated within the assumption of a uniformly charged sphere, and take into
account the symmetry effect induced by the difference between N and Z numbers
in a straightforward manner using the conventional parameterization. The
Coulomb interaction has repulsive effect, causing the wave functions to extend
further toward the nucleus exterior, resulting in an effectively larger
oscillator length parameter. The symmetry effect is attractive for protons in
neutron-rich nuclei and for neutrons in proton-rich nuclei, and repulsive for
the other cases. Therefore, three distinct oscillator parameters are
determined: one for protons, one for neutrons, and one isospin-invariant
version, which is obtained by subtracting the Coulomb and symmetry
contributions. Additionally, we explore the direct fit of the harmonic
oscillator wave functions to the eigenfunctions of the Hartree-Fock mean field
using the Skyrme interaction. Generally, this method agrees well with the
others for light nuclei, typically up to Ca. Beyond this nucleus,
however, the results begin to diverge over the orbits chosen for the fit. Only
the parameters values obtained for the last occupied states agree remarkably
well with the conventional ones throughout the mass range under consideration.Comment: 8 pages, 2 figure
Symmetry energy and neutron star properties constrained by chiral effective field theory calculations
We investigate the nuclear symmetry energy and neutron star properties using
a Bayesian analysis based on constraints from different chiral effective field
theory calculations using new energy density functionals that allow for large
variations at high densities. Constraints at high densities are included from
observations of GW170817 and NICER. In particular, we show that both NICER
analyses lead to very similar posterior results for the symmetry energy and
neutron star properties when folded into our equation of state framework. Using
the posteriors, we provide results for the symmetry energy and the slope
parameter, as well as for the proton fraction, the speed of sound, and the
central density in neutron stars. Moreover, we explore correlations of neutron
star radii with the pressure and the speed of sound in neutron stars. Our 95\%
credibility ranges for the symmetry energy , the slope parameter , and
the radius of a 1.4\, neutron star are
\,MeV, \,MeV, and \,km.
Our analysis for the proton fraction shows that larger and-or heavier neutron
stars are more likely to cool rapidly via the direct Urca process. Within our
equation of state framework a maximum mass of neutron stars indicates that the speed of sound needs to exceed the
conformal limit.Comment: 12 pages, 12 figure