36 research outputs found
Development of high-order realizable finite-volume schemes for quadrature-based moment method
Kinetic equations containing terms for spatial transport, gravity, fluid drag and particle-particle collisions can be used to model dilute gas-particle flows. However, the enormity of independent variables makes direct numerical simulation of these equations almost impossible for practical problems. A viable alternative is to reformulate the problem in terms of moments of velocity distribution. Recently, a quadrature-based moment method was derived by Fox for approximating solutions to kinetic equation for arbitrary Knudsen number. Fox also described 1st- and 2nd-order finite-volume schemes for solving the equations. The success of the new method is based on a moment-inversion algorithm that is used to calculate non-negative weights and abscissas from moments. The moment-inversion algorithm does not work if the moments are non-realizable, meaning they do not correspond to a distribution function. Not all the finite-volume schemes lead to realizable moments. Desjardins et al. showed that realizability is guaranteed with the 1 st-order finite-volume scheme, but at the expense of excess numerical diffusion. In the present work, the nonrealizability of the standard 2 nd-order finite-volume scheme is demonstrated and a generalized idea for the development of high-order realizable finite-volume schemes for quadrature-based moment methods is presented. This marks a significant improvement in the accuracy of solutions using the quadrature-based moment method as the use of 1st-order scheme to guarantee realizability is no longer a limitation
Ultracold neutrons, quantum effects of gravity and the Weak Equivalence Principle
We consider an extension of the recent experiment with ultracold neutrons and
the quantization of its vertical motion in order to test the Weak Equivalence
Principle. We show that an improvement on the energy resolution of the
experiment may allow to establish a modest limit to the Weak Equivalence
Principle and on the gravitational screening constant. We also discuss the
influence of a possible new interaction of Nature.Comment: Revtex4, 4 pages. Discussion on the equivalence principle altered.
Bound is improve
Nonlinear r-Modes in Neutron Stars: Instability of an unstable mode
We study the dynamical evolution of a large amplitude r-mode by numerical
simulations. R-modes in neutron stars are unstable growing modes, driven by
gravitational radiation reaction. In these simulations, r-modes of amplitude
unity or above are destroyed by a catastrophic decay: A large amplitude r-mode
gradually leaks energy into other fluid modes, which in turn act nonlinearly
with the r-mode, leading to the onset of the rapid decay. As a result the
r-mode suddenly breaks down into a differentially rotating configuration. The
catastrophic decay does not appear to be related to shock waves at the star's
surface. The limit it imposes on the r-mode amplitude is significantly smaller
than that suggested by previous fully nonlinear numerical simulations.Comment: Published in Phys. Rev. D Rapid Comm. 66, 041303(R) (2002
Metal enrichment of the intra-cluster medium over a Hubble time for merging and relaxed galaxy clusters
We investigate the efficiency of galactic mass loss, triggered by
ram-pressure stripping and galactic winds of cluster galaxies, on the chemical
enrichment of the intra-cluster medium (ICM). We combine N-body and
hydrodynamic simulations with a semi-numerical galaxy formation model. By
including simultaneously different enrichment processes, namely ram-pressure
stripping and galactic winds, in galaxy-cluster simulations, we are able to
reproduce the observed metal distribution in the ICM. We find that the mass
loss by galactic winds in the redshift regime z>2 is ~10% to 20% of the total
galactic wind mass loss, whereas the mass loss by ram-pressure stripping in the
same epoch is up to 5% of the total ram-pressure stripping mass loss over the
whole simulation time. In the cluster formation epochs z<2 ram-pressure
stripping becomes more dominant than galactic winds. We discuss the
non-correlation between the evolution of the mean metallicity of galaxy
clusters and the galactic mass losses. For comparison with observations we
present two dimensional maps of the ICM quantities and radial metallicity
profiles. The shape of the observed profiles is well reproduced by the
simulations in the case of merging systems. In the case of cool-core clusters
the slope of the observed profiles are reproduced by the simulation at radii
below ~300 kpc, whereas at larger radii the observed profiles are shallower. We
confirm the inhomogeneous metal distribution in the ICM found in observations.
To study the robustness of our results, we investigate two different
descriptions for the enrichment process interaction.Comment: 11 pages, 13 figures, accepted for publication in A&A, high
resolution version can be found at
<http://astro.uibk.ac.at/~wolfgang/kapferer.pdf
Towards a Realistic Neutron Star Binary Inspiral: Initial Data and Multiple Orbit Evolution in Full General Relativity
This paper reports on our effort in modeling realistic astrophysical neutron
star binaries in general relativity. We analyze under what conditions the
conformally flat quasiequilibrium (CFQE) approach can generate
``astrophysically relevant'' initial data, by developing an analysis that
determines the violation of the CFQE approximation in the evolution of the
binary described by the full Einstein theory. We show that the CFQE assumptions
significantly violate the Einstein field equations for corotating neutron stars
at orbital separations nearly double that of the innermost stable circular
orbit (ISCO) separation, thus calling into question the astrophysical relevance
of the ISCO determined in the CFQE approach. With the need to start numerical
simulations at large orbital separation in mind, we push for stable and long
term integrations of the full Einstein equations for the binary neutron star
system. We demonstrate the stability of our numerical treatment and analyze the
stringent requirements on resolution and size of the computational domain for
an accurate simulation of the system.Comment: 22 pages, 18 figures, accepted to Phys. Rev.
Metal enrichment of the intra-cluster medium by thermally and cosmic-ray driven galactic winds
We investigate the efficiency and time-dependence of thermally and cosmic ray
driven galactic winds for the metal enrichment of the intra-cluster medium
(ICM) using a new analytical approximation for the mass outflow. The spatial
distribution of the metals are studied using radial metallicity profiles and 2D
metallicity maps of the model clusters as they would be observed by X-ray
telescopes like XMM-Newton. Analytical approximations for the mass loss by
galactic winds driven by thermal and cosmic ray pressure are derived from the
Bernoulli equation and implemented in combined N-body/hydrodynamic cosmological
simulations with a semi-analytical galaxy formation model. Observable
quantities like the mean metallicity, metallicity profiles, and 2D metal maps
of the model clusters are derived from the simulations. We find that galactic
winds alone cannot account for the observed metallicity of the ICM. At redshift
the model clusters have metallicities originating from galactic winds
which are almost a factor of 10 lower than the observed values. For massive,
relaxed clusters we find, as in previous studies, a central drop in the
metallicity due to a suppression of the galactic winds by the pressure of the
ambient ICM. Combining ram-pressure stripping and galactic winds we find radial
metallicity profiles of the model clusters which agree qualitatively with
observed profiles. Only in the inner parts of massive clusters the observed
profiles are steeper than in the simulations. Also the combination of galactic
winds and ram-pressure stripping yields too low values for the ICM
metallicities. The slope of the redshift evolution of the mean metallicity in
the simulations agrees reasonably well with recent observations.Comment: 9 pages, 6 figures, accepted by A&
Three-dimensional general relativistic hydrodynamics II: long-term dynamics of single relativistic stars
This is the second in a series of papers on the construction and validation
of a three-dimensional code for the solution of the coupled system of the
Einstein equations and of the general relativistic hydrodynamic equations, and
on the application of this code to problems in general relativistic
astrophysics. In particular, we report on the accuracy of our code in the
long-term dynamical evolution of relativistic stars and on some new physics
results obtained in the process of code testing. The tests involve single
non-rotating stars in stable equilibrium, non-rotating stars undergoing radial
and quadrupolar oscillations, non-rotating stars on the unstable branch of the
equilibrium configurations migrating to the stable branch, non-rotating stars
undergoing gravitational collapse to a black hole, and rapidly rotating stars
in stable equilibrium and undergoing quasi-radial oscillations. The numerical
evolutions have been carried out in full general relativity using different
types of polytropic equations of state using either the rest-mass density only,
or the rest-mass density and the internal energy as independent variables. New
variants of the spacetime evolution and new high resolution shock capturing
(HRSC) treatments based on Riemann solvers and slope limiters have been
implemented and the results compared with those obtained from previous methods.
Finally, we have obtained the first eigenfrequencies of rotating stars in full
general relativity and rapid rotation. A long standing problem, such
frequencies have not been obtained by other methods. Overall, and to the best
of our knowledge, the results presented in this paper represent the most
accurate long-term three-dimensional evolutions of relativistic stars available
to date.Comment: 19 pages, 17 figure
Neutrino oscillations in curved spacetime: an heuristic treatment
We discuss neutrino oscillations in curved spacetime. Our heuristic approach
can accomodate matter effects and gravitational contributions to neutrino spin
precession in the presence of a magnetic field. By way of illustration, we
perform explicit calculations in the Schwarzschild geometry. In this case,
gravitational effects on neutrino oscillations are intimately related to the
redshift. We discuss how spacetime curvature could affect the resonance
position and adiabaticity of matter-enhanced neutrino flavor conversion.Comment: 7 pages, REVTeX and 1 included style file. Submitted to Phys. Rev.
Instabilities in the Ionization Zones Around the First Stars
We consider the evolution of the ionization zone around Population III stars
with in protogalaxies with at
redshifts , assuming that the dark matter profile is a modified
isothermal sphere. We study the conditions for the growth of instabilities in
the ionization zones. The Rayleigh-Taylor and thermal instabilities develop
efficiently in the ionization zones around 25-40 stars, while this
efficiency is lower for stars with . For more massive stars
(), the flux of ionizing photons is strong enough to
considerably reduce the gas density in the ionization zone, and the typical
lifetimes of stars ( Myr) are insufficient for the growth of
instabilities. The gas in a protogalaxy with with a 200
central star is completely ionized by the end of the star's lifetime;
in the case of a 120 central star, only one-third of the total mass
of gas is ionized. Thus, ionizing photons from stars with M_*\simlt 120
M_\odot cannot leave protogalaxies with M\simgt 10^7 M_\odot. If the masses
of the central stars are 25 and 40 , the gas in protogalaxies of this
mass remains essentially neutral. We discuss the consequences of the evolution
of the ionization zones for the propagation of the envelope after the supernova
explosions of the stars and the efficiency of enrichment of the intergalactic
medium in heavy elements.Comment: 11 pages, 4 figure
The neutron and its role in cosmology and particle physics
Experiments with cold and ultracold neutrons have reached a level of
precision such that problems far beyond the scale of the present Standard Model
of particle physics become accessible to experimental investigation. Due to the
close links between particle physics and cosmology, these studies also permit a
deep look into the very first instances of our universe. First addressed in
this article, both in theory and experiment, is the problem of baryogenesis ...
The question how baryogenesis could have happened is open to experimental
tests, and it turns out that this problem can be curbed by the very stringent
limits on an electric dipole moment of the neutron, a quantity that also has
deep implications for particle physics. Then we discuss the recent spectacular
observation of neutron quantization in the earth's gravitational field and of
resonance transitions between such gravitational energy states. These
measurements, together with new evaluations of neutron scattering data, set new
constraints on deviations from Newton's gravitational law at the picometer
scale. Such deviations are predicted in modern theories with extra-dimensions
that propose unification of the Planck scale with the scale of the Standard
Model ... Another main topic is the weak-interaction parameters in various
fields of physics and astrophysics that must all be derived from measured
neutron decay data. Up to now, about 10 different neutron decay observables
have been measured, much more than needed in the electroweak Standard Model.
This allows various precise tests for new physics beyond the Standard Model,
competing with or surpassing similar tests at high-energy. The review ends with
a discussion of neutron and nuclear data required in the synthesis of the
elements during the "first three minutes" and later on in stellar
nucleosynthesis.Comment: 91 pages, 30 figures, accepted by Reviews of Modern Physic