69 research outputs found
Unified description of neutron superfluidity in the neutron-star crust with analogy to anisotropic multi-band BCS superconductors
The neutron superfluidity in the inner crust of a neutron star has been
traditionally studied considering either homogeneous neutron matter or only a
small number of nucleons confined inside the spherical Wigner-Seitz cell.
Drawing analogies with the recently discovered multi-band superconductors, we
have solved the anisotropic multi-band BCS gap equations with Bloch boundary
conditions, thus providing a unified description taking consistently into
account both the free neutrons and the nuclear clusters. Calculations have been
carried out using the effective interaction underlying our recent
Hartree-Fock-Bogoliubov nuclear mass model HFB-16. We have found that even
though the presence of inhomogeneities lowers the neutron pairing gaps, the
reduction is much less than that predicted by previous calculations using the
Wigner-Seitz approximation. We have studied the disappearance of superfluidity
with increasing temperature. As an application we have calculated the neutron
specific heat, which is an important ingredient for modeling the thermal
evolution of newly-born neutron stars. This work provides a new scheme for
realistic calculations of superfluidity in neutron-star crusts.Comment: 15 pages, 31 figures, accepted for publication in Physical Review
Semi-classical equation of state and specific heats for neutron-star inner crust with proton shell corrections
An approach to the equation of state for the inner crust of neutron stars
based on Skyrme-type forces is presented. Working within the Wigner-Seitz
picture, the energy is calculated by the TETF (temperature-dependent extended
Thomas-Fermi) method, with proton shell corrections added self-consistently by
the Strutinsky-integral method. Using a Skyrme force that has been fitted to
both neutron matter and to essentially all the nuclear mass data, we find
strong proton shell effects: proton numbers = 50, 40 and 20 are the only
values possible in the inner crust, assuming that nuclear equilibrium is
maintained in the cooling neutron star right down to the ambient temperature.
Convergence problems with the TETF expansion for the entropy, and our way of
handling them, are discussed. Full TETF expressions for the specific heat of
inhomogeneous nuclear matter are presented. Our treatment of the electron gas,
including its specific heat, is essentially exact, and is described in detail.Comment: 41 pages, 6 figure
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Recent breakthroughs in Skyrme-Hartree-Fock-Bogoliubov mass formulas
We review our recent achievements in the construction of microscopic mass
tables based on the Hartree-Fock-Bogoliubov method with Skyrme effective
interactions. In the latest of our series of HFB-mass models, we have obtained
our best fit ever to essentially all the available mass data, by treating the
pairing more realistically than in any of our earlier models. The rms deviation
on the 2149 measured masses of nuclei with N and Z>8 has been reduced for the
first time in a mean field approach to 0.581 MeV. With the additional
constraint on the neutron-matter equation of state, this new force is thus very
well-suited for the study of neutron-rich nuclei and for the description of
astrophysical environments like supernova cores and neutron-star crusts.Comment: Proceedings of the Fifth International Conference on Exotic Nuclei
and Atomic Masses, September 7-13 2008, Ryn (Poland). To appear in the
European Physical Journal
The Effects of Reputation and Ethics on Budgetary Slack
This experimental study tests the effects on budgetary slack of two potential controls for opportunistic self-interestóreputation and ethics. I manipulate the level of information asymmetry between the subordinate and the superior regarding productive capability and measure the subordinateís reputation and ethical concerns regarding budgetary slack. In this setting, I examine how information asymmetry affects reputation and ethical concerns, and test the effects of these concerns on budgetary slack. Consistent with prior findings, subordinates restrict the slack in their budgets to well below the maximum under a slackinducing pay scheme, even after five periods of experience. Budgetary slack is negatively associated with a measure of ethical responsibility from a pre-experiment personality questionnaire as well as reputation and ethical concerns expressed in an exit questionnaire. Subordinates express lower reputation concerns as information asymmetry regarding productive capability increases, thereby reducing the superiorís ability to monitor the slack in their budget. Ethical concerns, however, are not diminished with increases in information asymmetry. These results suggest that reputation is a socially mediated control, whereas ethics is an internally mediated control for opportunistic self-interest
The r-process nucleosynthesis: a continued challenge for nuclear physics and astrophysics
The identification of the astrophysical site and the specific conditions in
which r-process nucleosynthesis takes place remain unsolved mysteries of
astrophysics. The present paper emphasizes some important future challenges
faced by nuclear physics in this problem, particularly in the determination of
the radiative neutron capture rates by exotic nuclei close to the neutron drip
line and the fission probabilities of heavy neutron-rich nuclei. These
quantities are particularly relevant to determine the composition of the matter
resulting from the decompression of initially cold neutron star matter. New
detailed r-process calculations are performed and the final composition of
ejected inner and outer neutron star crust material is estimated. We discuss
the impact of the many uncertainties in the astrophysics and nuclear physics on
the final composition of the ejected matter. The similarity between the
predicted and the solar abundance pattern for A > 140 nuclei as well as the
robustness of the prediction with varied input parameters makes this scenario
one of the most promising that deserves further exploration.Comment: 8 pages, contribution to the Nuclei in the Cosmos Conference, to
appear in Nucl. Phys.
Shell Corrections of Superheavy Nuclei in Self-Consistent Calculations
Shell corrections to the nuclear binding energy as a measure of shell effects
in superheavy nuclei are studied within the self-consistent Skyrme-Hartree-Fock
and Relativistic Mean-Field theories. Due to the presence of low-lying proton
continuum resulting in a free particle gas, special attention is paid to the
treatment of single-particle level density. To cure the pathological behavior
of shell correction around the particle threshold, the method based on the
Green's function approach has been adopted. It is demonstrated that for the
vast majority of Skyrme interactions commonly employed in nuclear structure
calculations, the strongest shell stabilization appears for Z=124, and 126, and
for N=184. On the other hand, in the relativistic approaches the strongest
spherical shell effect appears systematically for Z=120 and N=172. This
difference has probably its roots in the spin-orbit potential. We have also
shown that, in contrast to shell corrections which are fairly independent on
the force, macroscopic energies extracted from self-consistent calculations
strongly depend on the actual force parametrisation used. That is, the A and Z
dependence of mass surface when extrapolating to unknown superheavy nuclei is
prone to significant theoretical uncertainties.Comment: 14 pages REVTeX, 8 eps figures, submitted to Phys. Rev.
A survey of the parameter space of the compressible liquid drop model as applied to the neutron star inner crust
We present a systematic survey the range of predictions of the neutron star
inner crust composition, crust-core transition densities and pressures, and
density range of the nuclear `pasta' phases at the bottom of the crust provided
by the compressible liquid drop model in the light of current experimental and
theoretical constraints on model parameters. Using a Skyrme-like model for
nuclear matter, we construct baseline sequences of crust models by consistently
varying the density dependence of the bulk symmetry energy at nuclear
saturation density, , under two conditions: (i) that the magnitude of the
symmetry energy at saturation density is held constant, and (ii)
correlates with under the constraint that the pure neutron matter (PNM) EoS
satisfies the results of ab-initio calculations at low densities. Such baseline
crust models facilitate consistent exploration of the dependence of crustal
properties. The remaining surface energy and symmetric nuclear matter
parameters are systematically varied around the baseline, and different
functional forms of the PNM EoS at sub-saturation densities implemented, to
estimate theoretical `error bars' for the baseline predictions. Inner crust
composition and transition densities are shown to be most sensitive to the
surface energy at very low proton fractions and to the behavior of the
sub-saturation PNM EoS. Recent calculations of the energies of neutron drops
suggest that the low-proton-fraction surface energy might be higher than
predicted in Skyrme-like models, which our study suggests may result in a
greatly reduced volume of pasta in the crust than conventionally predicted.Comment: 37 Pages, 16 figures, accepted for publication in Astrophysical
Journal Supplement Serie
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