165 research outputs found
Numerical simulations of the magnetorotational instability in protoneutron stars: I. Influence of buoyancy
The magneto-rotational instability (MRI) is considered to be a promising
mechanism to amplify the magnetic field in fast rotating protoneutron stars. In
contrast to accretion disks, radial buoyancy driven by entropy and lepton
fraction gradients is expected to have a dynamical role as important as
rotation and shear. We investigate the poorly known impact of buoyancy on the
non-linear phase of the MRI, by means of three dimensional numerical
simulations of a local model in the equatorial plane of a protoneutron star.
The use of the Boussinesq approximation allows us to utilise a shearing box
model with clean shearing periodic boundary conditions, while taking into
account the buoyancy driven by radial entropy and composition gradients. We
find significantly stronger turbulence and magnetic fields in buoyantly
unstable flows. On the other hand, buoyancy has only a limited impact on the
strength of turbulence and magnetic field amplification for buoyantly stable
flows in the presence of a realistic thermal diffusion. The properties of the
turbulence are, however, significantly affected in the latter case. In
particular, the toroidal components of the magnetic field and of the velocity
become even more dominant with respect to the poloidal ones. Furthermore, we
observed in the regime of stable buoyancy the formation of long lived coherent
structures such as channel flows and zonal flows. Overall, our results support
the ability of the MRI to amplify the magnetic field significantly even in
stably stratified regions of protoneutron stars.Comment: 22 pages, 15 figures, accepted for publication in MNRA
Three-Dimensional Simulations of Core-Collapse Supernovae: From Shock Revival to Shock Breakout
We present 3D simulations of core-collapse supernovae from blast-wave
initiation by the neutrino-driven mechanism to shock breakout from the stellar
surface, considering two 15 Msun red supergiants (RSG) and two blue supergiants
(BSG) of 15 Msun and 20 Msun. We demonstrate that the metal-rich ejecta in
homologous expansion still carry fingerprints of asymmetries at the beginning
of the explosion, but the final metal distribution is massively affected by the
detailed progenitor structure. The most extended and fastest metal fingers and
clumps are correlated with the biggest and fastest-rising plumes of
neutrino-heated matter, because these plumes most effectively seed the growth
of Rayleigh-Taylor (RT) instabilities at the C+O/He and He/H composition-shell
interfaces after the passage of the SN shock. The extent of radial mixing,
global asymmetry of the metal-rich ejecta, RT-induced fragmentation of initial
plumes to smaller-scale fingers, and maximal Ni and minimal H velocities do not
only depend on the initial asphericity and explosion energy (which determine
the shock and initial Ni velocities) but also on the density profiles and
widths of C+O core and He shell and on the density gradient at the He/H
transition, which lead to unsteady shock propagation and the formation of
reverse shocks. Both RSG explosions retain a great global metal asymmetry with
pronounced clumpiness and substructure, deep penetration of Ni fingers into the
H-envelope (with maximum velocities of 4000-5000 km/s for an explosion energy
around 1.5 bethe) and efficient inward H-mixing. While the 15 Msun BSG shares
these properties (maximum Ni speeds up to ~3500 km/s), the 20 Msun BSG develops
a much more roundish geometry without pronounced metal fingers (maximum Ni
velocities only ~2200 km/s) because of reverse-shock deceleration and
insufficient time for strong RT growth and fragmentation at the He/H interface.Comment: 21 pages, 15 figures; revised version with minor changes in Sect.1;
accepted by Astron. Astrophy
The exact solution of the Riemann problem with non-zero tangential velocities in relativistic hydrodynamics
We have generalised the exact solution of the Riemann problem in special
relativistic hydrodynamics for arbitrary tangential flow velocities. The
solution is obtained by solving the jump conditions across shocks plus an
ordinary differential equation arising from the self-similarity condition along
rarefaction waves, in a similar way as in purely normal flow. The dependence of
the solution on the tangential velocities is analysed, and the impact of this
result on the development of multidimensional relativistic hydrodynamic codes
(of Godunov type) is discussed.Comment: 26 pages, 4 figures. Accepted for publication in Journal of Fluid
Mechanic
Exploring the relativistic regime with Newtonian hydrodynamics: II. An effective gravitational potential for rapid rotation
We present the generalization of a recently introduced modified gravitational
potential for self-gravitating fluids. The use of this potential allows for an
accurate approximation of general relativistic effects in an otherwise
Newtonian hydrodynamics code also in cases of rapid rotation. We test this
approach in numerical simulations of astrophysical scenarios related to compact
stars, like supernova core collapse with both a simplified and detailed
microphysical description of matter, and rotating neutron stars in equilibrium.
We assess the quality of the new potential, and demonstrate that it provides a
significant improvement compared to previous formulations for such potentials.
Newtonian simulations of compact objects employing such an effective
relativistic potential predict inaccurate pulsation frequencies despite the
excellent agreement of the collapse dynamics and structure of the compact
objects with general relativistic results. We analyze and discuss the reason
for this behavior.Comment: 15 pages, 12 figures, minor modification
The core helium flash revisited III. From Pop I to Pop III stars
Degenerate ignition of helium in low-mass stars at the end of the red giant
branch phase leads to dynamic convection in their helium cores. One-dimensional
(1D) stellar modeling of this intrinsically multi-dimensional dynamic event is
likely to be inadequate. Previous hydrodynamic simulations imply that the
single convection zone in the helium core of metal-rich Pop I stars grows
during the flash on a dynamic timescale. This may lead to hydrogen injection
into the core, and a double convection zone structure as known from
one-dimensional core helium flash simulations of low-mass Pop III stars. We
perform hydrodynamic simulations of the core helium flash in two and three
dimensions to better constrain the nature of these events. To this end we study
the hydrodynamics of convection within the helium cores of a 1.25 \Msun
metal-rich Pop I star (Z=0.02), and a 0.85 \Msun metal-free Pop III star (Z=0)
near the peak of the flash. These models possess single and double convection
zones, respectively. We use 1D stellar models of the core helium flash computed
with state-of-the-art stellar evolution codes as initial models for our
multidimensional hydrodynamic study, and simulate the evolution of these models
with the Riemann solver based hydrodynamics code Herakles which integrates the
Euler equations coupled with source terms corresponding to gravity and nuclear
burning. The hydrodynamic simulation of the Pop I model involving a single
convection zone covers 27 hours of stellar evolution, while the first
hydrodynamic simulations of a double convection zone, in the Pop III model,
span 1.8 hours of stellar life. We find differences between the predictions of
mixing length theory and our hydrodynamic simulations. The simulation of the
single convection zone in the Pop I model shows a strong growth of the size of
the convection zone due to turbulent entrainment. Hence we predict that for the
Pop I model a hydrogen injection phase (i.e. hydrogen injection into the helium
core) will commence after about 23 days, which should eventually lead to a
double convection zone structure known from 1D stellar modeling of low-mass Pop
III stars. Our two and three-dimensional hydrodynamic simulations of the double
(Pop III) convection zone model show that the velocity field in the convection
zones is different from that predicted by stellar evolutionary calculations.
The simulations suggest that the double convection zone decays quickly, the
flow eventually being dominated by internal gravity waves.Comment: 16 pages, 18 figures, submitted to Aa
Parallelized Solution Method of the Three-dimensional Gravitational Potential on the Yin-Yang Grid
We present a new method for solving the three-dimensional gravitational
potential of a density field on the Yin-Yang grid. Our algorithm is based on a
multipole decomposition and completely symmetric with respect to the two
Yin-Yang grid patches. It is particularly efficient on distributed-memory
machines with a large number of compute tasks, because the amount of data being
explicitly communicated is minimized. All operations are performed on the
original grid without the need for interpolating data onto an auxiliary
spherical mesh.Comment: 8 pages, 4 figures; two minor additions after refereeing; accepted by
Ap
The Formation of Disk Galaxies in a Cosmological Context: Structure and Kinematics
We present results concerning the internal structure and kinematics of disk
galaxies formed in cosmologically motivated simulations. The calculations
include dark matter, gas dynamics, radiative cooling, star formation, supernova
feedback and metal enrichment. The initial model is a rigidly rotating
overdense sphere with a mass of about 8 10^11 Msol which is perturbed by small
scale fluctuations according to a biased CDM power spectrum. Converging, Jeans
unstable and rapidly cooling regions are allowed to form stars. Via supernovae,
metal enriched gas is returned to the interstellar medium. {}From these initial
conditions a galaxy forms which shows the main properties of spiral galaxies: a
rotationally supported exponential disk which consists of young stars with
about solar metallicity, a slowly rotating halo of old metal poor stars, a
bulge of old metal rich stars and a slowly rotating extended halo of dark
matter. Bulge, stellar and dark halo are supported by an anisotropic velocity
dispersion and have a de Vaucouleurs surface density profile. The flattening of
the dark and stellar halo is too large to be explained by rotation only.
Whether the flattening of the bulge is caused by an anisotropic velocity
dispersion or by its rotation cannot be answered, because of the limited
numerical resolution due to gravitational softening. The velocity dispersion
and the thickness of the stellar disk increase with the age of the stars.
Considering only the young stellar component, the disk is cold (sigma=20
km/sec) and thin (z <1 kpc). The dynamical formation process ends after about
4\,Gyr, whenComment: 16 pages, compressed uu-encoded postscript file (185kB) no figures,
complete compressed postscript file via anonymous ftp
deep-thought.MPA-Garching.MPG.DE, pub/Preprints/disk.ps.Z, submitted to
MNRAS, Preprint MPA 81
"Mariage des Maillages": A new numerical approach for 3D relativistic core collapse simulations
We present a new 3D general relativistic hydrodynamics code for simulations
of stellar core collapse to a neutron star, as well as pulsations and
instabilities of rotating relativistic stars. It uses spectral methods for
solving the metric equations, assuming the conformal flatness approximation for
the three-metric. The matter equations are solved by high-resolution
shock-capturing schemes. We demonstrate that the combination of a finite
difference grid and a spectral grid can be successfully accomplished. This
"Mariage des Maillages" (French for grid wedding) approach results in high
accuracy of the metric solver and allows for fully 3D applications using
computationally affordable resources, and ensures long term numerical stability
of the evolution. We compare our new approach to two other, finite difference
based, methods to solve the metric equations. A variety of tests in 2D and 3D
is presented, involving highly perturbed neutron star spacetimes and
(axisymmetric) stellar core collapse, demonstrating the ability to handle
spacetimes with and without symmetries in strong gravity. These tests are also
employed to assess gravitational waveform extraction, which is based on the
quadrupole formula.Comment: 29 pages, 16 figures; added more information about convergence tests
and grid setu
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