107 research outputs found
Elastic properties of polycrystalline dense matter
Elastic properties of the solid regions of neutron star crusts and white
dwarfs play an important role in theories of stellar oscillations. Matter in
compact stars is presumably polycrystalline and, since the elastic properties
of single crystals of such matter are very anisotropic, it is necessary to
relate elastic properties of the polycrystal to those of a single crystal. We
calculate the effective shear modulus of polycrystalline matter with randomly
oriented crystallites using a self-consistent theory that has been very
successful in applications to terrestrial materials and show that previous
calculations overestimate the shear modulus by approximately 28%.Comment: Preprint NORDITA-2015-1
Nucleus--nucleus interactions in the inner crust of neutron stars
The interaction between nuclei in the inner crust of neutron stars consists
of two contributions, the so-called "direct" interaction and an "induced" one
due to density changes in the neutron fluid. For large nuclear separations
the contributions from nuclear forces to each of these terms are shown to be
nonzero. In the static limit they are equal in magnitude but have opposite
signs and they cancel exactly. We analyze earlier results on effective
interactions in the light of this finding. We consider the properties of
long-wavelength collective modes and, in particular, calculate the degree of
mixing between the lattice phonons and the phonons in the neutron superfluid.
Using microscopic theory, we calculate the net non-Coulombic contribution to
the nucleus--nucleus interaction and show that, for large , the leading term
is due to exchange of two phonons and varies as : it is an analog of the
Casimir--Polder interaction between neutral atoms.Comment: 11 pages, 4 figures, 3 table
Dynamics of the inner crust of neutron stars: hydrodynamics, elasticity and collective modes
We present calculations of the hydrodynamics of the inner crust of neutron
stars, where a superfluid neutron liquid coexists with a lattice of
neutron-rich nuclei. The long-wavelength collective oscillations are
combinations of phonons in the lattice and phonons in the superfluid neutrons.
Velocities of collective modes are calculated from information about effective
nucleon-nucleon interactions derived from Lattimer and Swesty's microscopic
calculations based on a compressible liquid drop picture of the atomic nuclei
and the surrounding neutrons.Comment: Preprint NORDITA-2013-1
Two-component superfluid hydrodynamics of neutron star cores
We consider the hydrodynamics of the outer core of a neutron star under
conditions when both neutrons and protons are superfluid. Starting from the
equation of motion for the phases of the wave functions of the condensates of
neutron pairs and proton pairs we derive the generalization of the Euler
equation for a onecomponent fluid. These equations are supplemented by the
conditions for conservation of neutron number and proton number. Of particular
interest is the effect of entrainment, the fact that the current of one nucleon
species depends on the momenta per nucleon of both condensates. We find that
the nonlinear terms in the Euler-like equation contain contributions that have
not always been taken into account in previous applications of superfluid
hydrodynamics. We apply the formalism to determine the frequency of
oscillations about a state with stationary condensates and states with a
spatially uniform counterflow of neutrons and protons. The velocities of the
coupled sound-like modes of neutrons and protons are calculated from properties
of uniform neutron star matter evaluated on the basis of chiral effective field
theory. We also derive the condition for the two-stream instability to occur.Comment: Final version. 9 pages, 5 figure
Dispersion and decay of collective modes in neutron star cores
We calculate the frequencies of collective modes of neutrons, protons and
electrons in the outer core of neutron stars. The neutrons and protons are
treated in a hydrodynamic approximation and the electrons are regarded as
collisionless. The coupling of the nucleons to the electrons leads to Landau
damping of the collective modes and to significant dispersion of the low-lying
modes. We investigate the sensitivity of the mode frequencies to the strength
of entrainment between neutrons and protons, which is not well characterized.
The contribution of collective modes to the thermal conductivity is evaluated.Comment: 10 pages, 4 figure
Turbulence in Binary Bose-Einstein Condensates Generated by Highly Non-Linear Rayleigh-Taylor and Kelvin-Helmholtz Instabilities
Quantum turbulence (QT) generated by the Rayleigh-Taylor instability in
binary immiscible ultracold 87Rb atoms at zero temperature is studied
theoretically. We show that the quantum vortex tangle is qualitatively
different from previously considered superfluids, which reveals deep relations
between QT and classical turbulence. The present QT may be generated at
arbitrarily small Mach numbers, which is a unique property not found in
previously studied superfluids. By numerical solution of the coupled
Gross-Pitaevskii equations we find that the Kolmogorov scaling law holds for
the incompressible kinetic energy. We demonstrate that the phenomenon may be
observed in the laboratory.Comment: Revised version. 7 pages, 8 figure
Superfluid liquid crystals: pasta phases in neutron star crusts
The pasta phases predicted to occur near the inner boundary of the crust of a
neutron star resemble liquid crystals, a smectic A in the case of sheet-like
nuclei (lasagna) and the columnar phase in the case of rod-like nuclei
(spaghetti). An important difference compared with usual liquid crystals is
that the nucleons are superfluid. We develop the hydrodynamic equations for
this system and use them to study collective oscillations. Nucleon
superfluidity leads to important qualitative differences in the spectra of
these oscillations and also increases their frequencies compared with ordinary
liquid crystals. We discuss a number of directions for future work.Comment: 7 page
Elastic properties of phases with nonspherical nuclei in dense matter
We consider the elastic constants of phases with nonspherical nuclei,
so-called pasta phases, predicted to occur in the inner crust of a neutron
star. First, we treat perfectly ordered phases and give numerical estimates for
lasagna and spaghetti when the pasta elements are spatially uniform: the
results are in order-of-magnitude agreement with the numerical simulations of
Caplan, Schneider, and Horowitz, Phys. Rev. Lett. 121, 132701 (2018). We then
turn to pasta phases without long-range order and calculate upper (Voigt) and
lower (Reuss) bounds on the effective shear modulus and find that the lower
bound is zero, but the upper bound is nonzero. To obtain better estimates, we
then apply the self-consistent formalism and find that this predicts that the
shear modulus of the phases without long-range order is zero if the pasta
elements are spatially uniform. In numerical simulations, the pasta elements
are found to be modulated spatially and we show that this modulation is crucial
to obtaining a nonzero elastic moduli for pasta phases without long-range
order. In the self-consistent formalism we find that, for lasagna, the
effective shear modulus is linear in the elastic constants that do not vanish
when the pasta elements are spatially uniform while, for spaghetti, it varies
as the square root of these elastic constants. We also consider the behavior of
the elastic constant associated with a homologous strain (hydrostatic
compression) of the structure of the pasta phases without long-range order.Comment: 9 pages, 6 figure
Parametric resonance of capillary waves at the interface between two immiscible Bose-Einstein condensates
We study parametric resonance of capillary waves on the interface between two
immiscible Bose-Einstein condensates pushed towards each other by an
oscillating force. Guided by analytical models, we solve numerically the
coupled Gross-Pitaevskii equations for two-component Bose-Einstein condensate
at zero temperature. We show that, at moderate amplitudes of the driving force,
the instability is stabilized due to non-linear modifications of the
oscillation frequency. When the amplitude of the driving force is large enough,
we observe detachment of droplets from the Bose-Einstein condensates, resulting
in generation of quantum vortices (skyrmions). We analytically investigate the
vortex dynamics, and conditions of quantized vortex generation.Comment: (Version 2) 11 resized figures. One new reference adde
- …