Gas accreting onto T Tauri stars should form shocks that are susceptible to the classical radiative shock instability. The instability should give rise to strong periodic modulation in the X-ray emission from the shock-heated plasma. Time series analysis of soft X-rays ...

Supercomputer simulations have been used in conjunction with analytic studies to investigate the central issue of astrophysical accretion-disk dynamics: the nature of the angular momentum transport. Simulations provide the means to investigate and experiment with candidate mechanisms, including global hydrodynamic instabilities, spiral ...

We use 2D numerical simulations to study the linear response of the stalled accretion shock during the first few hundred milliseconds after core bounce in core-collapse supernovae. We use these simulations to measure the growth rate and frequency of the Spherical Accretion Shock Instability, or ...

Context. Plasma accreting onto classical T Tauri stars (CTTS) is believed to impact the stellar surface at free-fall velocities, generating a shock. Current time-dependent models describing accretion shocks in CTTSs are one-dimensional, assuming that the plasma moves and transports energy only along magnetic field ...

We studied the standing accretion shock instability (SASI) for a core-collapse supernova explosion. SASI induces a nonspherically symmetric motion of a standing spherical shock wave. In order to investigate the growth of SASI, we solved the three-dimensional compressible Euler equations using ZEUS-MP/2 code based ...

A possible accretion model associated with the ionization instability of quasar disks is proposed to

A possible accretion model associated with the ionization instability of quasar disks is proposed to

Accretion describes the process by which matter in an astrophysical disk falls onto a central massive object. Accretion disks are present in many astrophysical situations including binary star systems, young stellar objects, and near black holes at the center of galaxies. Measurements from observations of these disks have shown that viscous processes are ...

The Vishniac instability is supposed to explain the fragmentation of the thin shell of shocked matter in the radiative phase of supernova remnants. However its implication and its consequence on the morphological evolution of stellar systems is not fully demonstrated. The present paper tackles this subject by numerical simulations and focus on the role of ...

The nonlinear evolution of the Parker instability in accretion disks and galactic gas disks is studied by using a two-dimensional magnetohydrodynamic (MHD) code. The gas layer is assumed to be located at some distance from a point mass which is the origin of gravity. The magnetic fields are assumed to be parallel to the disk plane in magnetostatic ...

We are developing GenASiS (General Astrophysical Simulation System) to study the explosion mechanism of core-collapse supernovae. The current version of GenASiS is suitable for large scale simulations and includes a mature implementation of magnetohydrodynamics as well as a general Poisson solver. As a science application of the code, we explore the capacity of the stationary ...

Two analyses pertaining to shock waves in a radiating gas are presented. The first study investigates the influence of an isothermal accretion shock on a spherical accretion flow. An 'isothermal shock' is the combination of a regular shock and a postshock...

We use time-dependent, two-dimensional hydrodynamic simulations including a passive magnetic field to quantify the amplification of progenitor magnetic fields by the strong shear created by the non-axisymmetric mode of the spherical accretion shock instability. During the linear growth phase of the instability, the ...

We examine the stability of a standing shock wave within a spherical accretion flow onto a gravitating star, in the context of core-collapse supernova explosions. Our focus is on the effect of nuclear dissociation below the shock on the linear growth, and nonlinear saturation, of nonradial oscillations of the ...

We examine the stability of a standing shock wave within a spherical accretion flow onto a gravitating star, in the context of core-collapse supernova explosions. Our focus is on the effect of nuclear dissociation below the shock on the linear growth, and nonlinear saturation, of nonradial oscillations of the ...

In the delayed explosion scenario of core-collapse supernovae, the accretion phase shows pronounced convective overturns and a low-multipole hydrodynamic instability, the standing accretion shock instability. These effects imprint detectable fast time variations on the emerging neutrino flux. ...

A critical component behind a supernova explosion is the spherical accretion shock instability, or SASI. Two theories describing this mechanism exist. An advective-acoustic mechanism says that entropy perturbations generated at the shock travel inward with the accretion flow at the free-fall ...

The gravitational accretion of gas onto a compact star moving supersonically through a uniform ambient medium is dynamically unstable in the restricted case of two-dimensional planar geometry (a cylindrical star). Numerical simulations in 3D (e.g., the series of papers by Ruffert) show some hint of instability, but not the dramatic flip-flop seen in 2D ...

Context. High resolution X-ray observations of classical T Tauri stars (CTTSs) show a soft X-ray excess due to high density plasma (ne = 1011-113 cm-3). This emission has been attributed to shock-heated accreting material impacting onto the stellar surface. Aims: We investigate the observability of the shock-heated ...

We present new results on the gravitational wave (GW) emission in the postbounce phase of nonrotating or slowly rotating core-collapse supernovae obtained from an extensive set of simulations with the 2D code BETHE/Hydro. Our calculations include the most recent presupernova stellar models, a finite-temperature nuclear equation of state and a prescription for neutrino cooling and heating. ...

The stationary accretion shock instability (SASI) plays an important role in modern simulations of core-collapse supernovae. With the intent to study magnetic field generation and the possible impact of magnetic fields during the crucial nonlinear phase leading to the explosion of massive stars, we have carried out high-resolution, ...

This thesis describes in detail a two-dimensional, axisymmetric computer code for calculating fully relativistic ideal gas hydrodynamics around a Kerr black hole. The aim is to study fully dynamic inviscid fluid accretion onto black holes, as well as to study the evolution and development of nonlinear instabilities in pressure supported ...

We present a linear stability analysis of a planar radiative shock with cooling function ..lambda..proportionalT/sup ..cap alpha../. We investigate an oscillatory instability similar to that found in numerical calculations of accretion onto degenerate dwarfs by Langer, Chanmugam, and Shaviv. Our analysis shows that multiple modes of ...

I outline particle simulations and theory of relativistic shock waves in an e/sup +-/ plasma. Magnetic reflection of particles is an essential role in the shock structure. Instability of the reflected particles in the shock front produces intense extraordinary mode radiation. Such shocks are ...

We revisit the flip-flop instability of two-dimensional planar accretion using high-fidelity numerical simulations. By starting from an initially steady-state axisymmetric solution, we are able to follow the growth of this overstability from small amplitudes. In the small-amplitude limit, before any transient accretion disk is formed, ...

The global nonlinear time-dependent evolution of the inertial-acoustic mode instability in accretion

In this article, I review the current understanding of the mechanism of core-collapse supernovae, one of the most energetic events in the present universe. I will not only summarize the neutrino-heating mechanism, the standard paradigm at present, but also pay special attention to the recent topics such as the standing accretion shock ...

Sep 27, 2002 ... Accretion Disk Simulation ... Instead, the graphic shows spiral shock waves in a three dimensional simulation of an accretion disk -- material ...

A stalled spherical accretion shock, such as that arising in core-collapse supernovae, is unstable to non-spherical perturbations. In three dimensions, this Standing Accretion Shock Instability can develop spiral modes that spin-up the protoneutron star. Here, we study these non-axisymmetric ...

A possible accretion model associated with the ionization instability of quasar disks is proposed to

We consider the consequences of magnetic buoyancy and the magnetic shearing instability (MSI) on the

We begin an exploration of the capacity of the stationary accretion shock instability (SASI) to generate magnetic fields by adding a weak, stationary, and radial (but bipolar) magnetic field, and in some cases rotation, to an initially spherically symmetric fluid configuration that models a stalled shock in the ...

A cloud of gas collapsing under gravity will fragment. We present a new theory for this process, in which layers of shocked gas fragment due to their gravitational instability. Our model explains why angular momentum does not inhibit the collapse process. The theory predicts that the fragmentation process produces objects which are significantly smaller ...

Context: High spectral resolution X-ray observations of classical T Tauri stars (CTTSs) demonstrate the presence of plasma at temperature T� 2-3� 106 K and density n_e� 1011-1013 cm-3, which are unobserved in non-accreting stars. Stationary models suggest that this emission is due to shock-heated accreting material, but do not ...

Rotation-powered radio pulsars are born with inferred initial rotation periods of order 300 ms (some as short as 20 ms) in core-collapse supernovae. In the traditional picture, this fast rotation is the result of conservation of angular momentum during the collapse of a rotating stellar core. This leads to the inevitable conclusion that pulsar spin is directly correlated with the rotation of the ...

A general numerical scheme is presented that allows a variety of analyses of an arbitrary set of hydrodynamic equations: a search for steady states; stability of steady states; nonlinear integration; search for specified nonlinear solutions; stability of nonlinear solutions. The scheme is an extension of the ''Henyey'' relaxation method and is especially useful ...

With the discovery that Alfven waves can be strongly unstable in a magnetized accretion disk, it can be expected to explain the turbulence necessary to drive accretion in these disks. The instability of non-axisymmetric Alfven waves is studied using analy...

An account is given of the waves and instabilities occurring at collisionless shocks, with attention

We present results of 2-D simulations of core-collapse supernovae in the period immediately following core bounce. We have discovered the rapid onset of instabilities that quickly develop into convection in the post-shock region. Convection in this region quickly erases the driving entropy gradients, leaving behind an anemic convective zone. These ...

Motivated by recent simulations of galaxy formation in which protogalaxies acquire their baryonic content through cold accretion, we study the gravitational fragmentation of cold streams flowing into a typical first galaxy. We use a one-zone hydrodynamical model to examine the thermal evolution of the gas flowing into a 108 M sun dark matter halo at redshift z = 10. The goal ...

Using three-dimensional (3D) simulations of neutrino-powered supernova explosions, we show that the hydrodynamical kick scenario proposed by Scheck et al. on the basis of two-dimensional (2D) models can yield large neutron star (NS) recoil velocities also in 3D. Although the shock stays relatively spherical, standing accretion-shock and convective ...

The numerical investigation of the Bondi-Hoyle accretion on to a moving black hole has a long history, both in Newtonian and in general-relativistic physics. By performing new two-dimensional and general-relativistic simulations on to a rotating black hole, we point out a novel feature, namely that quasi-periodic oscillations (QPOs) are naturally produced in the ...

A control law was designed, using constrained optimization techniques, to suppress an apparent shock

We investigate the effects of neutrino heating and alpha-particle recombination on the hydrodynamics of core-collapse supernovae. Our focus is on the nonlinear dynamics of the shock wave that forms in the collapse and the assembly of positive energy material below it. To this end, we perform time-dependent hydrodynamic simulations with FLASH2.5 in spherical and axial symmetry. ...

Steady state and time dependent calculations of a model for accretion instability are presented, in which matter accumulates in a cold torus and then undergoes a thermal instability causing the matter to heat and flow down onto the central star. The model...

... ON THE INTERACTION BETWEEN THE SHOCK WAVE ... to the interaction between shock waves and boundary-layer instabilities, although most ...

Supernova explosions are among the most dramatic in the universe. Type II supernovae follow core collapse of a massive star, while Type Ia supernovae are typically believed to be thermonuclear explosions of carbon-oxygen white dwarfs that have accreted enough material to initiate carbon burning. In both cases, the explosion dynamics are complicated by hydrodynamic ...

Instability of isolated planar shock waves JW Bates ... [21] GR Fowles, �Stimulated and spontaneous emission of acoustic waves from shock fronts,� ...

We consider the effects of ``photon bubble'' shock trains on the vertical structure of radiation pressure-dominated accretion disks. These density inhomogeneities are expected to develop spontaneously in radiation-dominated accretion disks where magnetic pressure exceeds gas pressure, even in the presence of magnetorotational ...

The Kelvin-Helmholtz instability is well known to be capable of converting well-ordered flows into more disordered, even turbulent, flows. As such it could represent a path by which the energy in, for example, bow shocks from stellar jets could be converted into turbulent energy thereby driving molecular cloud turbulence. We present the results of a suite ...

In the delayed explosion scenario of core-collapse supernovae (SNe), the accretion phase shows pronounced convective overturns and a low-multipole hydrodynamic instability, the standing accretion shock instability (SASI). These effects imprint detectable fast time variations on the emerging ...

A two dimensional numerical model forming two spiral shock fronts in accretion flow in close binary star system is presented. The position of these shocks is discussed. In the model, the position of the second shock front depends on the relation between t...

The transport of angular momentum in the outward direction is the fundamental requirement for accretion to proceed in an accretion disk. This objective can be achieved if the accretion flow is turbulent. Instabilities are one of the sources for the turbulence. We study a differentially rotating compressive flow in ...

Recent three-dimensional simulations of core-collapse supernovae have revealed the existence of non-axisymmetric modes of the Spherical Accretion Shock Instability, or SASI. Here we investigate the growth of these modes using two-dimensional simulations of the accretion flow in the equatorial plane of a ...

We study the nonlinear evolution of the Rossby wave instability in thin disks using global two-dimensional hydrodynamic simulations. The detailed linear theory of this nonaxisymmetric instability was developed earlier by Lovelace et al. and Li et al., who found that the instability can be excited when there is an extremum in the radial ...

Using three-dimensional hydrodynamics simulations, we have identified a robust instability of the stalled accretion shock in core-collapse supernovae that is able to generate a strong rotational flow in the vicinity of the accreting proto-neutron star (PNS). Sufficient angular momentum is deposited on the PNS to ...

We consider time-dependent accretion onto magnetized white dwarfs. A detailed description of a numerical method of solution to the hydrodynamical equations is given. The postshock flow is cooled by optically thin bremsstrahlung and is thermally unstable. As a result the shock height undergoes periodic oscillations. We consider the properties of this ...

Whether an efficient collisonless temperature equilibration mechanism exists for a two-temperature ion-electron plasma, with Ti> Te, is important for understanding astrophysical phenomena such as radiatively inefficient accretion flows and relativistic collisionless shocks in Gamma-ray bursts. Here we study whether the two-stream and Weibel ...

Whether an efficient collisonless temperature equilibration mechanism exists for a two-temperature ion-electron plasma, with Ti>Te, is important for understanding astrophysical phenomena such as radiatively inefficient accretion flows and relativistic collisionless shocks in GRBs. Here we study whether the two-stream and Weibel ...

The report deals with the question of whether mass accretion on a star can cause pulsational instability by itself. The principal result is a formula for the effect of accretion on the growth (or decay) rate of a radial pulsation mode. Application of this...

The influence of accretion disk instabilities on the UV fluxes of Seyfert 1 galaxies was studied. The observed characteristic timescales of variability are consistent with the interpretation that most of the UV radiation originates in an accretion disk, a...

This presentation describes the behavior of matter in environments with extreme magnetic and gravitational fields, explains the instability/stability of accretion disks in certain systems, and discusses how emergent radiation affects accretion flow. Magne...

In the accretion disk around a black hole or a neutron star, the radiative viscosity will much exceed the viscosity of the fully ionized gas. The Reynolds number, depending strongly on the accretion rate, will reach a minimum in the zone where the bulk of the energy is released. Thermal instability will set in, altering the local ...

Thermal models and radiometric ages for meteorites show that the peak temperatures inside their parent bodies were closely linked to their accretion times. Most iron meteorites come from bodies that accreted <0.5 Myr after CAIs formed and were melted by 26Al and 60Fe, probably inside 2 AU. Rare carbon-rich differentiated meteorites like ureilites ...

The current status of the physics of accretion disks in close binary stars is reviewed, with an emphasis on the hydrodynamic overreflection instability, which is a factor leading to the accretion disk turbulence. The estimated turbulent viscosity coefficients are in good agreement with observations and explain the high angular momentum ...

Apr 12, 2011 ... The characteristics of a particular planetary system depend upon the ... planets are formed from dynamic instabilities in accretion disks. ...

The instability of the flow along the accretion line in the two-dimensional Bondi-Hoyle-Lyttleton type accretion flow is studied. By means of numerical simulations, the nonlinear regime of the instability is explored, thus extending a previous work that dealt with the linear regime using the WKB approximation. In ...

During the core-collapse of a massive star, the spherical accretion shock which stalls at about 150 km above the surface of the newly formed neutron star is hydrodynamically unstable. A perturbative approach has allowed us to disentangle the effects of neutrino-driven convection from those of the advective-acoustic instability. The ...

A model of the X-ray radiation emitted from the accretion column of a compact object in AM Herculis binaries, using Monte Carlo methods is presented. The accretion column in the model has cylindrical symmetry. Above a shocked region in the column there is...

... Title : An Experimental Study of Nonstationary Instabilities of Planar Shock Waves in Ionizing Argon. Descriptive Note : Technical note. ...

We observe the Richtmyer-Meshkov (RM) instability of a perturbed, shock-accelerated interface between different gases. Instability growth of a singly shocked interface is observed to be consistent with previous experimental data. Late-time growth visually...

(substantial changes to section 3.2, otherwise minor) We present an analysis of the hydrodynamic stability of a cold slab bounded by two accretion shocks. Previous numerical work has shown that when the Mach number of the shock is large the slab is unstable. Here we show that to linear order both the bending and breathing modes of such ...

Motivated by recent simulations of galaxy formation in which protogalaxies acquire their baryonic content through cold accretion, we study the gravitational fragmentation of cold streams flowing into a typical first galaxy. We use a one-zone hydrodynamical model to examine the thermal evolution of the gas flowing into a 10{sup 8} M{sub sun} dark matter halo at redshift z = 10. ...

... scattering particles and causing dissipation, waves in a collisionless shock play ... extending to frequencies well above the ion plasma frequency, and ...

We have modeled the gas temperature structure in unstable C-type shocks and obtained predictions for

A previous report that 22 Vul is a zeta Aur--type eclipsing binary is supported by our discovery of evidence for an accretion shock cone in 22 Vul similar to those found in zeta Aurigae and 32 Cygni.

We consider the problem of the flow produced by the accretion of gas from a rotating cloud onto a point gravitational source. It is shown that, for a cloud initially in uniform rotation, the effect of accretion is to produce a Taylor column in the cloud parallel to the rotation axis. Only gas within the column is accreted; the gas ...

The water ice mantles on interstellar grains trap volatile molecules, such as CO and CH3OH, with an efficiency that depends on the amorphous or crystalline structure of the ice. The ice structure therefore affects the composition of comets formed from the icy grains. We present a detailed study of the processing of mantled grains by shock waves in protoplanetary disks. The ...

X-ray emitting MHD accretion shocks in classical T Tauri stars. Case for moderate to high plasma-? values

The known properties of water and ice over a wide range of pressures and temperatures are applied to

We present an analytical description of the energetics of the population of cosmic accretion shocks, for a concordance cosmology. We calculate how the shock-processed accretion power and mass current are distributed among different shock Mach numbers, and how they evolve with cosmic time. We ...

We present an analytical description of the energetics of the population of cosmic accretion shocks, for a concordance cosmology (?m+??=1). We calculate how the shock-processed accretion power and mass current are distributed among different shock Mach numbers and how they evolve with cosmic ...

We study the oscillatory instability of white dwarf radiative accretion shocks discovered by Langer, Chanmugam, and Shaviv. We extend previous works by examining spherical shocks dominated by: (1) bremsstrahlung and Compton cooling; and (2) bremsstrahlung and Compton cooling when the effects of electron thermal ...

The concept of shock wave instability leading to shock pressure amplification is investigated with respect to materials properties and thermodynamic equilibrium. It is concluded that a materials property requirement (negative macroscopic Grueneisen parame...

... REINFORCING MATERIALS, SHEAR PROPERTIES ... MATERIALS, COMPARISON, SENSITIVITY, INCLUSIONS ... STRUCTURES, SHOCK ...

A core collapse supernova marks the death of a star over 8 times the size of the sun. Sometimes in the aftermath of these explosions a spinning, magnetized, neutron star can be left behind, also known as a pulsar. It has recently been discovered that pulsar spins can arise through a spiral spherical accretion shock instability (SASI) ...

This paper discusses the solutions for stationary, axisymmetric, transonic, adiabatic flows with polytropic, rotating fluid configurations of small transverse thickness in an arbitrarily chosen potential. Special attention is paid to the formation of the standing shocks in the case of black hole accretion and winds. The possibility of three types of ...

Instabilities which occur in shock wave or impact experiments are noted to be of two kinds. In one-dimensional experiments the wave may break up into multiple waves due to plastic yield or phase transitions. The indication that such an instability will oc...

Simplifications that have emerged in the descriptions of stellar unstable collapse to a neutron star are discussed. The neutral current weak interaction leads to almost complete neutrino trapping in the collapse and to an electron fraction Y/sub e/ congruent to 0.35 in equilibrium with trapped electron neutrinos and ''iron'' nuclei. A soft equation of state ...

The mechanism for core collapse supernova explosions remains undefined in detail and perhaps even in broad brush. Past multidimensional simulations point to the important role neutrino transport, fluid instabilities, rotation, and magnetic fields play, or may play, in generating core collapse supernova explosions, but the fundamental question as to whether or not these events ...

The linear growth rate and saturation level of magnetic fields for Weibel instabilities driven by ion temperature anisotropy, defined as ?=(T?/T?)-1 where T? and T? are ion temperatures perpendicular and parallel to the wave vector, are derived in the small ? limit. It is shown that the ratio of the saturated magnetic energy to the initial ion energy scales as the fourth power ...

We present two-dimensional hydrodynamic simulations of stellar core collapse and develop the framework for a detailed analysis of the energetic aspects of neutrino-powered supernova explosions. Our results confirm that the neutrino-heating mechanism remains a viable explanation of the explosion of a wider mass range of supernova progenitors with iron cores, but the explosion sets in later and ...

Instability of a planar shock front perturbed by a corrugated interface is analyzed, where the perturbation wavelength is along the shock front plane. The presented analysis involves the effects of the features on the shock front, which is different from a general method presented by D'yakov and Kontorovich, where ...

A numerical simulation of two-dimensional compressible Navier-Stokes equations using a high-order weighted essentially nonoscillatory finite difference shock capturing scheme is carried out in this paper, to study the effect of shock waves on the development of Rayleigh-Taylor instability. Shocks with different ...

Star formation is such a complex process that accurate numerical tools are needed to quantitatively examine the mass distribution and accretion of fragments in collapsing, turbulent, magnetized gas clouds. To enable a numerical treatment of this regime, we implemented sink particles in the adaptive mesh refinement (AMR) hydrodynamics code FLASH. Sink particles are created in ...

Planetary accretion has been considered as a process to heat planets. Some fraction of the kinetic energy of incoming planetesimals is trapped to heat the planetary interior (Kaula, 1979; Davies, 1984). Moreover, blanketing effect of a primary atmosphere (Hayashi et al., 1979; Sasaki, 1990) or a degassed atmosphere (Abe and Matsui, 1986; Zahnle et al., 1988) would raise the ...

Context. Classical T Tauri stars (CTTS) are young, late-type objects, that still accrete matter from a circumstellar disk. Analytical treatments and numerical simulations predict instabilities of the accretion shock on the stellar surface. Aims: We search for variability on timescales below a few minutes in the ...

We investigate explosive nucleosynthesis in a delayed neutrino-driven, supernova explosion aided by standing accretion shock instability (SASI), based on two-dimensional hydrodynamic simulations of the explosion of a 15 Msolar star. We take into accounts neutrino heating and cooling as well as change in electron fraction due to weak ...

We present the results of numerical experiments in which we study how asphericities induced by the growth of the standing accretion shock instability (SASI) produce gravitational waveforms in the postbounce phase of core-collapse supernovae. To obtain the neutrino-driven explosions, we parameterize the neutrino fluxes emitted from the ...

By performing axisymmetric hydrodynamic simulations of core-collapse supernovae with spectral neutrino transport based on the isotropic diffusion source approximation scheme, we support the assumption that the neutrino-heating mechanism aided by the standing accretion shock instability (SASI) and convection can initiate an explosion of ...

The polarized radiation from accretion shocks with a three-dimensional structure are calculated for the first time. It is shown that, because of inhomogeneities along the flow lines, accretion columns with a uniform accretion rate across the column can also produce flat optical/IR spectra as has been noted ...

numerical and we give an example of this in � 3. For example, a wave propagating along a shock front, may the differential Mach number across the layer is least. The growth rate of the instability is negligible in the pre-shock in the post-shock region. These are related to waves on the shock ...

We present two-dimensional (axisymmetric) neutrino-hydrodynamic simulations of the long-time accretion phase of a 15 M_&sun; progenitor star after core bounce and before the launch of a supernova explosion, when non-radial hydrodynamic instabilities like convection occur in different regions of the collapsing stellar core and the standing ...

planet :: smoking gun : caught in the actsmoking gun : caught in the act Most simulations of disk-planetDisk Instability vsvs. Core Accretion. Core Accretion Formed planet : forming planet ::Formed planet : formingMost simulations of disk-planet interactions start with fully-formed planetinteractions start with fully

Steady state and time dependent calculations of a model for accretion instability are presented, in which matter accumulates in a cold torus and then undergoes a thermal instability causing the matter to heat and flow down onto the central star. The model is compared to the observations of dwarf novae and certain examples of both hard ...

A linear theory is developed for thermal convective instability in radiating gas dominated by radiation pressure. Analysis demonstrates how the magnetic field and differential rotation will affect the onset of thermal convection in the interior of a black hole accretion disk.

The evolution of thick accretion tori around black holes was investigated. Using three-dimensional computer models, it is shown that isentropic, pressure supported tori with constant specific angular momentum (Omega = r sup - 2) are unstable. Nonaxisymmet...

We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It produces tall, thin tongues of plasma that penetrate the ...

We present the first results of a global axisymmetric simulation of accretion on to rotating magnetized stars from a turbulent accretion disc, where the turbulence is driven by the magnetorotational instability (MRI). We observed that the angular momentum is transported outwards by the magnetic stress and accretion ...

We present the first results of a global axisymmetric simulation of accretion on to rotating magnetized stars from a turbulent accretion disc, where the turbulence is driven by the magnetorotational instability (MRI). We observed that the angular momentum is transported outwards by the magnetic stress and accretion ...

invariance with respect to shock Mach number for fluid mixing statistics, such as growth rate and volume� cates into a transmitted shock and reflected wave. This stage is known as the shock refraction stage. At the end of the refraction stage, both the transmitted shock and reflected wave detach from the material

We have performed a series of experiments examining shock propagation in low density aerogels. High-pressure ((approx)100 kbar) shock waves are produced by detonating high explosives. Radiography is used to obtain a time sequence imaging of the shocks as ...

A plasma instability which may lead to the thermal coupling of ions and electrons in two-temperature

Icing; Swept wings; Ice scallop formations; Lobster tails; Cross flow vortices; Cross flow instability; Swept wing ice accretions. NUMBER OF PAGES: ...

Nov 19, 2004 ... In this scenario the plasma instabilities that can occur during the accretion process (Rayleigh-Taylor, Kelvin-Helmoltz, ...

Mar 7, 2000 ... Accretion-Ejection Instability and QPO in Black-hole binaries ... RXTE Study of the Cyclotron Resonance Scattering Feature in Her X-1 ...

Icing; Swept wings; Ice scallop formations; Lobster tails; Cross flow vortices; Cross flow instability; Swept wing ice accretions. NUMBER OF PAGES: ...

Highly nonstationary accretion onto a magnetized neutron star from a surrounding accretion disc is considered. Nonstationary accretion has been considered before in terms of droplets or rain or as a necessary consequence of instabilities in the accretion column flow. Here, the extreme of very ...

A model for emission of the hard photons of gamma bursts is presented. The model assumes accretion at nearly the Eddington limited rate onto a neutron star without magnetic a field. Initially soft photons are heated as they are compressed between the accreting matter and the star. A large electric field due to relatively small charge separation is required ...

Shock degassing was a major factor in releasing the gases present in planetesimals while the earth was in its accretion phase. The atmosphere was formed as the earth grew, and it was of the oxidizing type. As much as 30--45% may have been contributed on the earth by shock degassing, and no less than 25--40% on Venus, but ...

We present a collection of results in theoretical astrophysical hydrodynamics. In order (1), we discuss the role of turbulence in the early stages of planet formation, showing how turbulence in the gas component of proto-planetary disks can hydrodynamically and gravitationally stir the dust from which rocky planets form. We show that turbulence can prevent gravitational collapse of the dust into ...

instability, two-streaming, Weibel instabilities) created in the shock are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic ...

Plasma waves and their associated instabilities (e.g., Buneman, Weibel and other two-stream instabilities) created in collisionless shocks are responsible ...

There have been major advances in the theory of magnetic reconnection and of magnetic instability, with important implications for the observations, as follows: (1) Fast and slow magnetic shock waves are produced by the magnetohydrodynamics of reconnectio...

The stability of magnetic fields within the accretion disks is examined, including the effects of magnetic buoyancy. Rayleigh-Taylor, and Kelvin-Helmholtz instabilities. The purpose of the study is to examine the efficacy of turbulent accretion (..cap alpha..-) disk models which depend upon ''magnetic ...

We investigate numerically the role of thermal instability (TI) as a generator of density structures in the interstellar medium (ISM), both by itself and in the context of a globally turbulent medium. Simulations of the instability alone show that the condenstion process which forms a dense phase (``clouds'') is highly dynamical, and that the boundaries of ...

As a class, the AM Her objects exhibit temporal variability on time scales ranging from seconds to years. Most of the variations can be adequately described by ''shot noise'' models (Cordova and Mason 1982). Exceptions to this are the strictly periodic features modulated on the orbital periods of the systems (typically several hours) and the one to two second ...

We consider a simple gravitational heating mechanism for the long-term quenching of cooling flows and star formation in massive dark matter haloes hosting elliptical galaxies and clusters. We showed earlier that the virial shock heating in haloes >=1012Msolar triggers natural quenching in 1012-1013Msolar haloes. Our present analytic estimates and simple simulations argue ...

The AM Her systems are widely believed to be mass transfer binaries containing a white dwarf primary accreting from a red dwarf secondary. The magnetic field of the white dwarf is so strong that it prevents the formation of an accretion disk and funnels the accretion flow into the polar caps of the white dwarf. The ...

A dusty plasma cloud in space has a gravitational instability which allows star formation even orders of magnitude below the Jeans limit for gravitational collapse. This instability leads to a stellesimal accretion which is analogous to the planetesimal a...

Differentially rotating disks are subject to the axisymmetric instability for perfectly conducting plasma in the presence of poloidal magnetic fields. For nonaxisymmetric perturbations, the authors find localized unstable eigenmodes whose eigenfunction is...

The effects of heat conduction and magnetic fields on the growth of thermal instabilities in cooling flows are examined using a time-dependent hydrodynamics code. It is found that, for magnetic field strengths of roughly 1 micro-Gauss, magnetic pressure forces can completely suppress shocks from forming in thermally unstable entropy perturbations with ...

This paper presents global solutions of adiabatic accretion flows with isothermal shocks in Kerr black hole geometry. It is known that, in the previously studied cases, where the flow including the shock is either entirely adiabatic or entirely isothermal, there can be no more than one stable shock solution, and ...

... Injection into the Magnetosphere: Eigenmode Description of the Hot Ion Cyclotron Beam-Whistler Mode Instability in Parallel Shock Waves,. ...

... INSTABILTIES OF PLANAR SHOCK WAVES IN IONIZIG ARGON by WL Buchanan ... nh number of half-waves in the shock-front geometry p pressure ...

. There is a class of dynamos where the field is caused by a velocity field which, in turn, is driven by the generated magnetic field. Numerical threedimensional simulations of this type of process are reviewed. This process is here discussed in connection with accretion discs, but we point out that similar mechanisms have also been suggested in the context of solar and ...

It is shown that an ionized disk in a vertical magnetic field (e.g., the dipole field of the central object for an accretion or planetary disk) is subject to spiral instabilities of low azimuthal wavenumber, driven by differential rotation. The amplification is due to the Swing mechanism already known for the spiral instability of ...

is excited from the interaction between a shock wave and a fluid interface across which fluid density, the interface receives the impact of two shock waves, the incident shock wave and the reflected wave the shock front or the Mach number of the shock together with the state before ...

Mass accretion onto (proto-)stars at high accretion rates \\dot{M}_* > 10^{-4} M_{&sun;} yr^{-1} is expected in massive star formation. We study the evolution of massive protostars at such high rates by numerically solving the stellar structure equations. In this paper, we examine the evolution via disk accretion. We consider a ...

Solid recovery impact-induced volatile loss experiments on the Murchison C2M meteorite indicate that

Solid recovery impact-induced volatile loss experiments on the Murchison C2M meteorite indicate that

UCDs with bright radio emission show radio bursts. ? Electron Cyclotron Maser ... Diagnostic for accretion shock: high-density (~ 1012 cm-3) ...

of stalled accretion shock. - Convection in the gain region, low l ? (Herant, Benz & Colgate 1992) ... hydrostatic equilibrium. (Chandrasekhar 1961) ...

Jul 20, 2011 ... (bremsstrahlung, cyclotron, blackbody) from accretion mound in shock. ... Cyclotron Resonant Scattering Features CRSFs ("Cyclotron Lines") ...

L is the maximal distance, which the proton can cover during the cluster age. Cluster mass. Mass of gas shell between. accretion shock and virial radius: ...

The first observations of the interaction of the Richtmyer-Meshkov (RM) instability with reflected shock and rarefaction waves in laser-driven targets are reported. The RM growth is started by a shock wave incident upon a rippled interface between low-den...

A series of shock tube experiments performed in the 1970s at the Institute for Aerospace Studies, University of Toronto, led in the discovery of instabilities in relaxing shock structures in noble gases under hypervelocity conditions. The instabilities we...

3D magnetohydrodynamic simulations were performed in a study of the nonlinear evolution of the Parker instability in galactic gas disks and/or accretion disks, as well as in emerging flux regions (EFR) of the sun. The initial magnetic fields are parallel to one of the horizontal coordinates in magnetostatic equilibrium. The effect of coupling between the ...

Part I: Concepts of Accretion Disks: Chap. 1: Introduction, 1.1 Accretion Energy - Historical Origin, { Accretion-Disk Paradigm - Active Universe, 1.3 Accretion-Powered Objects - Observational Reviews, 1.4 X-Ray Binaries and Ultra-Luminous X-Ray Sources, 1.5 Active Galactic Nuclei, 1.6 Present Paradigm, Chap. 2: ...

... developed in the target after the shock ... INSTABILITY, COMPUTERIZED SIMULATION, LASER BEAMS. Subject Categories : LASERS AND MASERS ...

... instabilities and optical shocks for propagation along nonlinear optical fibers, and (4) the dynamics of bistable optical switches coupled through ...

The continuum point of view is emphasized. Subjects included the governing equations for the motion of the fluid, waves of small amplitude, magnetohydrostatics, instability, and shock theory. (auth)

The effect of a magnetic field on a thermal instability has been studied in a radiatively cooling region behind an interstellar shock of moderate propagation velocity (?10 km s-1). It is shown that the presence of a magnetic field of a few microgauss is very effective in preventing the thermal instability from building-up density ...

... It is suggested that under shock conditions the Rayleigh-Taylor instability of interfacial mixing plays a dominant role in potential chemical processes ...

OUTFLOWS FROM DYNAMO-ACTIVE PROTOSTELLAR ACCRETION DISCS BRIGITTA VON REKOWSKI1 , AXEL BRANDENBURG2, Newcastle, UK Abstract. An axisymmetric model of a cool, dynamo-active accretion disc is applied by the disc dynamo driven by the magneto-rotational instability and some form of outflow is indeed seen

This is a report of the progress during the past year of the shock-tube study of the Richtmyer-Meshkov instability, initiated under the sponsorship of the Lawrence Livermore National Laboratory in September, 1982. The purpose of this research program, as stated in the original proposal, is: to investigate the nonlinear processes initiated by ...

Massive stars produce so much light that the radiation pressure they exert on the gas and dust around them is stronger than their gravitational attraction, a condition that has long been expected to prevent them from growing by accretion. We present three-dimensional radiation-hydrodynamic simulations of the collapse of a massive prestellar core and find that radiation ...

The nonlinear evolution and spatial propagation of the thermal instability in accretion disks in galactic nuclei are investigated. Integrations of the vertical structure of the disks are described for different alpha prescriptions, and the thermal stability is examined. Global time-dependent calculations of the unstable disks are performed which show that ...

The radial-azimuthal instability of a hot two-temperature accretion disc with advection is analyzed. After obtaining the dispersion equation, we find that advection and viscous force have an influence on the stability of acoustic modes, but not on the stability of thermal and viscous modes. We also find azimuthal perturbations to affect the stability of ...

We report experimental observation of an instability in a Couette-Taylor flow of a polymer fluid in a thin gap between two coaxially rotating cylinders in a regime where their angular velocity decreases with the radius while the specific angular momentum increases with the radius. In the considered regime, neither the inertial Rayleigh instability nor the ...

We report experimental observation of an instability in a Couette-Taylor flow of a polymer fluid in a thin gap between two coaxially rotating cylinders in a regime where their angular velocity decreases with the radius while the specific angular momentum increases with the radius. In the considered regime, neither the inertial Rayleigh instability nor the ...

We perform axisymmetric simulations of the magnetorotational collapse of very massive stars in full general relativity. Our simulations are applicable to the collapse of supermassive stars with masses M > or approx. 10{sup 3}M{sub {center_dot}} and to very massive Population III stars. We model our initial configurations by n=3 polytropes, uniformly rotating near the mass-shedding limit and ...

The corrugation instability of magnetic shock waves in a relativistically hot plasma with photon gas has been investigated. Conditions under which spontaneous amplification of surface corrugations on the discontinuity occurs are obtained. The corrugation instability may cause the condensation of the plasma number density, as well as ...

Observations of the radiative shock waves produced during the late stages of supernova remnant evolution cannot be understood in the context of steady state shock models. As a result, several more complicated scenarios have been suggested. For example, it has been proposed that several shocks are producing the emission or that one ...

The properties of cyclotron emission from accretion shocks in AM Herculis binaries are studied which

We study the standing dissipative and non-dissipative shocks and the isentropic compression waves systematically in the thin, rotating, adiabatic astrophysical flows, such as accretion or winds, near the black holes and the neutron stars. The flow is assu...

The linear instabilities induced by the electrostatic fields and gradients of equilibrium parameters in a plasma shock front are analyzed for the plasma shock structure. Small perturbations as well as the steady-state shock structure are described by a set of coupled two-fluid and Poisson equations. The dispersion ...

Previous work on axisymmetric shock solutions in hybrid model flows is extended to the case when the shocks are nonaxisymmetric. The properties of the sonic surface as well as the shocks are characterized analytically. The shocks are assumed to be self-similar, and the flow everywhere is assumed to be in ...

Bounds are placed on the growth rates of dynamical instability in shear flows in accretion discs and tori. Ft is shown that the critical layer lies in the flow even when the boundaries are free for flows in accretion discs, cylinders and isentropic, constant angular momentum tori (for which the relativistic effects are also ...

Context. High resolution X-ray spectroscopy has revealed soft X-rays from high density plasma in classical T Tauri stars (CTTSs), probably arising from the accretion shock region. However, the mass accretion rates derived from the X-ray observations are consistently lower than those derived from UV/optical/NIR studies. Aims: We aim to ...

Spiral shocks have been discovered in hot accretion disks in dwarf novae during outbursts, using Doppler maps of HeI and Balmer emission lines. Here, we present the eclipse light curves of these lines and we simulate the parameters that affect dilution of the spiral shock light by the ambient symmetric disk emission. Taking into ...

The development of solar wind shock models with tensor plasma pressure and the comparison of some of the shock models with the satellite data from Pioneer 6 through Pioneer 9 are reported. Theoretically, difficulties were found in non-turbulent fluid shock models for tensor pressure plasmas. For microscopic shock ...

The purpose of this research program is to investigate fluid dynamic instabilities and mixing initiated by the interaction of shock waves with interfaces between light and heavy gases. In particular, the nonlinear stage of shock-initiated Rayleigh-Taylor instability (also known as the Richtmeyer-Meshkov ...

By analyzing the fourth-order dispersion relation, the influence of coupled radial and azimuthal perturbations on the stability of a radiation-pressure-dominated accretion disk is investigated. If the azimuthal perturbations are considered, we find that the stability properties of disk are different from that in purely radial perturbation case. For a standard alpha disk model, ...

Plasma heating due to the ion instability and the modified two-stream instability is examined for quasi-perpendicular subcritical shocks. Electron and ion heating is investigated as a function of upstream electron to ion temperature ratio and plasma beta using second-order heating rates. A simple shock model is ...

Bolometric luminosities and black hole (BH) masses are estimated by various methods for a sample of young radio galaxies with known ages. We find that the ages are positively correlated with the bolometric luminosities in these young radio galaxies. This positive correlation is consistent with the theoretical prediction based on the radiation pressure instability of ...

This thesis consists of two parts. In the first hart, we present the development of instruments for high-speed infrared and optical photometry optimized for pulsar observations. In the second part, we present observational studies of pulsars, mainly on accretion- powered pulsars but also on rotation-powered pulsars. For the instrument development, we first built the Cornell ...

Young stars can produce X-rays by different mechanisms: Coronal activity, accretion shocks and shocks in outflows. We propose to observe two targets within one field-of-view (FOV): SU Aur is accreting mass from a disk and shows violent coronal activity. We will study the time evolution of stellar flares on time ...

We describe in detail our two-dimensional, axisymmetric computer code for calculating fully relativistic ideal gas hydrodynamics around a Kerr black hole. This code is being used to study fully dynamic inviscid fluid accretion onto black holes, as well as the evolution and developement of nonlinear instabilities in pressure-supported ...

Shock waves that occur in stellar atmospheres have their origin in some hydrodynamic instability of the atmosphere itself or of the stellar interior. In luminous early-type stars these two possibilities are represented by shocks due to an unstable radiati...

Understanding the suface stability of metals undergoing dynamic fracture at shock breakout is important to several applications in metals processing. The advantages of using the Pegasus II facility to investigate the phenomena occurring at shock break out...

In the standard Richtmyer-Meshkov instability, perturbations at a shocked interface grow after the passage of a shock. Freeze-out refers to the phenomenon whereby the perturbations do not grow, i.e., freeze-out, after the passage of a shock. This is fairl...

Recent high spectral resolution X-ray observations of some CTTSs show the presence of high density plasma (ne=10^{11}-10^{13} cm^{-3}) at temperature T=2-3 MK. This plasma is likely heated up by an accretion shock on the star surface. We investigate this issue by an accurate modelling of the impact of an accretion stream onto the ...

Young T Tauri stars exhibit strong solar-type magnetic activity, with extremely high temperature coronae and energetic flares. In a few systems discovered with Chandra and XMM-Newton there is also evidence for X-ray emission produced by shocks associated with magnetically channeled accretion. A recent 489 ksec Chandra HETG/ACIS-S observation of the ...