391 research outputs found

### Quasi-normal modes of superfluid neutron stars

We study non-radial oscillations of neutron stars with superfluid baryons, in
a general relativistic framework, including finite temperature effects. Using a
perturbative approach, we derive the equations describing stellar oscillations,
which we solve by numerical integration, employing different models of nucleon
superfluidity, and determining frequencies and gravitational damping times of
the quasi-normal modes. As expected by previous results, we find two classes of
modes, associated to superfluid and non-superfluid degrees of freedom,
respectively. We study the temperature dependence of the modes, finding that at
specific values of the temperature, the frequencies of the two classes of
quasi-normal modes show avoided crossings, and their damping times become
comparable. We also show that, when the temperature is not close to the avoided
crossings, the frequencies of the modes can be accurately computed by
neglecting the coupling between normal and superfluid degrees of freedom. Our
results have potential implications on the gravitational wave emission from
neutron stars.Comment: 16 pages, 7 figures, 2 table

### Dissipation in relativistic superfluid neutron stars

We analyze damping of oscillations of general relativistic superfluid neutron
stars. To this aim we extend the method of decoupling of superfluid and normal
oscillation modes first suggested in [Gusakov & Kantor PRD 83, 081304(R)
(2011)]. All calculations are made self-consistently within the finite
temperature superfluid hydrodynamics. The general analytic formulas are derived
for damping times due to the shear and bulk viscosities. These formulas
describe both normal and superfluid neutron stars and are valid for oscillation
modes of arbitrary multipolarity. We show that: (i) use of the ordinary
one-fluid hydrodynamics is a good approximation, for most of the stellar
temperatures, if one is interested in calculation of the damping times of
normal f-modes; (ii) for radial and p-modes such an approximation is poor;
(iii) the temperature dependence of damping times undergoes a set of rapid
changes associated with resonance coupling of neighboring oscillation modes.
The latter effect can substantially accelerate viscous damping of normal modes
in certain stages of neutron-star thermal evolution.Comment: 25 pages, 9 figures, 1 table, accepted for publication in MNRA

### Neutron star inner crust: reduction of shear modulus by nuclei finite size effect

The elasticity of neutron star crust is important for adequate interpretation
of observations. To describe elastic properties one should rely on theoretical
models. The most widely used is Coulomb crystal model (system of point-like
charges on neutralizing uniform background), in some works it is corrected for
electron screening. These models neglect finite size of nuclei. This
approximation is well justified except for the innermost crustal layers, where
nuclei size becomes comparable with the inter-nuclear spacing. Still, even in
those dense layers it seems reasonable to apply the Coulomb crystal result, if
one assumes that nuclei are spherically symmetric: Coulomb interaction between
them should be the same as interaction between point-like charges. This
argument is indeed correct, however, as we point here, shear of crustal lattice
generates (microscopic) quadrupole electrostatic potential in a vicinity of
lattice cites, which induces deformation on the nuclei. We analyze this problem
analytically within compressible liquid drop model, using ionic spheroid model
(which is generalization of well known ion sphere model). In particular, for
ground state crust composition the effective shear modulus is reduced for a
factor of $1-u^{5/3}/(2+3\,u-4\,u^{1/3})$, where u is the filling factor (ratio
of the nuclei volume to the volume of the cell). This result is universal and
does not depend on the applied nucleon interaction model. For the innermost
layers of inner crust u~0.2 leading to reduction of the shear modulus by ~25%,
which can be important for correct interpretation of quasi-periodic
oscillations in the tails of magnetar flares.Comment: 7 pages, submitted to MNRAS on Sept.

### Seismic signatures of strange stars with crust

We study acoustic oscillations (eigenfrequencies, velocity distributions,
damping times) of normal crusts of strange stars. These oscillations are very
specific because of huge density jump at the interface between the normal crust
and the strange matter core. The oscillation problem is shown to be
self-similar. For a low (but non-zero) multipolarity l the fundamental mode
(without radial nodes) has a frequency ~300 Hz and mostly horizontal
oscillation velocity; other pressure modes have frequencies >=20 kHz and almost
radial oscillation velocities. The latter modes are similar to radial
oscillations (have approximately the same frequencies and radial velocity
profiles). The oscillation spectrum of strange stars with crust differs from
the spectrum of neutron stars. If detected, acoustic oscillations would allow
one to discriminate between strange stars with crust and neutron stars and
constrain the mass and radius of the star.Comment: 15 pages, 3 figures, Accepted for publication in MNRA

### Shear viscosity of degenerate electron matter

We calculate the partial electron shear viscosity $\eta_{ee}$ limited by
electron-electron collisions in a strongly degenerate electron gas taking into
account the Landau damping of transverse plasmons. The Landau damping strongly
suppresses $\eta_{ee}$ in the domain of ultrarelativistic degenerate electrons
and modifies its %asymptotic temperature behavior. The efficiency of the
electron shear viscosity in the cores of white dwarfs and envelopes of neutron
stars is analyzed.Comment: 16 pages, 4 figures, accepted to Journal of Physics

### Coulomb tunneling for fusion reactions in dense matter: Path integral Monte Carlo versus mean field

We compare Path Integral Monte Carlo calculations by Militzer and Pollock
(Phys. Rev. B 71, 134303, 2005) of Coulomb tunneling in nuclear reactions in
dense matter to semiclassical calculations assuming WKB Coulomb barrier
penetration through the radial mean-field potential. We find a very good
agreement of two approaches at temperatures higher than ~1/5 of the ion plasma
temperature. We obtain a simple parameterization of the mean field potential
and of the respective reaction rates. We analyze Gamow-peak energies of
reacting ions in various reaction regimes and discuss theoretical uncertainties
of nuclear reaction rates taking carbon burning in dense stellar matter as an
example.Comment: 13 pages, 7 figures, to appear in Phys. Rev.

### Hardware and Software for Determining the Thermal Properties of Rocks Under Close to Stratified Conditions

An instrument for determining the nonlinear thermal properties of rocks under natural conditions is proposed. Software is developed for processing the temperature readings, obtained from the instrument sensors, using mainly the theory of the solution of inverse problems for a parabolic-type nonlinear equation. Practical examples of the determination of the nonlinear thermal properties of some materials are presented. Â© 2014 Springer Science+Business Media New York

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