Showing papers on "Shock wave published in 2008"

The MAPPINGS III Library of Fast Radiative Shock Models

Abstract: We present a new library of fully-radiative shock models calculated with the MAPPINGS iii shock and photoionization code. The library consists of grids of models with shock velocities in the range vs=100-1000 km s −1 and magnetic parameters B/ p n of 10 −4 -10 µG cm 3/2 for five different atomic abundance sets, and for a pre-shock density of 1.0 cm −3 . Additionally, Solar abundance model grids have been calculated for densities of 0.01, 0.1, 10, 100, and 1000 cm −3 with the same range in vs and B/ p n. Each model includes components of both the radiative shock and its photoionized precursor, ionized by the EUV and soft X-ray radiation generated in the radiative gas. We present the details of the ionization structure, the column densities, and the luminosities of the shock and its precursor. Emission line ratio predictions are separately given for the shock and its precursor as well as for the composite shock+precursor structure to facilitate comparison with observations in cases where the shock and its precursor are not resolved. Emission line ratio grids for shock and shock+precursor are presented on standard line ratio diagnostic diagrams, and we compare these grids to observations of radio galaxies and a sample of AGN and star forming galaxies from the Sloan Digital Sky Survey. This library is available online, along with a suite of tools to enable the analysis of the shocks and the easy creation of emission line ratio diagnostic diagrams. These models represent a significant increase in parameter space coverage over previously available models, and therefore provide a unique tool in the diagnosis of emission by shocks. Subject headings: hydrodynamics - shock waves - ISM: abundances,- Galaxies: Nuclei, Galaxies: Seyfert - infrared: ISM, Ultraviolet: ISM, X-rays: ISM

Particle acceleration in relativistic collisionless shocks: Fermi process at last?

Abstract: We present evidence that relativistic shocks propagating in unmagnetized plasmas can self-consistently accelerate particles. We use long-term two-dimensional particle-in-cell simulations to study the well-developed shock structure in unmagnetized pair plasma. The particle spectrum downstream of such a shock consists of two components: a relativistic Maxwellian, with a characteristic temperature set by the upstream kinetic energy of the flow, and a high-energy tail, extending to energies >100 times that of the thermal peak. This high-energy tail is best fitted as a power law in energy with index –2.4 ± 0.1, modified by an exponential cutoff. The cutoff moves to higher energies with time of the simulation, leaving a larger power-law range. The number of particles in the tail is ~1% of the downstream population, and they carry ~10% of the kinetic energy in the downstream region. Investigating the trajectories of particles in the tail, we find that the energy gains occur as particles bounce between the upstream and downstream regions in the magnetic fields generated by the Weibel instability. We compare this mechanism to the first-order Fermi acceleration and set a lower limit on the efficiency of the shock acceleration process.

On the Structure of Relativistic Collisionless Shocks in Electron-Ion Plasmas

Abstract: Relativistic collisionless shocks in electron-ion plasmas are thought to occur in the afterglow phase of gamma-ray bursts (GRBs) and in other environments where relativistic flows interact with the interstellar medium. A particular regime of shocks in an unmagnetized plasma has generated much interest for GRB applications. In this Letter, we present ab initio particle-in-cell simulations of unmagnetized relativistic electron-ion shocks. Using long-term 2.5-dimensional simulations with ion-electron mass ratios from 16 to 1000, we resolve the shock formation and reach a steady state shock structure beyond the initial transient. We find that even at high ion-electron mass ratios initially unmagnetized shocks can be effectively mediated by the ion Weibel instability with a typical shock thickness of ~20 ion skin depths. Upstream of the shock, the interaction with merging ion current filaments heats the electron component, so that the postshock flow achieves near-equipartition between the ions and electrons, with the electron temperature reaching 50% of the ion temperature. This energy exchange helps to explain the large electron energy fraction inferred from GRB afterglow observations.

S/WAVES: The radio and plasma wave investigation on the STEREO Mission

Abstract: This paper introduces and describes the radio and plasma wave investigation on the STEREO Mission: STEREO/WAVES or S/WAVES. The S/WAVES instrument includes a suite of state-of-the-art experiments that provide comprehensive measurements of the three components of the fluctuating electric field from a fraction of a hertz up to 16 MHz, plus a single frequency channel near 30 MHz. The instrument has a direction finding or goniopolarimetry capability to perform 3D localization and tracking of radio emissions associated with streams of energetic electrons and shock waves associated with Coronal Mass Ejections (CMEs). The scientific objectives include: (i) remote observation and measurement of radio waves excited by energetic particles throughout the 3D heliosphere that are associated with the CMEs and with solar flare phenomena, and (ii) in-situ measurement of the properties of CMEs and interplanetary shocks, such as their electron density and temperature and the associated plasma waves near 1 Astronomical Unit (AU). Two companion papers provide details on specific aspects of the S/WAVES instrument, namely the electric antenna system (Bale et al., Space Sci. Rev., 2007) and the direction finding technique (Cecconi et al., Space Sci. Rev., 2007).

Analysis of shock motion in shockwave and turbulent boundary layer interaction using direct numerical simulation data

Abstract: Direct numerical simulation data of a Mach 2.9, 24○ compression ramp configuration are used to analyse the shock motion. The motion can be observed from the animated DNS data available with the online version of the paper and from wall-pressure and mass-flux signals measured in the free stream. The characteristic low frequency is in the range of (0.007–0.013) U∞/δ, as found previously. The shock motion also exhibits high-frequency, of O(U∞/δ), small-amplitude spanwise wrinkling, which is mainly caused by the spanwise non-uniformity of turbulent structures in the incoming boundary layer. In studying the low-frequency streamwise oscillation, conditional statistics show that there is no significant difference in the properties of the incoming boundary layer when the shock location is upstream or downstream. The spanwise-mean separation point also undergoes a low-frequency motion and is found to be highly correlated with the shock motion. A small correlation is found between the low-momentum structures in the incoming boundary layer and the separation point. Correlations among the spanwise-mean separation point, reattachment point and the shock location indicate that the low-frequency shock unsteadiness is influenced by the downstream flow. Movies are available with the online version of the paper.

Origin of Coronal Shock Waves

Abstract: The basic idea of the paper is to present transparently and confront two different views on the origin of large-scale coronal shock waves, one favoring coronal mass ejections (CMEs), and the other one preferring flares. For this purpose, we first review the empirical aspects of the relationship between CMEs, flares, and shocks (as manifested by radio type II bursts and Moreton waves). Then, various physical mechanisms capable of launching MHD shocks are presented. In particular, we describe the shock wave formation caused by a three-dimensional piston, driven either by the CME expansion or by a flare-associated pressure pulse. Bearing in mind this theoretical framework, the observational characteristics of CMEs and flares are revisited to specify advantages and drawbacks of the two shock formation scenarios. Finally, we emphasize the need to document clear examples of flare-ignited large-scale waves to give insight on the relative importance of flare and CME generation mechanisms for type II bursts/Moreton waves.

Entropy production in collisions of gravitational shock waves and of heavy ions

Abstract: We calculate the area of a marginally trapped surface formed by a head-on collision of gravitational shock waves in AdS{sub D}. We use this to obtain a lower bound on the entropy produced after the collision. A comparison to entropy production in heavy-ion collisions is included. We also discuss an O(D-2) remnant of conformal symmetry, which is present in a class of gravitational shockwave collisions in AdS{sub D} and which might be approximately realized (with D=5) in central heavy-ion collisions.

Long-Term Evolution of Magnetic Turbulence in Relativistic Collisionless Shocks: Electron-Positron Plasmas

Abstract: We study the long term evolution of magnetic fields generated by an initially unmagnetized collisionless relativistic e+e− shock. Our two-dimensional particle-in-cell numerical simulations show that downstream of such a Weibel-mediated shock, particle distributions are approximately isotropic, relativistic Maxwellians, and the magnetic turbulence is highly intermittent spatially. The nonpropagating magnetic fields decay in amplitude and do not merge. The fields start with magnetic energy density ~ 0.1-0.2 of equipartition, but rapid downstream decay drives the fields to much smaller values, below ~10−3 of equipartition after ~103 skin depths. To construct a theory to follow field decay to these smaller values, we hypothesize that the observed damping is a variant of Landau damping. The model is based on the small value of the downstream magnetic energy density, which only weakly perturbs particle orbits, for homogeneous turbulence. Using linear kinetic theory, we find a simple analytic form for the damping rates for small-amplitude, subluminous electromagnetic fields. Our theory predicts that overall magnetic energy decays as (ωpt)−q with q ~ 1, which compares with simulations. However, our theory predicts overly rapid damping of short-wavelength modes. Magnetic trapping of particles within the highly spatially intermittent downstream magnetic structures may be the origin of this discrepancy and may allow for some of this initial magnetic energy to persist. Absent additional physical processes that create longer wavelength, more persistent fields, we conclude that initially unmagnetized relativistic shocks in electron-positron plasmas are unable to form persistent downstream magnetic fields. These results put interesting constraints on synchrotron models for the prompt and afterglow emission from GRBs. We also comment on the relevance of these results for relativistic electron-ion shocks.

Multidimensional supernova simulations with approximative neutrino transport II. Convection and the advective-acoustic cycle in the supernova core

Abstract: Performing two-dimensional hydrodynamic simulations including a detailed treatment of the equation of state of the stellar plasma and for the neutrino transport and interactions, we investigate here the interplay between different kinds of non-radial hydrodynamic instabilities that can play a role during the postbounce accretion phase of collapsing stellar cores. The convective mode of instability, which is driven by the negative entropy gradients caused by neutrino heating or by variations in the shock strength in transient phases of shock expansion and contraction, can be identified clearly by the development of typical Rayleigh-Taylor mushrooms. However, in those cases where the gas in the postshock region is rapidly advected towards the gain radius, the growth of such a buoyancy instability can be suppressed. In this situation the shock and postshock flow can nevertheless develop non-radial asymmetry with an oscillatory growth in the amplitude. This phenomenon has been termed “standing (or spherical) accretion shock instability” (SASI). It is shown here that the SASI oscillations can trigger convective instability, and like the latter, they lead to an increase in the average shock radius and in the mass of the gain layer. Both hydrodynamic instabilities in combination stretch the advection time of matter accreted through the neutrino-heating layer and thus enhance the neutrino energy deposition in support of the neutrino-driven explosion mechanism. A rapidly contracting and more compact nascent neutron star turns out to be favorable for explosions, because the accretion luminosity and neutrino heating are greater and the growth rate of the SASI is higher. Moreover, we show that the oscillation period of the SASI observed in our simulations agrees with the one estimated for the advective-acoustic cycle (AAC), in which perturbations are carried by the accretion flow from the shock to the neutron star and pressure waves close an amplifying global feedback loop. A variety of other features in our models, as well as differences in their behavior, can also be understood on the basis of the AAC hypothesis. The interpretation of the SASI in our simulations as a purely acoustic phenomenon, however, appears difficult.

SiO line emission from C-type shock waves : interstellar jets and outflows

Abstract: We study the production of SiO in the gas phase of molecular outflows, through the sputtering of Si-bearing material in refractory grain cores, which are taken to be olivine. We calculate also the rotational line spectrum of the SiO. The sputtering is driven by neutral particle impact on charged grains, in steady-state C-type shock waves, at the speed of ambipolar diffusion. The emission of the SiO molecule is calculated by means of an LVG code. A grid of models, with shock speeds in the range 20 s and preshock gas densities 104 H cm-3 , has been generated. We compare our results with those of an earlier study (Schilke et al. 1997). Improvements in the treatment of the coupling between the charged grains and the neutral fluid lead to narrower shock waves and lower fractions of Si (10%) being released into the gas phase. Erosion of grain cores is significant (1%) only for C-type shock speeds v s > 25 km s-1 , given the adopted properties of olivine. More realistic assumptions concerning the initial fractional abundance of O2 lead to SiO formation being delayed, so that it occurs in the cool, dense postshock flow. Good agreement is obtained with recent observations of SiO line intensities in the L1157 and L1448 molecular outflows. The inferred temperature, opacity, and SiO column density in the emission region differ significantly from those estimated by means of LVG “slab” models. The fractional abundance of SiO is deduced and found to be in the range 4 10-8 3 10-7 . Observed line profiles are wider than predicted and imply multiple, unresolved shock regions within the beam.

Survey of Aerodynamic Drag Reduction at High Speed by Energy Deposition

Abstract: A selected survey of aerodynamic drag reduction at high speed is presented. The dimensionless governing parameters are described for energy deposition in an ideal gas. The types of energy deposition are divided into two categories. First, energy deposition in a uniform supersonic flow is discussed. Second, energy deposition upstream of a simple aerodynamic body is examined. Both steady and unsteady (pulsed) energy deposition are examined for both categories, as well as the conditions for the formation of shock waves and recirculation regions. The capability of energy deposition to reduce drag is demonstrated experimentally. Areas for future research are briefly discussed.

Shock-Wave Exploration of the High-Pressure Phases of Carbon

Abstract: The high–energy density behavior of carbon, particularly in the vicinity of the melt boundary, is of broad scientific interest and of particular interest to those studying planetary astrophysics and inertial confinement fusion. Previous experimental data in the several hundred gigapascal pressure range, particularly near the melt boundary, have only been able to provide data with accuracy capable of qualitative comparison with theory. Here we present shock-wave experiments on carbon (using a magnetically driven flyer-plate technique with an order of magnitude improvement in accuracy) that enable quantitative comparison with theory. This work provides evidence for the existence of a diamond-bc8-liquid triple point on the melt boundary.

Three-dimensional simulations of standing accretion shock instability in core-collapse supernovae

Abstract: We have studied nonaxisymmetric standing accretion shock instabilities, or SASI, using three-dimensional (3D) hydrodynamical simulations. This is an extension of our previous study of axisymmetric SASI. We have prepared a spherically symmetric and steady accretion flow through a standing shock wave onto a proto-neutron star, taking into account a realistic equation of state and neutrino heating and cooling. This unperturbed model is meant to represent approximately the typical postbounce phase of core-collapse supernovae. We then added a small perturbation (~1%) to the radial velocity and computed the ensuing evolutions. Both axisymmetric and nonaxisymmetric perturbations have been imposed. We have applied mode analysis to the nonspherical deformation of the shock surface, using spherical harmonics. We have found that (1) the growth rates of SASI are degenerate with respect to the azimuthal index m of the spherical harmonics Ylm, just as expected for a spherically symmetric background; (2) nonlinear mode couplings produce only m = 0 modes for axisymmetric perturbations, whereas m≠ 0 modes are also generated in the nonaxisymmetric cases, according to the selection rule for quadratic couplings; (3) the nonlinear saturation level of each mode is lower in general for 3D than for 2D, because a larger number of modes contribute to turbulence in 3D; (4) low-l modes are dominant in the nonlinear phase; (5) equipartition is nearly established among different m modes in the nonlinear phase; (6) spectra with respect to l obey power laws with a slope slightly steeper for 3D; and (7) although these features are common to the models with and without a shock revival at the end of the simulation, the dominance of low- l modes is more remarkable in the models with a shock revival.

SPECIALIST: A dynamic speed limit control algorithm based on shock wave theory

Abstract: In literature there are several approaches to eliminate shock waves on freeways by means of dynamic speed limits. Most of them incorporate control systems that have a high computational complexity or that contain parameters without direct physical interpretation, which may make the application in real life difficult. Here we present an approach called SPECIALIST that is based on shock wave theory, and that has parameters with clear physical meaning. The clear interpretation of the parameters leads to an intuitive and insightful formulation of the tuning guidelines. One of the most important features related to the parameter tuning is that the stability of the traffic flow can be ensured by selecting a proper maximum density that is allowed to occur in the speed-controlled area. In addition, other parameters can be tuned for more robust behavior of the algorithm. We first present the theory of shock wave resolution, and next we develop a practical control algorithm based on this theory. A unique feature of the algorithm is that it first judges the solvability of a shock wave and only starts controlling the speed limits if the shock wave is classified as solvable. The algorithm is demonstrated with a simulation example, and it is shown that its performance is similar to existing approaches.

SiO line emission from interstellar jets and outflows: silicon-containing mantles and non-stationary shock waves

Abstract: Context. We study the production and emission of SiO and H 2 in the gas phase of molecular outflows, extending previous work in which we considered steady-state C-type shock waves and assumed the silicon to be present only in the cores of silicate grains. Aims. We place constraints on the physical parameters of the pre-shock region, using recent observations of SiO and observations of molecular hydrogen. We show the effects of introducing SiO-containing mantles and of varying the age of the shock wave. We consider simultaneously the emission of SiO and H 2 from the young L1157 outflow. Methods. The molecular outflows are studied by means of a code that can generate stationary C- and J-type shock models and approximate non-stationary solutions, which combine these two types of shock wave. The emission of molecular hydrogen is computed by this code, whereas the SiO emission is computed by means of a separate LVG model, which uses the calculated physical and chemical profiles. A grid of models has been computed, with shock speeds in the range 10 ≤ u s < 35 km s -1 and pre-shock gas densities 10 4 ≤ n H ≤ 10 6 cm -3 . A wide range of magnetic field strengths has been investigated, from 45 pG to about 600 pG. Results. We illustrate our results by means of observational data obtained on the blue lobe of the L1157 outflow. Given the combinations of pre-shock densities and shock velocities necessary to fit the H 2 observations, we find that the erosion only of the silicate material in the grains cores cannot account for the observed SiO line intensities. We investigate the possiblity that a fraction of the SiO is present initially in the grain mantles, and we succeed in constraining this fraction. Introducing even a few percent of the silicon (as SiO) into the mantles is sufficient to increase the SiO line widths and fluxes by an order of magnitude. With this assumption, it is possible to find a non-stationary shock model that provides a reasonable fit of the observations of both H 2 and SiO. Conclusions. With a few percent of the silicon present initially in the grain mantles, good agreement is obtained with recent observations of SiO line integrated line intensities for a pre-shock density n H = 10 4 cm - and a shock speed v s = 20 km s -1 . The magnetic field strength and the shock age are not well constrained by the observations of either H 2 or SiO. We show that CO observations (in particular, with the Herschel satellite) could provide further discrimination between the models.

Nonrelativistic collisionless shocks in unmagnetized electron-ion plasmas

Abstract: We show that the Weibel-mediated collisionless shocks are driven at nonrelativistic propagation speed (0.1c < V < 0.45c) in unmagnetized electron-ion plasmas by performing two-dimensional particle-in-cell simulations. It is shown that the profiles of the number density and the mean velocity in the vicinity of the shock transition region, which are normalized by the respective upstream values, are almost independent of the upstream bulk velocity, i.e., the shock velocity. In particular, the width of the shock transition region is ~100 ion inertial lengths, independent of the shock velocity. For these shocks the energy density of the magnetic field generated by the Weibel-type instability within the shock transition region reaches typically 1%-2% of the upstream bulk kinetic energy density. This mechanism probably explains the robust formation of collisionless shocks, for example, driven by young supernova remnants, with no assumption of an external magnetic field in the universe.

Diffusive Shock Acceleration with Magnetic Amplification by Nonresonant Streaming Instability in Supernova Remnants

Abstract: We investigate the diffusive shock acceleration in the presence of the nonresonant streaming instability introduced by Bell. The numerical MHD simulations of the magnetic field amplification combined with the analytical treatment of cosmic-ray acceleration permit us to calculate the maximum energy of particles accelerated by high-velocity supernova shocks. The estimates for the Cas A, Kepler, SN 1006, and Tycho historical supernova remnants are given. We also found that the amplified magnetic field is preferentially oriented perpendicular to the shock front downstream of the fast shock. This explains the origin of the radial magnetic fields observed in young supernova remnants.

Three-dimensional simulations of the atmosphere of an AGB star

Abstract: Context. Winds of asymptotic giant branch stars are assumed to be driven by radiation pressure on dust. Previously, this process has been modeled with detailed time-dependent simulations of atmospheres and winds assuming spherically symmetric flows. In such models kinetic energy is injected by a variable inner boundary (“piston”) simulating the effects of stellar pulsation. However, the dynamical processes in these atmospheres – convection and pulsations – are actually three-dimensional. Aims. We present and analyze first 3D radiation hydrodynamics simulations of the convective interior and the atmosphere of a typical AGB star. In particular, we check whether the piston description in the 1D wind models is compatible with the 3D results. Methods. We used two different RHD codes, one (CO5BOLD) to produce 3D models of the outer convective envelope and the inner atmosphere of an AGB star, the other to describe the atmosphere and the wind acceleration region, including dust formation and non-grey radiative transfer, but assuming spherically symmetric flows. From the movements of stellar surface layers in the 3D models, we derived a description for the variable inner boundary in the 1D models. Results. The 3D models show large convection cells and pulsations that give rise to roughly spherically expanding shock waves in the atmosphere, levitating material into regions which are cool enough to allow for dust formation. The atmospheric velocity fields have amplitudes and time scales close to the values that are necessary to start dust formation in the 1D wind models. Conclusions. The convection cells in the 3D simulations are so large that the associated shock fronts appear almost spherical, justifying the assumption of spherical symmetry and the use of a piston boundary condition in the context of wind models. Nevertheless, certain non-radial structures exist in the dust shell developing in the 3D simulations which should be detectable with current interferometric techniques.

Fundamentals of Shock Wave Propagation in Solids

Abstract: Mechanical Principles.- Plane Longitudinal Shocks.- Material Response I: Principles.- Material Response II: Inviscid Compressible Fluids.- Material Response III: Elastic Solids.- Material Response IV: Elastic-Plastic and Elastic-Viscoplastic Solids.- Weak Elastic Waves.- Finite-amplitude Elastic Waves.- Elastic-Plastic and Elastic-Viscoplastic Waves.- Porous Solids.- Spall Fracture.- Steady Detonation Waves.

Ultrafast X-ray Thomson Scattering of Shock-Compressed Matter

Abstract: Spectrally resolved scattering of ultrafast K-α x-rays has provided experimental validation of the modeling of the compression and heating of shocked matter. The elastic scattering component has characterized the evolution and coalescence of two shocks launched by a nanosecond laser pulse into lithium hydride with an unprecedented temporal resolution of 10 picoseconds. At shock coalescence, we observed rapid heating to temperatures of 25,000 kelvin when the scattering spectra show the collective plasmon oscillations that indicate the transition to the dense metallic plasma state. The plasmon frequency determines the material compression, which is found to be a factor of 3, thereby reaching conditions in the laboratory relevant for studying the physics of planetary formation.

Two-Fluid Magnetohydrodynamic Simulations of Converging H I Flows in the Interstellar Medium. I. Methodology and Basic Results

Abstract: We develop an unconditionally stable numerical method for solving the coupling between two fluids (frictional forces/heatings, ionization, and recombination), which can adequately solve the evolution of a partially ionized medium from weak-coupling to strong-coupling regimes. By using two-dimensional two-fluid magnetohydrodynamical simulations based on this method, we investigate the dynamical condensation process of thermally unstable gas that is provided by the shock waves in a weakly ionized and magnetized interstellar medium. If we neglect the effect of magnetic field, it is known that condensation driven by thermal instability can generate high-density clouds whose physical condition corresponds to molecular clouds (precursor of molecular clouds). In this paper we study the effect of magnetic field on the evolution of supersonic converging H I flows and focus on the case in which the orientation of magnetic field to converging flows is orthogonal. We show that the magnetic pressure gradient parallel to the flows prevents the formation of high-density and high-column density clouds, but instead generates fragmented, filamentary H I clouds. With this restricted geometry, magnetic field drastically diminishes the opportunity of fast molecular cloud formation directly from the warm neutral medium, in contrast to the case without magnetic field.

Numerical investigation of three-dimensional cloud cavitation with special emphasis on collapse induced shock dynamics

Abstract: The aim of the present investigation is to model and analyze compressible three-dimensional (3D) cavitating liquid flows with special emphasis on the detection of shock formation and propagation. We recently developed the conservative finite volume method CATUM (Cavitation Technische Universitat Munchen), which enables us to simulate unsteady 3D liquid flows with phase transition at all Mach numbers. The compressible formulation of the governing equations together with the thermodynamic closure relations are solved by a modified Riemann approach by using time steps down to nanoseconds. This high temporal resolution is necessary to resolve the wave dynamics that leads to acoustic cavitation as well as to detect regions of instantaneous high pressure loads. The proposed two-phase model based on the integral average properties of thermodynamic quantities is first validated against the solution of the Rayleigh–Plesset equation for the collapse of a single bubble. The computational fluid dynamics tool CATUM is then applied to the numerical simulation of the highly unsteady two-phase flow around a 3D twisted hydrofoil. This specific hydrofoil allows a detailed study of sheet and cloud cavitation structures related to 3D shock dynamics emerging from collapsing vapor regions. The time dependent development of vapor clouds, their shedding mechanism, and the resulting unsteady variation of lift and drag are discussed in detail. We identify instantaneous local pressure peaks of the order of 100 bar, which are thought to be responsible for the erosive damage of the surface of the hydrofoil.

Production of Magnetic Turbulence by Cosmic Rays Drifting Upstream of Supernova Remnant Shocks

Abstract: We present results of two- and three-dimensional particle-in-cell simulations of magnetic turbulence production by isotropic cosmic-ray ions drifting upstream of supernova remnant shocks. The studies aim at testing recent predictions of a strong amplification of short-wavelength magnetic field and at studying the subsequent evolution of the magnetic turbulence and its back-reaction on cosmic-ray trajectories. For our parameters an oblique filamentary mode grows more rapidly than nonresonant parallel modes analytically found in the limit -->? ?i, and the growth rate is slower than is estimated for the parallel plane wave mode. The evolved oblique filamentary mode was also observed in MHD simulations to dominate in the nonlinear phase, when the structures are already isotropic. We thus confirm the generation of the turbulent magnetic field due to the drift of cosmic-ray ions in the upstream plasma, but as our main result find that the amplitude of the turbulence saturates at about -->?B/B ~ 1. The back-reaction of the magnetic turbulence on the particles leads to an alignment of the bulk flow velocities of the cosmic rays and the background medium, which accounts for the saturation of the instability at moderate amplitudes of the magnetic field. Previously published MHD simulations have assumed a constant cosmic-ray current and no energy or momentum flux in the cosmic rays, which excludes a back-reaction of the generated magnetic field on cosmic rays, and thus the saturation of the field amplitude is artificially suppressed. This may explain the continued growth of the magnetic field in the MHD simulations. A strong magnetic field amplification to amplitudes -->?B B0 has not been demonstrated yet.

Shock Breakout Emission from a Type Ib/c Supernova: XRT 080109/SN 2008D

Abstract: The X-ray transient 080109, associated with SN 2008D, can be attributed to the shock breakout emission from a normal Type Ib/c supernova. If the observed emission is interpreted as thermal emission, the temperature and radiated energy are close to expectations, considering that scattering dominates absorption processes so that spectrum formation occurs deep within the photosphere. The X-ray emission observed at ~10 days is attributed to inverse Compton scattering of photospheric photons with relativistic electrons produced in the interaction of the supernova with the progenitor wind. A simple model for the optical/ultraviolet emission from shock breakout is developed and applied to SN 1987A, SN 1999ex, SN 2008D, and SN 2006aj, all of which have optical emission observed at -->t ~ 1 day. The emission from the first three can plausibly be attributed to shock breakout emission. The photospheric temperature is most sensitive to the radius of the progenitor star core and the radii in these cases are in line with expectations from stellar evolution. The early optical/ultraviolet observations of SN 2006aj cannot be accommodated by a nonrelativistic shock breakout model in a straightforward way.

Shock-induced collapse of a gas bubble in shockwave lithotripsy.

Abstract: The shock-induced collapse of a pre-existing nucleus near a solid surface in the focal region of a lithotripter is investigated. The entire flow field of the collapse of a single gas bubble subjected to a lithotripter pulse is simulated using a high-order accurate shock- and interface-capturing scheme, and the wall pressure is considered as an indication of potential damage. Results from the computations show the same qualitative behavior as that observed in experiments: a re-entrant jet forms in the direction of propagation of the pulse and penetrates the bubble during collapse, ultimately hitting the distal side and generating a water-hammer shock. As a result of the propagation of this wave, wall pressures on the order of 1 GPa may be achieved for bubbles collapsing close to the wall. The wall pressure decreases with initial stand-off distance and pulse width and increases with pulse amplitude. For the stand-off distances considered in the present work, the wall pressure due to bubble collapse is larger than that due to the incoming shockwave; the region over which this holds may extend to ten initial radii. The present results indicate that shock-induced collapse is a mechanism with high potential for damage in shockwave lithotripsy.

Two-fluid mhd simulations of converging hi flows in the interstellar medium. i: methodology and basic results

Abstract: We develop an unconditionally stable numerical method for solving the coupling between two fluids (frictional forces/heatings, ionizatio n, and recombination), and investigate the dynamical condensation process of thermally unstable gas that is provided by the shock waves in a weakly ionized and magnetized interstellar medium by using two-dimensional two-fluid magnetohydrodynamical si mulations. If we neglect the effect of magnetic field, it is known that condensation dr iven by thermal instability can generate high density clouds whose physical condition corresponds to molecular clouds (precursor of molecular clouds). In this paper, we study the effect of magnetic field on the evolution of supersonic converging HI flows and focus on the case in which the orientation of magnetic field to converging flows is orthogonal. We show that the magnetic pressure gradient parallel to the flows pre vents the formation of high density and high column density clouds, but instead generates fragmented, filamentary HI clouds. With this restricted geometry, magnetic field dra stically diminishes the opportunity of fast molecular cloud formation directly from the warm neutral medium, in contrast to the case without magnetic field.

Spatial Structure and Collisionless Electron Heating in Balmer-dominated Shocks

Abstract: Balmer-dominated shocks in supernova remnants (SNRs) produce strong hydrogen lines with a two-component profile composed of a narrow contribution from cold upstream hydrogen atoms and abroad contribution from hydrogen atoms that have undergone charge transfer reactions with hot protons. Observations of emission lines from edgewise shocks in SNRs can constrain the gas velocity and collisionless electron heating at the shock front. Downstream hydrogen atoms engage in charge transfer, excitation, and ionization reactions, defining an interaction region called the shock transition zone. The properties of hot hydrogen atoms produced by charge transfers (called broad neutrals) are critical for accurately calculating the structure and radiation from the shock transition zone. This paper is the third in a series describing the kinetic, fluid, and emission properties of Balmer-dominated shocks, and it is the first to properly treat the effect of broad neutral kinetics on the shock transition zone structure. We use our models to extract shock parameters from observations of Balmer-dominated SNRs. We find that the inferred shock velocities and electrontemperaturesarelower thanthoseof previouscalculations by 1500 km s � 1 . This effect is primarily due to the fact that excitation by proton collisions and charge transfer to excitedlevelsfavorthehigh-speedpartof theneutralhydrogenvelocitydistribution.Ourresultshaveastrongdependence on the ratio of the electron to proton temperatures, � � Te/Tp, which allows us to construct a relation � (vs) between the temperature ratio and the shock velocity.We compare our calculations to previous results byGhavamian and coworkers. Subject headingg shock waves — supernova remnants Online material: color figures