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1.
A new hydrodynamics code aimed at astrophysical applications has been developed. The new code and algorithms are presented along with a comprehensive suite of test problems in one, two, and three dimensions. The new code is shown to be robust and accurate, equalling or improving upon a set of comparison codes. Fyris Alpha will be made freely available to the scientific community.  相似文献   

2.
The results of a diffusive radiation transport experiment in a simple geometry are presented. The experiment depends primarily on two variables, the target density and the temperature drive, which are characterized well. The experiment is designed to verify and validate radiation transport in codes. The codes can then be used to model astrophysical systems. The results of the experiments are found to be in good agreement with simulation results.  相似文献   

3.
A series of experiments is underway using the Omega laser to examine radiative shocks of astrophysical relevance. In these experiments, the laser accelerates a thin layer of low-Z material, which drives a strong shock into xenon gas. One-dimensional numerical simulations using the HYADES radiation hydrodynamics code predict that radiation cooling will cause the shocked xenon to collapse spatially, producing a thin layer of high density (i.e., a collapsed shock). Preliminary experimental results show a less opaque layer of shocked xenon than would be expected assuming that all the xenon accumulates in the layer and that the X-ray source is a pure Kα source. However, neither of these assumptions is strictly correct. Here we explore whether radial mass and/or energy transport may be significant to the dynamics of the system. We report the results of two-dimensional numerical simulations using the ZEUS-2D astrophysical fluid dynamics code. Particular attention is given to the simulation method.  相似文献   

4.
We propose a new method to conserve the total energy to round-off error in grid-based codes for hydrodynamic simulations with self-gravity. A formula for the energy flux due to the work done by the self-gravitational force is given, so the change in total energy can be written in conservative form. Numerical experiments with the code Athena show that the total energy is indeed conserved with our new algorithm and the new algorithm is second order accurate. We have performed a set of tests that show the numerical errors in the traditional, non-conservative algorithm can affect the dynamics of the system. The new algorithm only requires one extra solution of the Poisson equation, as compared to the traditional algorithm which includes self-gravity as a source term. If the Poisson solver takes a negligible fraction of the total simulation time, such as when FFTs are used, the new algorithm is almost as efficient as the original method. This new algorithm is useful in Eulerian hydrodynamic simulations with self-gravity, especially when results are sensitive to small energy errors, as for radiation pressure dominated flow.  相似文献   

5.
The simulation or solution of the supernova remnant evolution may be scaled from one interstellar environment to another. We systematically examine this scalability, the use of which is so far still very limited in astrophysical literature. We show how the scalability is affected by various constraints imposed by physical processes and initial conditions, and demonstrate the use of the scaling as a powerful tool to explore the interdependence among relevant parameters, based on a minimum set of simulations. In particular, we devise a scaling scheme that can be used to adaptively generate numerous seed remnants and plant them into 3D hydrodynamic simulations of the supernova-dominated interstellar medium.  相似文献   

6.
The growing field of Laboratory Astrophysics seeks to study the extreme environments found in many astrophysical events in the controlled setting of a laboratory. In addition to the opportunity to perform basic research into the nature and properties of materials in astrophysical environments, laboratory astrophysics experiments serve beautifully for validating calculations performed by simulation codes designed to model astrophysical phenomena. I present results from our ongoing validation effort for FLASH, a parallel adaptive-mesh hydrodynamics code for the compressible, reactive flows of astrophysical environments. The first test case is a laser-driven shock propagating though a multilayer target introducing Rayleigh–Taylor and Richtmyer–Meshkov fluid instabilities at the material interfaces. The second is an accelerating fluid interface that is subject to the Rayleigh–Taylor instability. We found good agreement between simulations and experiment for the multilayer target case, but disagreement between experiment and simulation in the Rayleigh–Taylor case. I discuss our findings and possible reasons for the disagreement.  相似文献   

7.
In smoothed particle hydrodynamics (SPH) codes with a large number of particles, star formation as well as gas and metal restitution from dying stars can be treated statistically. This approach allows one to include detailed chemical evolution and gas re-ejection with minor computational effort. Here we report on a new statistical algorithm for star formation and chemical evolution, especially conceived for SPH simulations with large numbers of particles, and for parallel SPH codes.
For the sake of illustration, we also present two astrophysical simulations obtained with this algorithm, implemented into the Tree-SPH code by Lia & Carraro .
In the first simulation, we follow the formation of an individual disc-like galaxy, predict the final structure and metallicity evolution, and test resolution effects. In the second simulation we simulate the formation and evolution of a cluster of galaxies, to demonstrate the capabilities of the algorithm in investigating the chemo-dynamical evolution of galaxies and of the intergalactic medium in a cosmological context.  相似文献   

8.
The behavior of dispersive Alfvén waves (DAWs), including inertial and kinetic Alfvén waves, in astrophysical plasmas of very low, intermediate, and low pressure is investigated in the hydrodynamic approximation. New full solutions are obtained. Our results are analyzed and compared with those from the kinetic approach. It is shown that one general solution for the DAWs in plasmas of very low, intermediate, and low pressure can be obtained in the framework of the hydrodynamic approach, as opposed to the kinetic one. In the very low damping region, the kinetic and hydrodynamic solutions agree very well; but there are parameter regions where the solutions are essentially different. The influence of the astrophysical medium parameters on the DAW behavior and properties is analyzed. All main wave characteristics—the dispersion, damping, polarization, density perturbations, and charge density perturbations—are obtained, whose the consideration is very important for the observation and detection of these waves, as well as for a more correct understanding of their behavior and role in various astrophysical processes taking place in the cosmic medium.  相似文献   

9.
The behavior of dispersive Alfven waves (DAWs) in astrophysical plasmas of finite and high pressure, which have not been considered thus far, is studied in the hydrodynamic approximation. The results are analyzed and compared with those obtained in the kinetic approach. It is shown that one general solution for DAWs in plasmas of finite and high pressure can be obtained using the hydrodynamic approach in contrast to the kinetic one. Kinetic and hydrodynamic solutions correspond to each other very well in a domain with weakly damped DAWs; however, solutions may differ appreciably in some parameter domains, especially in high-pressure plasma. The effect of parameters of the astrophysical medium on the DAW behavior and properties is analyzed. All the main wave characteristics were determined: dispersion, damping, polarization, density perturbations, and charge density perturbations. Since finite-pressure plasma is one of the most frequently encountered states of astrophysical plasma, it is very important to take into account specific features in behavior of these waves for their detecting and a more correct understanding of their behavior and the role they play in different astrophysical processes that occur in space environment.  相似文献   

10.
The low-redshift evolution of the intergalactic medium is investigated using hydrodynamic cosmological simulations. The assumed cosmological model is a critical density cold dark matter universe. The imposed uniform background of ionizing radiation has the amplitude, shape and redshift evolution as computed from the observed quasar luminosity function by Haardt &38; Madau. We have analysed simulated Lyman-α spectra using Voigt-profile fitting, mimicking the procedure with which quasar spectra are analysed. Our simulations reproduce the observed evolution of the number of Lyman-α absorption lines over the whole observed interval of z  = 0.5 to 4. In particular, our simulations show that the decrease in the rate of evolution of Lyman-α absorption lines at z  ≤ 2, as observed by the Hubble Space Telescope , can be explained by the steep decline in the photoionizing background resulting from the rapid decline in quasar numbers at low redshift.  相似文献   

11.
The interaction of an astrophysical shock with a cloud typically occurs at high Reynolds number, and in such cases will be highly turbulent. However, the formation of fully developed turbulence is usually prevented by the artificial viscosity inherent in hydrodynamical simulations. Upstream structures mean that the flow behind the shock is also likely to be turbulent, as it sweeps over such inhomogeneities. We study the nature of adiabatic shock-cloud interactions using a subgrid compressible k–ε turbulence model.  相似文献   

12.
We give an expression for the radiation produced by a uniformly charged particle when it traverses normally a semi-infinite boundary between two media, both of the same constant; refractive indexn, measured in the appropriate rest frames of the media. The media are taken to slip relative to each other with constant velocity parallel to the boundary.We compute the differential power output and show that (a) the emitted radiation has a flat spectrum up to a frequency such thatn can no longer be considered constant; (b) the angular dependence of the emitted radiation is peaked at an angle to the direction of motion of the particle; (c) there is a back-scattered component to the radiation. in view of the complexity of the analytic formulae for the differential power output we give some numerical examples forn>1 andn<1 to illustrate the different angular dependences of the power output in both cases.Since high energy charged particles are currently thought to be produced in the magnetospheres of pulsars, and since such particles must then escape from the environment of the pulsar by traversing the differentially shearing magnetosphere, it would seem that the simple calculations reported here illustrate a new mechanism for radiation production which is of astrophysical interest.  相似文献   

13.
Photoionised plasmas are common in astrophysical environments and new high resolution spectra from such sources have been recorded in recent years by the Chandra and XMM-Newton satellites. These provide a wealth of spectroscopic information and have motivated recent efforts aimed at obtaining a detailed understanding of the atomic-kinetic and radiative characteristics of photoionised plasmas. The Z-pinch facility at the Sandia National Laboratories is the most powerful terrestrial source of X-rays and provides an opportunity to produce photoionised plasmas in a well characterised radiation environment. We present modelling work and experimental design considerations for a forthcoming experiment at Sandia in which X-rays from a collapsing Z-pinch will be used to photoionise low density neon contained in a gas cell. View factor calculations were used to evaluate the radiation environment at the gas cell; the hydrodynamic characteristics of the gas cell were examined using the Helios-CR code, in particular looking at the heating, temperature and ionisation of the neon and the absorption of radiation. Emission and absorption spectra were also computed, giving estimates of spectra likely to be observed experimentally.  相似文献   

14.
A wide variety of astrophysical objects, ranging from newly formed stars to accreting black holes, produce ionizing radiation. Nearby material will reprocess these photons into other forms of light. Depending on the geometry, we will observe such gas through either an emission or absorption line spectrum. Such clouds are detected across the electromagnetic spectrum and detailed analysis can reveal such vital parameters as the composition of the gas and the form of the ionizing radiation field. The gas is in a non-equilibrium state due to its low density and the presence of a wide range of non-thermal energy sources. The spectrum is typically analyzed with the help of large-scale numerical simulations. This paper describes recent developments of Cloudy, a code that performs such simulations, and outlines applications to star forming regions, active galactic nuclei, and laboratory plasmas.  相似文献   

15.
A rigorous theoretical investigation on the characteristics of dust-ion-acoustic (DIA) shock waves in an unmagnetized multi component electron-positron-ion dusty plasma (consisting of inertial ions, electrons of two distinct temperatures referred to as low and high temperature superthermal electrons where superthermality is introduced via the κ-type of nonthermal distribution, Boltzmann distributed positrons, and negatively charged immobile dust grains) has been made both theoretically and analytically. The hydrodynamic equation for inertial ions has been used to derive the Burgers equation. The influence of superthermal electrons, Maxwellian positrons and ion kinematic viscosity, which are found in this investigation, significantly modify the basic features of DIA shock waves, are briefly discussed. The present investigation can be very effective for studying and understanding the basic characteristics of shock wave propagation through different astrophysical situations where distinct temperature superthermal electrons dominate the wave dynamics.  相似文献   

16.
The interaction of supersonic plasma jets with dense gases and plasmas has been studied experimentally and theoretically. Collimated plasma jets were generated from the laser pulse interaction with solid targets. The jet propagates with the velocity exceeding 400 km/s and transports the energy of a few kJ/cm2. The interaction of such a jet with an Ar and He gases at various pressures has been studied by using optical and X-ray diagnostics. Qualitative estimates and numerical simulations with a radiative hydrodynamic code explain a sequence of physical processes during the interaction. Experimental and numerical results show that, by changing ambient material, the working surface structure changes from an adiabatic outflow to a radiative cooling jet. The applications of this phenomenon to astrophysical conditions and the inertial confinement fusion are discussed.  相似文献   

17.
In this paper we report the results of axisymmetric relativistic magnetohydrodynamic (MHD) simulations for the problem of a Kerr black hole immersed in a rarefied plasma with 'uniform' magnetic field. The long-term solution shows properties that are significantly different from those of the initial transient phase studied recently by Koide. The topology of magnetic field lines within the ergosphere is similar to that of the split-monopole model with a strong current sheet in the equatorial plane. Closer inspection reveals a system of isolated magnetic islands inside the sheet and ongoing magnetic reconnection. No regions of negative hydrodynamic 'energy at infinity' are seen inside the ergosphere and the so-called MHD Penrose process does not operate. However, the rotational energy of the black hole continues to be extracted via the purely electromagnetic Blandford–Znajek mechanism. In spite of this, no strong relativistic outflows from the black hole are seen to be developing. Combined with results of other recent simulations, our results signal a potential problem for the standard MHD model of relativistic astrophysical jets should they be found at distances as small as a few tens of gravitational radii from the central black hole.  相似文献   

18.
It has recently been realized that the Weibel instability plays a major role in the formation and dynamics of astrophysical shocks of gamma-ray bursts and supernovae. Thanks to technological advances in the recent years, experimental studies of the Weibel instability are now possible in laser-plasma interaction devices. We, thus, have a unique opportunity to model and study astrophysical conditions in laboratory experiments – a key goal of the Laboratory Astrophysics program. Here we briefly review the theory of strong non-magnetized collisionless GRB and SN shocks, emphasizing the crucial role of the Weibel instability and discuss the properties of radiation emitted by (isotropic) electrons moving through the Weibel-generated magnetic fields, which is referred to as the jitter radiation. We demonstrate that the jitter radiation field is anisotropic with respect to the direction of the Weibel current filaments and that its spectral and polarization characteristics are determined by microphysical plasma parameters. We stress that the spectral analysis can be used for accurate diagnostics of the plasma conditions in laboratory experiments and in astrophysical GRB and SN shocks.  相似文献   

19.
We compare simulations of the Lyman α forest performed with two different hydrodynamical codes, gadget-2 and enzo . A comparison of the dark matter power spectrum for simulations run with identical initial conditions show differences of 1–3 per cent at the scales relevant for quantitative studies of the Lyman α forest. This allows a meaningful comparison of the effect of the different implementations of the hydrodynamic part of the two codes. Using the same cooling and heating algorithm in both codes, the differences in the temperature and the density probability distribution function are of the order of 10 per cent. The differences are comparable to the effects of box size and resolution on these statistics. When self-converged results for each code are taken into account, the differences in the flux power spectrum – the statistics most widely used for estimating the matter power spectrum and cosmological parameters from Lyman α forest data – are about 5 per cent. This is again comparable to the effects of box size and resolution. Numerical uncertainties due to a particular implementation of solving the hydrodynamic or gravitational equations appear therefore to contribute only moderately to the error budget in estimates of the flux power spectrum from numerical simulations. We further find that the differences in the flux power spectrum for enzo simulations run with and without adaptive mesh refinement are also of the order of 5 per cent or smaller. The latter require 10 times less CPU time making the CPU time requirement similar to that of a version of gadget-2 that is optimized for Lyman α forest simulations.  相似文献   

20.
High‐fidelity spectroscopy presents challenges for both observations and in designing instruments. High‐resolution and high‐accuracy spectra are required for verifying hydrodynamic stellar atmospheres and for resolving intergalactic absorption‐line structures in quasars. Even with great photon fluxes from large telescopes with matching spectrometers, precise measurements of line profiles and wavelength positions encounter various physical, observational, and instrumental limits. The analysis may be limited by astrophysical and telluric blends, lack of suitable lines, imprecise laboratory wavelengths, or instrumental imperfections. To some extent, such limits can be pushed by forming averages over many similar spectral lines, thus averaging away small random blends and wavelength errors. In situations where theoretical predictions of lineshapes and shifts can be accurately made (e.g., hydrodynamic models of solar‐type stars), the consistency between noisy observations and theoretical predictions may be verified; however this is not feasible for, e.g., the complex of intergalactic metal lines in spectra of distant quasars, where the primary data must come from observations. To more fully resolve lineshapes and interpret wavelength shifts in stars and quasars alike, spectral resolutions on order R = 300 000 or more are required; a level that is becoming (but is not yet) available. A grand challenge remains to design efficient spectrometers with resolutions approaching R = 1 000 000 for the forthcoming generation of extremely large telescopes (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

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