Evolution of cold shock-bounded slabs

The stability and evolution of cold, shock-bounded slabs is studied using numerical hydrodynamic simulations. We confirm the analysis of Vishniac (1994) [ApJ, 428, 186], who showed that such slabs are unstable if they are perturbed by a displacement larger than their width. The growth rate of this nonlinear thin shell instability (NTSI) is found to increase with decreasing wavelength, in qualitative agreement with Vishniac's analysis. The NTSI saturates when the bending angle becomes large and the growth in the width of the slab pinches off the perturbation. After saturation, the slab remains greatly extended with an average density much less than the original slab density, supported primarily by supersonic turbulence within the slab. Linear perturbations are also found to be unstable in that they can lead to turbulent flow within the slab, although this response to linear perturbations is distinct from, and much less violent than the NTSI.Richard McCray     

Theoretical and numerical studies of the Vishniac instability in supernova remnants

Vishniac instability has been theoretically studied in supernova remnants where it is supposed to explain the fragmentation of the interstellar medium. However its role is not fully demonstrated in these astrophysical objects. Conditions and assumptions required for the instability growth are explained in detail in the present paper. In addition the HYDRO-MUSCL hydrodynamic code has been used to simulate this instability in order to compare the numerical growth rate with the Vishniac analytical solution.     

Analytical Study of Supernova Remnant Non-Stationary Expansions

We study analytically the Rayleigh–Taylor instability in expanding supernova gas shell. The instability appears at the inner shell surface accelerated by blowing pulsar wind. The most dangerous perturbations correspond to wavelengths comparable to the shell thickness. We analyze the fragility of the supernova remnant shell in function of the initial perturbation amplitude and the shell thickness.     

Stellar clustering as induced by a supernova

When the shock wave from a supernova expands, it sweeps up not only interstellar matter but also magnetic field. The field is greatly amplified by compression and will provide the dominant pressure during the cool radiative phase of an expanding supernova shell. We examine a hydromagnetic instability in this system (a form of the Parker instability) and find that it will concentrate gas at intervals of the order of parsecs. The length and time scales make the instability promising as an explanation of the stellar clustering that is seen in Canis Major R1.     

Acceleration of a Young Supernova Remnant by an Associated Pulsar

We present a numerical model of the action of a pulsar on its associated supernova remnant. The expansion of the blast wave in the progenitor star has first been considered until radiation pressure within the ejected material becomes negligible due to expansion. By assuming that expansion is ballistic and that the ejecta is opaque to the pulsar 's magnetodipole radiation, the model produces a radiation-filled cavity which grows around the pulsar and contributes to power the dynamics of the forming supernova remnant. The interface between the cavity and the ejecta has been modelled as a thin shell which, depending on the initial spin frequency of the pulsar, can sweep through the ejecta and reach the blast wave. Results for the evolution of the shell indicate that it may strike the front most part of the shocked gas some 60 years after the explosion. Such pulsar-supernova remnant interactions are proposed to form the base of a new subclassification of pulsar-filled supernova remnants. This revised version was published online in July 2006 with corrections to the Cover Date.     

Non-Stationary Rayleigh-Taylor Instabilities in Pulsar Wind Interaction with a Supernova Shell

The Rayleigh-Taylor instability (RTI) plays an important role in the dynamics of several astronomical objects, in particular, in the supernovae (SN) evolution. In the present paper we examine the dynamics of a shell (representing a type II SN remnant) blown-up by a wind emitted by a central pulsar. The shell is accelerated by the pulsar wind and its inner surface experiences the RTI. We develop an analytical approach by using a specific transformation into the coordinate frame co-moving with the SN ejecta. We first derive a non-stationary spherically symmetric solution describing an expansion of a gas shell under the pressure of a central source (pulsar). Then, we analyze its 3D stability with respect to a small perturbation on the inner shell surface. The dispersion relation is derived in the co-moving reference frame. The growth rate of the perturbation is found and its temporal evolution is discussed. We compare our result with the previous published studies and apply it to the Crab nebula evolution.     

Building the Fast Galactic Dynamo

In this paper we demonstrate the importance of cosmic rays for the dynamics of the interstellar medium. We present the first 3D-MHD numerical simulations of the Parker instability triggered by cosmic rays accelerated in randomly distributed supernova remnants. We show that in the presence of galactic rotation a net radial magnetic field is produced as a result of the cosmic ray injection and Coriolis force. This process provides a possibility of very efficient magnetic field amplification within the general frame of so called fast galactic dynamo proposed by Parker (1992).     

On the Stability of Colliding Flows: Radiative Shocks, ThinShells, and Supersonic Turbulence

High-resolution numerical simulations reveal the turbulent character of the interaction zone of colliding, radiative, hypersonic flows. As the shocked gas cools radiatively, the cooled matter is squeezed into thin, high density shells. The remaining kinetic energy causes supersonic turbulence within these shells, before it is finally dissipated by internal shocks and vortex cascades. The density is far from homogeneous. High density filaments and large voids coexist. Its mean value is significantly below the stationary value. Similarly, areas with supersonic velocities are found next to subsonic regions. The mean velocity is slightly below or above the sound speed. While quasi uniform flow motions are observed on smaller scales the large scale velocity distribution is isotropic. Part of the turbulent shell is occupied by relatively uniform flow-patches, resembling coherent structures. Astronomical implications of the turbulent interaction zone are multifarious. It probably drives the X-ray variability in colliding wind binaries as well as the surprising dust formation on orbital scales in some WR-binaries. It lets us understand the knotty appearance of wind-driven structures as planetary and WR-ring nebulae, symbiotics, supernova remnants, galactic supperbubbles. Also, WR and other radiatively driven, clumpy winds, advection dominated accretion, cooling flows and molecular cloud dynamics in star-forming regions may carry its stamp This revised version was published online in July 2006 with corrections to the Cover Date.     

Self-consistent stability analysis of spherical shocks

In this paper, we study self-similar solutions, and their linear stability as well, describing the flow within a spherical shell with finite thickness, expanding according to a power law of time, t ^q , where q>0. The shell propagates in a medium with initially uniform density and it is bounded by a strong shock wave at its outer border while the inner face is submitted to a time-dependent uniform pressure. For q=2/5, the well-known Sedov–Taylor solution is recovered. In addition, although both accelerated and decelerated shells can be unstable against dynamic perturbations, they exhibit highly different behaviors. Finally, the dispersion relation derived earlier by Vishniac (Vishniac, E.T. in Astrophys. J. 274:152, 1983) for an infinitely thin shell is obtained in the limit of an isothermal shock wave.     

MHD simulations of rayleigh-taylor instability in young supernova remnants

Radio observations shows that young supernova remnants such as Tycho and Cas A generally exhibit a circular clumpy shell. This shell shows a radial magnetic field whose equipartition strength is 2 to 3 orders of magnitude higher than the interstellar field. A simple compression of the ambient field by the shock can explain neither of these observations. We show that the Rayleigh-Taylor instability which occurs at the ejecta/ISM interface can explain these observations. We have done MHD simulations of the instability in the shell of Type-I supernova remnants for the first time by utilizing moving grid technique. Our simulation shows that Rayleigh-Taylor and Kelvin-Helmholtz instabilities amplify ambient magnetic fields locally and produce the clumpy radio shell. Strong magnetic field lines draped around the Rayleigh-Taylor fingers produce the radial B-vector polarization, whereas thermal bremsstrahlung from the dense fingers themselves produce the clumpy X-ray emission.     

Magnetorotational supernovae. Magnetorotational instability. Jet formation

2D numerical simulations of magnetorotational (MR) supernova mechanism are described. It is shown that magnetic field is amplified due to the differential rotation after core collapse. When magnetic pressure reaches some level, a compression wave starts to move outwards. Moving along steeply decreasing density profile the compression wave transforms quickly into fast MHD shock. The magnetorotational instability (MRI) was found in our simulations. MRI leads to the exponential growth of the components of the magnetic field. The MRI significantly reduces MR supernova explosion time. Configuration of the initial magnetic field qualitatively defines the shape of MR supernova explosion. For the quadrupole-like initial poloidal field the MR supernova explosion develops mainly along equatorial plane, the dipole-like initial field results in MR supernova developing as mildly collimated jet along axis of rotation. The explosion energy of MR supernova found in our simulations is ∼0.5–0.6×10⁵¹ erg.     

Simulations of mixed-morphology supernova remnants with anisotropic thermal conduction

We explore the role of anisotropic thermal conduction on the evolution of supernova remnants (SNRs) through interstellar media with a range of densities via numerical simulations. We find that a remnant expanding in a dense environment can produce centre-bright hard X-ray emission within 20 kyr, and centre-bright soft X-ray emission within 60 kyr of the supernova event. In a more tenuous environment, the appearance of a centre-bright structure in hard X-rays is delayed until about 60 kyr. The soft X-ray emission from such a remnant may not become centre bright during its observable lifetime. This can explain the observations that show that mixed-morphology SNRs preferentially occur close to denser, molecular environments. Remnants expanding into denser environments tend to be smaller, making it easier for thermal conduction to make large changes in the temperatures of their hot gas bubbles. We show that the lower temperatures make it very favourable to use high-stage ions as diagnostics of the hot gas bubbles in SNRs. In particular, the distribution of O  viii transitions from shell bright at early epochs to centre bright at later epochs in the evolution of an SNR expanding in a dense interstellar medium when the physics of thermal conduction is included.     

Interfacial instabilities driven by self-gravity: first numerical results

The evolution of the interstellar medium (ISM) is driven by a variety of phenomena, including turbulence, shearing flows, magnetic fields and the thermal properties of the gas. Among the most important forces at work is self-gravity, which ultimately drives protostellar collapse. As part of an ongoing study of instabilities in the ISM, Hunter, Whitaker & Lovelace have discovered another process driven by self-gravity: the instability of an interface of discontinuous density. Theory predicts that this self-gravity driven interfacial instability persists in the static limit and in the absence of a constant background acceleration. Disturbances to a density interface are found to grow on a time-scale of the order of the free-fall time, even when the perturbation wavelength is much less than the Jeans length. Here we present the first numerical simulations of this instability. The theoretical growth rate is confirmed and the non-linear morphology displayed. The self-gravity interfacial instability is shown to be fundamentally different from the Rayleigh–Taylor instability, although both exhibit similar morphologies under the condition of a high density contrast, such as is commonly found in the ISM. Such instabilities are a possible mechanism by which observed features, such as the pillars of gas seen near the boundaries of interstellar clouds, are formed.     

Charged shell supported by a phantom energy

This paper discusses the evolution of a thin spherically symmetric self gravitating phantom shell around the charged shell. The general equations describing the motion of shell with a general form of equation of state are derived. The different types of space-time R _± and T _± regions and shell motion are classified depending on the parameters of the problem. The mechanical stability analysis of this spherically symmetric thin shell with charge in Reissner-Nordstrom (RN) to linearized spherically symmetric perturbation about static equilibrium solution is carried out.     

The Triggered Star Formation in Rotating Disks

The gravitational instability of expanding shells triggering the formation of clouds and stars is analyzed. Disks with different scale-heights, ambient and shell velocity dispersions, mid-plane densities, rotation rates and shear rates are explored with three dimensional numerical simulations in the thin shell approximation. Three conditions for the shell collapse are specified: the first is that it happens before a significant blow-out, the second requires that the shell collapses before it is distorted by Coriolis forces and shear, and the third requires that the internal pressure in the accumulated gas is small and the fragmentation is achieved within the expansion time. The gas-rich and slowly rotating galaxies are the best sites of the triggered star formation, concluding that its importance has been much larger at the times of galaxy formation compared to the present epoch. This revised version was published online in September 2006 with corrections to the Cover Date.     

The influence of gravitational acceleration on the supernova-driven Parker instability

Within a framework of 2D magnetohydrodynamic (MHD) simulations, we explore the dynamical regimes initiated by a supernova explosion in a magnetized stratified interstellar medium (ISM). We concentrate on the formation of large-scale magnetic structures and outflows connected with the Parker instability. For the sake of simplicity we only show models with a fixed explosion energy corresponding to a single supernova (SN) occurring in host galaxies with different fixed values of the gravitational acceleration g and different ratios of specific heats. We show that in general, depending on these two parameters, three different regimes are possible: a slowly growing Parker instability on time-scales much longer than the galactic rotation period for small g; the Parker instability growing at roughly the rotation period, which for ratios of specific heats larger than one is accompanied by an outflow resulting from the explosion for intermediate g; and a rapidly growing instability and a strong blow-out flow for large g . By means of numerical simulations and analytical estimates we show that the explosion energy and gravitational acceleration which separate the three regimes scale as Eg ²∼constant in the 2D case. We expect that in the 3D case this scaling law is Eg ³∼constant . Our simulations demonstrate furthermore that a single SN explosion can lead to the growth of multiple Parker loops in the disc and large-scale magnetic field loops in the halo, extending over 2–3 kpc horizontally and up to 3 kpc vertically above the mid-plane of the disc.     

The dynamical coupling of cosmic rays and magnetic field in galactic disks

In this paper we demonstrate the importance of cosmic rays for the dynamics of the interstellar medium. We present the first 3D-MHD numerical simulations of the Parker instability triggered by cosmic rays accelerated in supernova remnants. We show that in the presence of galactic rotation a net radial magnetic field is produced as a result of the cosmic ray injection. This process provides a very efficient magnetic field amplification within the general frame of so called fast galactic dynamo proposed by Parker (1992). This revised version was published online in September 2006 with corrections to the Cover Date.     

Thermal instability of coronal neutral sheets and the formation of quiescent prominences

It is argued that the quiscent prominences are a natural consequence of the formation and thermal instability of current sheets in the corona. Thus observation and theory of prominences can give vital information on the presence of currents and the topology of magnetic fields in the corona. Conversely by developing the theory of the structure and evolution of current sheets under coronal conditions we can attempt to gain a comprehensive understanding of the structure, evolution, and mass and energy balance of quiescent prominences. A stability analysis for coronal material permeated by a vertical magnetic field rooted in the photosphere, indicates that a condensation will take the form of a thin vertical wedge of cool matter. The development of a finite condensation is followed and it is shown that photospheric line tying is only important in the initial stages. A perturbation analysis of vertical motions at the neutral sheet shows that thermal instability can lead to overstable oscillations. Cooling of coronal material can lead to both upward and downward mass motions, and gravitational energy release is important to the thermal balance of prominences. Relevant optical and radio observations are discussed. Synoptic observations of the development of active regions and magnetic fields are needed to test the basic hypothesis of the formation of prominences from neutral sheets.     

Density wave formation in differentially rotating disk galaxies: Hydrodynamic simulation of the linear regime

Most rapidly and differentially rotating disk galaxies, in which the sound speed (thermal velocity dispersion) is smaller than the orbital velocity, display graceful spiral patterns. Yet, over almost 240 yr after their discovery in M51 by Charles Messier, we still do not fully understand how they originate. In this first paper of a series, the dynamical behavior of a rotating galactic disk is examined numerically by a high-order Godunov hydrodynamic code. The code is implemented to simulate a two-dimensional flow driven by an internal Jeans gravitational instability in a nonresonant wave–“fluid” interaction in an infinitesimally thin disk composed of stars or gas clouds. A goal of this work is to explore the local and linear regimes of density wave formation, employed by Lin, Shu, Yuan and many others in connection with the problem of spiral pattern of rotationally supported galaxies, by means of computer-generated models and to compare those numerical results with the generalized fluid-dynamical wave theory. The focus is on a statistical analysis of time-evolution of density wave structures seen in the simulations. The leading role of collective processes in the formation of both the circular and spiral density waves (“heavy sound”) is emphasized. The main new result is that the disk evolution in the initial, quasilinear stage of the instability in our global simulations is fairly well described using the local approximation of the generalized wave theory. Certain applications of the simulation to actual gas-rich spiral galaxies are also explored.     

宇宙线的超新星遗迹起源

宇宙线的起源是高能天体物理的核心问题之一.一直以来,超新星爆发被认为是能谱膝区以下宇宙线的主要来源.多波段观测表明,超新星遗迹有能力加速带电粒子至亚PeV (10~(15)eV)能量.扩散激波加速被认为是最有效的天体高能粒子加速机制之一,而超新星遗迹的大尺度激波正好为这一机制提供平台.近年来,一系列较高精度的地面和空间实验极大地推动了对宇宙线以及超新星遗迹的研究.新的观测事实挑战着传统的扩散激波加速模型以及其在银河系宇宙线超新星遗迹起源学说上的应用,深化了人们对宇宙高能现象的认识.结合超新星遗迹辐射能谱的时间演化特性,构建的时间依赖的超新星遗迹粒子加速模型,不仅能够解释200 GV附近宇宙线的能谱反常,还自然地形成能谱膝区,甚至可以将超新星遗迹粒子加速对宇宙线能谱的贡献延伸至踝区.该模型预期超新星遗迹中粒子的输运行为表现为湍流扩散,这需要未来的观测以及与粒子输运相关的等离子体数值模拟工作来进一步验证.