WorldWideScience

Sample records for accretion shock instability

  1. Numerical Analysis on Standing Accretion Shock Instability with Neutrino Heating in the Supernova Cores

    Ohnishi, Naofumi; Kotake, Kei; Yamada, Shoichi

    2005-01-01

    We have numerically studied the instability of the spherically symmetric standing accretion shock wave against non-spherical perturbations. We have in mind the application to the collapse-driven supernovae in the post bounce phase, where the prompt shock wave generated by core bounce is commonly stalled. We take an experimental stand point in this paper. Using spherically symmetric, completely steady, shocked accretion flows as unperturbed states, we have clearly observed both the linear grow...

  2. Links between the shock instability in core-collapse supernovae and asymmetric accretions of envelopes

    Takahashi, Kazuya; Yamamoto, Yu; Yamada, Shoichi

    2016-01-01

    The explosion mechanism of core-collapse supernovae has not been fully understood yet but multi-dimensional fluid instabilities such as standing accretion shock instability (SASI) and convection are now believed to be crucial for shock revival. Another multi-dimensional effect that has been recently argued is the asymmetric structures in progenitors, which are induced by violent convections in silicon/oxygen layers that occur before the onset of collapse, as revealed by recent numerical simulations of the last stage of massive star evolutions. Furthermore, it has been also demonstrated numerically that accretions of such non-spherical envelopes could facilitate shock revival. These two multi-dimensional may hence hold a key to successful explosions. In this paper, we performed a linear stability analysis of the standing accretion shock in core-collapse supernovae, taking into account non-spherical, unsteady accretion flows onto the shock to clarify the possible links between the two effects. We found that suc...

  3. Numerical Analysis of Standing Accretion Shock Instability with Neutrino Heating in Supernova Cores

    Ohnishi, Naofumi; Kotake, Kei; Yamada, Shoichi

    2006-04-01

    We have numerically studied the instability of the spherically symmetric standing accretion shock wave against nonspherical perturbations. We have in mind the application to collapse-driven supernovae in the postbounce phase, where the prompt shock wave generated by core bounce is commonly stalled. We take an experimental standpoint in this paper. Using spherically symmetric, completely steady, shocked accretion flows as unperturbed states, we have clearly observed both the linear growth and the subsequent nonlinear saturation of the instability. In so doing, we have employed a realistic equation of state, together with heating and cooling via neutrino reactions with nucleons. We have performed a mode analysis based on the spherical harmonics decomposition and found that the modes with l=1,2 are dominant not only in the linear regime but also after nonlinear couplings generate various modes and saturation occurs. By varying the neutrino luminosity, we have constructed unperturbed states both with and without a negative entropy gradient. We have found that in both cases the growth of the instability is similar, suggesting that convection does not play a dominant role, which also appears to be supported by the recent linear analysis of the convection in accretion flows by Foglizzo et al. The oscillation period of the unstable l=1 mode is found to fit better with the advection time rather than with the sound crossing time. Whatever the cause may be, the instability favors a shock revival.

  4. Jittering-jets explosion triggered by the standing accretion shock instability

    Papish, Oded; Soker, Noam

    2015-01-01

    We show that the standing accretion shock instability (SASI) that has been used to ease the shock revival in core collapse supernovae (CCSNe) neutrino-driven explosion models, might play a much more decisive role in supplying the stochastic angular momentum required to trigger an explosion with jittering jets. To play a minor role in neutrino-based explosion models, the kinetic energy of the gas inside the stalled shock associated with the transverse (non-radial) motion should be about more than ten percent of the energy of the accreted gas. We find that this implies a stochastic angular momentum that can reach about five percent of the Keplerian specific angular momentum around the newly born neutron star. Such an accretion flow leaves an open conical region along the poles with an average opening angle of about 5 degrees. The outflow from the open polar region powers an explosion according to the jittering-jets model.

  5. Numerical Analysis on Standing Accretion Shock Instability with Neutrino Heating in the Supernova Cores

    Ohnishi, N; Yamada, S; Ohnishi, Naofumi; Kotake, Kei; Yamada, Shoichi

    2006-01-01

    We have numerically studied the instability of the spherically symmetric standing accretion shock wave against non-spherical perturbations. We have in mind the application to the collapse-driven supernovae in the post bounce phase, where the prompt shock wave generated by core bounce is commonly stalled. We take an experimental stand point in this paper. Using spherically symmetric, completely steady, shocked accretion flows as unperturbed states, we have clearly observed both the linear growth and the subsequent nonlinear saturation of the instability. In so doing, we have employed a realistic equation of state together with heating and cooling via neutrino reactions with nucleons. We have done a mode analysis based on the spherical harmonics decomposition and found that the modes with l=1, 2 are dominant not only in the linear regime, but also after the nonlinear couplings generate various modes and the saturation occurs. Varying the neutrino luminosity, we have constructed the unperturbed states both with ...

  6. Inelastic Neutrino Reactions with Light Nuclei and Standing Accretion Shock Instability in Core-Collapse Supernovae

    Furusawa, S.; Nagakura, H.; Sumiyoshi, K.; Yamada, S.

    2016-01-01

    We perform numerical experiments to investigate the influence of inelastic neutrino reactions with light nuclei on the standing accretion shock instability. The time evolutions of shock waves are calculated with a simple light-bulb approximation for the neutrino transport and a multi-nuclei equation of state. The neutrino absorptions and inelastic interactions with deuterons, tritons, helions and alpha particles are taken into account in the hydrodynamical simulations in addition to the ordinary charged-current interactions with nucleons. Axial symmetry is assumed but no equatorial symmetry is imposed. We show that the heating rates of deuterons reach as high as ∼ 10% of those of nucleons around the bottom of the gain region. On the other hands, alpha particles heat the matter near the shock wave, which is important when the shock wave expands and density and temperature of matter become low. It is also found that the models with heating by light nuclei have different evolutions from those without it in non-linear evolution phase. The matter in the gain region has various densities and temperatures and there appear regions that are locally rich in deuterons and alpha particles. These results indicate that the inelastic reactions of light nuclei, especially deuterons, should be incorporated in the simulations of core-collapse supernovae.

  7. Pulsar spins from an instability in the accretion shock of supernovae.

    Blondin, John M; Mezzacappa, Anthony

    2007-01-01

    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 progenitor star. So far, however, stellar theory has not been able to explain the distribution of pulsar spins, suggesting that the birth rotation is either too slow or too fast. Here we report 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. Sufficient angular momentum is deposited on the proto-neutron star to generate a final spin period consistent with observations, even beginning with spherically symmetrical initial conditions. This provides a new mechanism for the generation of neutron star spin and weakens, if not breaks, the assumed correlation between the rotational periods of supernova progenitor cores and pulsar spin. PMID:17203055

  8. Pulsar spins from an instability in the accretion shock of supernovae

    Blondin, J M; Blondin, John M.; Mezzacappa, Anthony

    2006-01-01

    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 progenitor star. So far, however, stellar theory has not been able to explain the distribution of pulsar spins, suggesting that the birth rotation is either too slow or too fast. Here we report 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. Sufficient angular momentum is deposited on the proto-neutron star to generate a final spin period consistent with observations, even beginning with spherically symmetrical initial conditions. This provides a new mechanism for the generation of neu...

  9. Gravitational Radiation from Standing Accretion Shock Instability in Core-Collapse Supernovae

    Kotake, K; Yamada, S; Kotake, Kei; Ohnishi, Naofumi; Yamada, Shoichi

    2006-01-01

    We perform long-term two dimensional axisymmetric simulations in the postbounce phase of core-collapse supernovae to study how the asphericities induced by the growth of the standing accretion shock instability (SASI) produce the gravitational waveforms. To obtain the neutrino-driven explosions, we parameterize the neutrino fluxes emitted from the central protoneutron star and approximate the neutrino transfer by a light-bulb scheme. We find that the waveforms due to the anisotropic neutrino emissions show the monotonic increase with time, whose amplitudes are up to two order-of-magnitudes larger than the ones from the convective matter motions outside the protoneutron stars. We point out that the amplitudes begin to become larger when the growth of the SASI enters the nonlinear phase, in which the deformation of the shocks and the neutrino anisotropy become large. From the spectrum analysis of the waveforms, we find that the amplitudes from the neutrinos are dominant over the ones from the matter motions at ...

  10. Ringed accretion disks: instabilities

    Pugliese, D

    2016-01-01

    We analyze the possibility that several instability points may be formed, due to the Paczy\\'nski mechanism of violation of mechanical equilibrium, in the orbiting matter around a supermassive Kerr black hole. We consider recently proposed model of ringed accretion disk, made up by several tori (rings) which can be corotating or counterrotating relative to the Kerr attractor due to the history of the accretion process. Each torus is governed by the general relativistic hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. We prove that the number of the instability points is generally limited and depends on the dimensionless spin of the rotating attractor.

  11. Ringed Accretion Disks: Instabilities

    Pugliese, D.; Stuchlík, Z.

    2016-04-01

    We analyze the possibility that several instability points may be formed, due to the Paczyński mechanism of violation of mechanical equilibrium, in the orbiting matter around a supermassive Kerr black hole. We consider a recently proposed model of a ringed accretion disk, made up by several tori (rings) that can be corotating or counter-rotating relative to the Kerr attractor due to the history of the accretion process. Each torus is governed by the general relativistic hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. We prove that the number of the instability points is generally limited and depends on the dimensionless spin of the rotating attractor.

  12. THE INFLUENCE OF INELASTIC NEUTRINO REACTIONS WITH LIGHT NUCLEI ON THE STANDING ACCRETION SHOCK INSTABILITY IN CORE-COLLAPSE SUPERNOVAE

    Furusawa, Shun; Nagakura, Hiroki; Yamada, Shoichi [Advanced Research Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555 (Japan); Sumiyoshi, Kohsuke, E-mail: furusawa@heap.phys.waseda.ac.jp [Numazu College of Technology, Ooka 3600, Numazu, Shizuoka 410-8501 (Japan)

    2013-09-01

    We perform numerical experiments to investigate the influence of inelastic neutrino reactions with light nuclei on the standing accretion shock instability (SASI). The time evolution of shock waves is calculated with a simple light-bulb approximation for the neutrino transport and a multi-nuclei equation of state. The neutrino absorptions and inelastic interactions with deuterons, tritons, helions, and alpha particles are taken into account in the hydrodynamical simulations. In addition, the effects of ordinary charged-current interactions with nucleons is addressed in the simulations. Axial symmetry is assumed but no equatorial symmetry is imposed. We show that the heating rates of deuterons reach as high as {approx}10% of those of nucleons around the bottom of the gain region. On the other hand, alpha particles are heated near the shock wave, which is important when the shock wave expands and the density and temperature of matter become low. It is also found that the models with heating by light nuclei evolve differently in the non-linear phase of SASI than do models that lack heating by light nuclei. This result is because matter in the gain region has a varying density and temperature and therefore sub-regions appear that are locally rich in deuterons and alpha particles. Although the light nuclei are never dominant heating sources and they work favorably for shock revival in some cases and unfavorably in other cases, they are non-negligible and warrant further investigation.

  13. Influences of inelastic neutrino reactions with light nuclei on standing accretion shock instability in core collapse supernovae

    Furusawa, Shun; Sumiyoshi, Kohsuke; Yamada, Shoichi

    2013-01-01

    We perform numerical experiments to investigate the influence of inelastic neutrino reactions with light nuclei on the standing accretion shock instability. The time evolutions of shock waves are calculated with a simple light-bulb approximation for the neutrino transport and a multi-nuclei equation of state. The neutrino absorptions and inelastic interactions with deuterons, tritons, helions and alpha particles are taken into account in the hydrodynamical simulations in addition to the ordinary charged-current interactions with nucleons. Axial symmetry is assumed but no equatorial symmetry is imposed. We show that the heating rates of deuterons reach as high as ~ 10% of those of nucleons around the bottom of the gain region. On the other hands, alpha particles heat the matter near the shock wave,which is important when the shock wave expands and density and temperature of matter become low. It is also found that the models with heating by light nuclei have different evolutions from those without it in non-li...

  14. Limits on the spin up of stellar-mass black holes through a spiral stationary accretion shock instability

    Moreno Méndez, Enrique; Cantiello, Matteo

    2016-04-01

    The spin of a number of black holes (BHs) in binary systems has been measured. In the case of BHs found in low-mass X-ray binaries (LMXBs) the observed values are in agreement with some theoretical predictions based on binary stellar evolution. However, using the same evolutionary models, the calculated spins of BHs in high-mass X-ray binaries (HMXBs) fall short compared to the observations. A possible solution to this conundrum is the accretion of high-specific-angular-momentum material after the formation of the BH, although this requires accretion above the Eddington limit. Another suggestion is that the observed high values of the BHs spin could be the result of an asymmetry during Core Collapse (CC). The only available energy to spin up the compact object during CC is its binding energy. A way to convert it to rotational kinetic energy is by using a Standing Accretion Shock Instability (SASI), which can develop during CC and push angular momentum into the central compact object through a spiral mode (m = 1). Here we study the CC-SASI scenario and discuss, in the case of LMXBs and HMXBs, the limits for the spin of a stellar-mass BHs. Our results predict a strong dichotomy in the maximum spin of low-mass compact objects and massive BHs found in HMXBs. The maximum spin value (|a⋆|) for a compact object near the mass boundary between BHs and NSs is found to be somewhere between 0.27 and 0.38, depending on whether secular or dynamical instabilities limit the efficiency of the spin up process. For more massive BHs, such as those found in HMXBs, the natal spin is substantially smaller and for MBH > 8M⊙ spin is limited to values |a⋆| ≲ 0.05. Therefore we conclude that the observed high spins of BHs in HMXBs cannot be the result of a CC-SASI spin up.

  15. Instabilities of advection-dominated accretion flows

    Chen, X

    1996-01-01

    Accretion disk instabilities are briefly reviewed. Some details are given to the short-wavelength thermal instabilities and the convective instabilities. Time-dependent calculations of two-dimensional advection-dominated accretion flows are presented.

  16. Instabilities of Advection-Dominated Accretion Flows

    Chen, Xingming

    1996-01-01

    Accretion disk instabilities are briefly reviewed. Some details are given to the short-wavelength thermal instabilities and the convective instabilities. Time-dependent calculations of two-dimensional advection-dominated accretion flows are presented.

  17. Instability in Shocked Granular Gases

    Sirmas, Nick; Falle, Sam; Radulescu, Matei

    2013-01-01

    Shocks in granular media, such as vertically oscillated beds, have been shown to develop instabilities. Similar jet formation has been observed in explosively dispersed granular media. Our previous work addressed this instability by performing discrete-particle simulations of inelastic media undergoing shock compression. By allowing finite dissipation within the shock wave, instability manifests itself as distinctive high density non-uniformities and convective rolls within the shock structur...

  18. Shock instability in dissipative gases

    Radulescu, Matei I.; Sirmas, Nick

    2011-01-01

    Previous experiments have revealed that shock waves in thermally relaxing gases, such as ionizing, dissociating and vibrationally excited gases, can become unstable. To date, the mechanism controlling this instability has not been resolved. Previous accounts of the D'yakov-Kontorovich instability, and Bethe-Zel'dovich-Thompson behaviour could not predict the experimentally observed instability. To address the mechanism controlling the instability, we study the propagation of shock waves in a ...

  19. Interaction of Accretion Shocks with Winds

    Kinsuk Acharya; Sandip K. Chakrabarti; D. Molteni

    2002-03-01

    Accretion shocks are known to oscillate in presence of cooling processes in the disk. This oscillation may also cause quasi-periodic oscillations of black holes. In the presence of strong winds, these shocks have oscillations in vertical direction as well.We show examples of shock oscillations under the influence of both the effects. When the shocks are absent and the flow is cooler, the wind becomes weaker and the vertical oscillation becomes negligible.

  20. Instability in Shocked Granular Gases

    Sirmas, Nick; Radulescu, Matei

    2013-01-01

    Shocks in granular media, such as vertically oscillated beds, have been shown to develop instabilities. Similar jet formation has been observed in explosively dispersed granular media. Our previous work addressed this instability by performing discrete-particle simulations of inelastic media undergoing shock compression. By allowing finite dissipation within the shock wave, instability manifests itself as distinctive high density non-uniformities and convective rolls within the shock structure. In the present study we have extended this work to investigate this instability at the continuum level. We modeled the Euler equations for granular gases with a modified cooling rate to include an impact velocity threshold necessary for inelastic collisions. Our results showed a fair agreement between the continuum and discrete-particle models. Discrepancies, such as higher frequency instabilities in our continuum results may be attributed to the absence of higher order effects.

  1. Instability in shocked granular gases

    Shocks in granular media, such as vertically oscillated beds, have been shown to develop instabilities. Similar jet formation has been observed in explosively dispersed granular media. Our previous work addressed this instability by performing discrete-particle simulations of inelastic media undergoing shock compression. By allowing finite dissipation within the shock wave, instability manifests itself as distinctive high density non-uniformities and convective rolls within the shock structure. In the present study we have extended this work to investigate this instability at the continuum level. We modeled the Euler equations for granular gases with a modified cooling rate to include an impact velocity threshold necessary for inelastic collisions. Our results showed a fair agreement between the continuum and discrete-particle models. Discrepancies, such as higher frequency instabilities in our continuum results may be attributed to the absence of higher order effects.

  2. Nonlinear Instabilities in Shock-Bounded Slabs

    Vishniac, E T

    1993-01-01

    (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 a slab are stable. However, nonlinear effects will tend to soften the restoring forces for bending modes, and when the slab displacement is comparable to its thickness this gives rise to a nonlinear instability. The growth rate of the instability, above this threshold but for small bending angles, is $\\sim c_sk (k\\eta)^{1/2}$, where $\\eta$ is the slab displacement. When the bending angle is large the slab will contain a local vorticity comparable to $c_s/L$, where $L$ is the slab thickness. We discuss the implications of this work for gravitational instabilities of slabs. Finally, we examine the cases of a decelerating slab bounded by a single shock and a stationary slab bounde...

  3. Gravitational Instability in Neutrino Dominated Accretion Disks

    We revisit the vertical structure of neutrino-dominated accretion flows (NDAFs) in spherical coordinates under a boundary condition based on a mechanical equilibrium. The solutions show that the NDAF is significantly geometrically thick. The Toomre parameter is determined by the mass accretion rate and the viscosity parameter, which is defined as Q = cSΩ/πGΣ, where cS, Ω and Σ are the sound speed, angular velocity and surface density, respectively. According to the distribution of the Toomre parameter, the possible fragments of the disk may appear near the disk surface in the outer region. These possible outflows originating from the gravitational instability of the disk may account for the late-time flares in gamma-ray bursts. (geophysics, astronomy, and astrophysics)

  4. Standing Shocks in Viscous Accretion Flows around Black Holes

    GU Wei-Min; LU Ju-Fu

    2005-01-01

    @@ We study the problem of standing shocks in viscous accretion flows around black holes.We parameterize such a flow with two physical constants, namely the specific angular momentum accreted by the black hole j and the energy quantity K.By providing the global dependence of shock formation in the j - K parameter space, we show that a significant parameter region can ensure solutions with shocks of different types, namely Rankine-Hugoniot shocks, isothermal shocks, and more realistically, mixed shocks.

  5. Generalized Shock Solutions for Hydrodynamic Black Hole Accretion

    Das, Tapas Kumar

    2002-01-01

    For the first time, {\\it all} available pseudo-Schwarzschild potentials are exhaustively used to investigate the possibility of shock formation in hydrodynamic, invicid, black hole accretion discs. It is shown that a significant region of parameter space spanned by important accretion parameters allows shock formation for flow in {\\it all} potentials used in this work. This leads to the conclusion that the standing shocks are essential ingredients in accretion discs around non-rotating black ...

  6. Parametric study of flow patterns behind the standing accretion shock wave for core-collapse supernovae

    In this study, we conduct three-dimensional hydrodynamic simulations systematically to investigate the flow patterns behind the accretion shock waves that are commonly formed in the post-bounce phase of core-collapse supernovae. Adding small perturbations to spherically symmetric, steady, shocked accretion flows, we compute the subsequent evolutions to find what flow pattern emerges as a consequence of hydrodynamical instabilities such as convection and standing accretion shock instability for different neutrino luminosities and mass accretion rates. Depending on these two controlling parameters, various flow patterns are indeed realized. We classify them into three basic patterns and two intermediate ones; the former includes sloshing motion (SL), spiral motion (SP), and multiple buoyant bubble formation (BB); the latter consists of spiral motion with buoyant-bubble formation (SPB) and spiral motion with pulsationally changing rotational velocities (SPP). Although the post-shock flow is highly chaotic, there is a clear trend in the pattern realization. The sloshing and spiral motions tend to be dominant for high accretion rates and low neutrino luminosities, and multiple buoyant bubbles prevail for low accretion rates and high neutrino luminosities. It is interesting that the dominant pattern is not always identical between the semi-nonlinear and nonlinear phases near the critical luminosity; the intermediate cases are realized in the latter case. Running several simulations with different random perturbations, we confirm that the realization of flow pattern is robust in most cases.

  7. Intermittent activity of radio sources. Accretion instabilities and jet precession

    Kunert-Bajraszewska, M.; Janiuk, A.; Siemiginowska, A.; Gawronski, M.

    2010-01-01

    We consider the radiation pressure instability operating on short timescales 10^3 - 10^6 years in the accretion disk around a supermassive black hole as the origin of the intermittent activity of radio sources. We test whether this instability can be responsible for short ages (

  8. Magnetic Instability in Accretion Disks with Anomalous Viscosity

    ZHOU Ai-Ping; LI Xiao-Qing

    2004-01-01

    @@ Using the new model of anomalous viscosity, we investigate the magnetic instability in the accretion disks and give the dispersion formula. On the basis of the dispersion relation obtained, it is numerically shown that the instability condition of viscous accretion disk is well consistent with that of the ideal accretion disk, namely there would be magneto-rotational instability in the presence of a vertical weak magnetic field. For a given distance R from the centre of the disk, the growth rate in the anomalous case deviates from the ideal case more greatly when the vertical magnetic field is smaller. The large viscosity limits to the instability. In the two cases, the distributions of growth rate with wave number k approach each other when the magnetic field increases. It greatly represses the effect of viscosity.

  9. Standing Rankine-Hugoniot Shocks in Black Hole Accretion Discs

    GU Wei-Min; LU Ju-Fu

    2004-01-01

    @@ We study the problem of standing shocks in viscous disc-like accretion flows around black holes. For the first time we parametrize such a flow with two physical constants, namely the specific angular momentum accreted by the black hole j and the energy quantity K. By providing the global dependence of shock formation in the j - K parameter space, we show that a significant parameter region can ensure solutions with Rankine-Hugoniot shocks; and that the possibilities of shock formation are the largest for inviscid flows, decreasing with increasing viscosity, and ceasing to exist for a strong enough viscosity. Our results support the view that the standing shock is an essential ingredient in black hole accretion discs and is a general phenomenon in astrophysics, and that there should be a continuous change from the properties of inviscid flows to those of viscous ones.

  10. X-Ray Spectroscopy of Accretion Shocks in Young Stars

    Brickhouse, Nancy S.

    2011-01-01

    High resolution X-ray spectroscopy of accreting young stars is providing new insights into the physical conditions of the shocked plasma. While young stars exhibit exceedingly active coronae (>10 MK) with highly energetic flares, the relatively low temperature ( 3 MK), high density (>1012 cm-3) accretion shock can only be clearly distinguished at high spectral resolution. The nearby Classical T Tauri star TW Hydrae was the first to show evidence of accretion using 50 ks with the Chandra High Energy Transmission Grating (HETG). More recently a Chandra HETG Large Program (489 ks obtained over the course of one month) on TW Hydrae has found evidence for a new type of coronal structure. In the standard model, the accreting gas shocks near the atmosphere of the star and gently settles onto the surface as it slows down and cools. On TW Hydrae the observed post-shock region is not this predicted settling flow, since its mass is 30 times the mass of material that passes through the shock. Instead the stellar atmosphere must be heated to soft X-ray emitting temperatures. Of the CTTS systems observed with the gratings on Chandra and XMM-Newton not all show the accretion shock signature; however, all of them show excess soft X-ray emission related to accretion. The production of highly charged ions in the proximity of both open and closed magnetic field lines has important implications for coronal heating, winds and jets in the presence of accretion. This work is supported by the Chandra X-ray Observatory through a NASA contract with the Smithsonian Astrophysical Observatory.

  11. Radiative Shocks in Rotating Accretion Flows around Black Holes

    Okuda, T; Toscano, E; Molteni, D

    2004-01-01

    It is well known that the rotating accretion flows around black holes form shock waves close to the black holes, after the flow passes through the outer sonic point and can be virtually stopped by the centrifugal force. We examine numerically such shock waves in 1D and 2D accretion flows, taking account of the cooling and heating of gas and the radiation transport. The numerical results show that the shock location shifts outward compared with that in the adiabatic solutions and that the more rarefied ambient density leads to the more outward shock position. In the 2D-flow, we find an intermediate frequency QPO behavior of the shock location as is observed in the black hole candidate GRS 1915+105.

  12. Physical and radiative properties of the first core accretion shock

    Commerçon, Benoît; Chabrier, Gilles; Chièze, Jean-Pierre

    2011-01-01

    Radiative shocks play a dominant role in star formation. The accretion shocks on the first and second Larson's cores involve radiative processes and are thus characteristic of radiative shocks. In this study, we explore the formation of the first Larson's core and characterize the radiative and dynamical properties of the accretion shock, using both analytical and numerical approaches. We develop both numerical RHD calculations and a semi-analytical model that characterize radiative shocks in various physical conditions, for radiating or barotropic fluids. Then, we perform 1D spherical collapse calculations of the first Larson's core, using a grey approximation for the opacity of the material. We consider three different models for radiative transfer, namely: the barotropic approximation, the FLD approximation and the more complete M1 model. We investigate the characteristic properties of the collapse and of the first core formation. Comparison between the numerical results and our semi-analytical model shows...

  13. Global MHD Simulations of Accretion Disks in Cataclysmic Variables. I. The Importance of Spiral Shocks

    Ju, Wenhua; Stone, James M.; Zhu, Zhaohuan

    2016-06-01

    We present results from the first global 3D MHD simulations of accretion disks in cataclysmic variable (CV) systems in order to investigate the relative importance of angular momentum transport via turbulence driven by the magnetorotational instability (MRI) compared with that driven by spiral shock waves. Remarkably, we find that even with vigorous MRI turbulence, spiral shocks are an important component of the overall angular momentum budget, at least when temperatures in the disk are high (so that Mach numbers are low). In order to understand the excitation, propagation, and damping of spiral density waves in our simulations more carefully, we perform a series of 2D global hydrodynamical simulations with various equation of states, both with and without mass inflow via the Lagrangian point (L1). Compared with previous similar studies, we find the following new results. (1) The linear wave dispersion relation fits the pitch angles of spiral density waves very well. (2) We demonstrate explicitly that mass accretion is driven by the deposition of negative angular momentum carried by the waves when they dissipate in shocks. (3) Using Reynolds stress scaled by gas pressure to represent the effective angular momentum transport rate {α }{eff} is not accurate when mass accretion is driven by non-axisymmetric shocks. (4) Using the mass accretion rate measured in our simulations to directly measure α defined in standard thin-disk theory, we find 0.02≲ {α }{eff}≲ 0.05 for CV disks, consistent with observed values in quiescent states of dwarf novae. In this regime, the disk may be too cool and neutral for the MRI to operate and spiral shocks are a possible accretion mechanism. However, we caution that our simulations use unrealistically low Mach numbers in this regime and, therefore, future models with more realistic thermodynamics and non-ideal MHD are warranted.

  14. Local Dynamical Instabilities in Magnetized, Radiation Pressure Supported Accretion Disks

    Blaes, Omer M; Blaes, Omer; Socrates, Aristotle

    2000-01-01

    We present a general linear dispersion relation which describes the coupled behavior of magnetorotational, photon bubble, and convective instabilities in weakly magnetized, differentially rotating accretion disks. We presume the accretion disks to be geometrically thin and supported vertically by radiation pressure. We fully incorporate the effects of a nonzero radiative diffusion length on the linear modes. In an equilibrium with purely vertical magnetic field, the vertical magnetorotational modes are completely unaffected by compressibility, stratification, and radiative diffusion. However, in the presence of azimuthal fields, which are expected in differentially rotating flows, the growth rate of all magnetorotational modes can be reduced substantially below the orbital frequency. This occurs if diffusion destroys radiation sound waves on the length scale of the instability, and the magnetic energy density of the azimuthal component exceeds the non-radiative thermal energy density. While sluggish in this c...

  15. Accretion of supersonic winds onto black holes in 3D: stability of the shock cone

    Gracia-Linares, M

    2015-01-01

    Using numerical simulations we present the accretion of supersonic winds onto a rotating black hole in three dimensions. We study five representative directions of the wind with respect to the axis of rotation of the black hole and focus on the evolution and stability of the high density shock cone that is formed during the process. We explore both, the regime in which the shock cone is expected to be stable in order to confirm previous results obtained with two dimensional simulations, and the regime in which the shock cone is expected to show a flip-flop type of instability. The methods used to attempt triggering the instability were first the accumulation of numerical errors and second the explicit application of a perturbation on the velocity field after the shock-cone was formed. The result is negative, that is, we did not find the flip-flop instability within the parameter space we explored, which includes cases that are expected to be unstable.

  16. Aerodynamic instability of a cylinder with thin ice accretion

    Gjelstrup, Henrik; Georgakis, Christos

    2009-01-01

    selected. This was then used in the generation of a generalized ice profile. The generalized ice profile was selected so as to depict with a fair degree of representation the most commonly observed ice accretion on the Great Belt East Bridge. Subsequently, the generalized ice profile was manufactured by...... use of rapid prototyping. Next, a series of static wind tunnel tests were undertaken to determine the aerodynamic force coefficients of the rapidly prototyped hanger sectional model. Finally the aerodynamic force coefficients (drag, lift and moment), found from the static wind tunnel tests, were used...... to determine the potential for aerodynamic instability of the hanger through application of the quasi-steady theory developed by Gjelstrup et al. [9-10]. The application of the theoretical model yield regions of expected aerodynamic instability in which the observed vibrations of the Great Belt East...

  17. Instability of an accretion disk with a magnetically driven wind

    Cao, X.; Spruit, H. C.

    2002-04-01

    We present a linear analysis of the stability of accretion disks in which angular momentum is removed by the magnetic torque exerted by a centrifugally driven wind. The effects of the dependence of the wind torque on field strength and inclination, the sub-Keplerian rotation due to magnetic forces, and the compression of the disk by the field are included. A WKB dispersion relation is derived for the stability problem. We find that the disk is always unstable if the wind torque is strong. The growth time scale of the instability can be as short as the orbital time scale. The instability is mainly the result of the sensitivity of the mass flux to changes in the inclination of the field at the disk surface. Magnetic diffusion in the disk stabilizes if the wind torque is small.

  18. Global MHD Simulations of Accretion Disks in Cataclysmic Variables (CVs): I. The Importance of Spiral Shocks

    Ju, Wenhua; Zhu, Zhaohuan

    2016-01-01

    We present results from the first global 3D MHD simulations of accretion disks in Cataclysmic Variable (CV) systems in order to investigate the relative importance of angular momentum transport via turbulence driven by the magnetorotational instability (MRI) compared to that driven by spiral shock waves. Remarkably, we find that even with vigorous MRI turbulence, spiral shocks are an important component to the overall angular momentum budget, at least when temperatures in the disk are high (so that Mach numbers are low). In order to understand the excitation, propagation, and damping of spiral density waves in our simulations more carefully, we perform a series of 2D global hydrodynamical simulations with various equation of states and both with and without mass inflow via the Lagrangian point (L1). Compared with previous similar studies, we find the following new results. 1) Linear wave dispersion relation fits the pitch angles of spiral density waves very well. 2) We demonstrate explicitly that mass accreti...

  19. Instability in stratified accretion flows under primary and secondary perturbations

    Nasraoui, S.; Salhi, A.; Lehner, T.

    2015-04-01

    We consider horizontal linear shear flow (shear rate denoted by Λ ) under vertical uniform rotation (ambient rotation rate denoted by Ω0 ) and vertical stratification (buoyancy frequency denoted by N ) in unbounded domain. We show that, under a primary vertical velocity perturbation and a radial density perturbation consisting of a one-dimensional standing wave with frequency N and amplitude proportional to w0sin(ɛ N x /w0) ≈ɛ N x (≪1 ) , where x denotes the radial coordinate and ɛ a small parameter, a parametric instability can develop in the flow, provided N2>8 Ω0(2 Ω0-Λ ) . For astrophysical accretion flows and under the shearing sheet approximation, this implies N2>8 Ω02(2 -q ) , where q =Λ /Ω0 is the local shear gradient. In the case of a stratified constant angular momentum disk, q =2 , there is a parametric instability with the maximal growth rate (σm/ɛ ) =3 √{3 }/16 for any positive value of the buoyancy frequency N . In contrast, for a stratified Keplerian disk, q =1.5 , the parametric instability appears only for N >2 Ω0 with a maximal growth rate that depends on the ratio Ω0/N and approaches (3 √{3 }/16 )ɛ for large values of N .

  20. Cures for the Expansion Shock and the Shock Instability of the Roe Scheme

    Li, Xue-song; Gu, Chun-wei

    2016-01-01

    A common defect of the Roe scheme is the production of non-physical expansion shock and shock instability. An improved method with several advantages was presented to suppress the shock instability. However, this method cannot prevent expansion shock and is incompatible with the traditional curing method for expansion shock. Therefore, the traditional curing mechanism is analyzed. The discussion explains the effectiveness of the traditional curing method and identifies several defects, one of which leads to incompatibility between curing the shock instability and expansion shock. Consequently, a new improved Roe scheme is proposed in this study. This scheme is concise, easy to implement, low computational cost, and robust. More importantly, the scheme can simultaneously cure the shock instability and expansion shock without additional costs.

  1. 3D numerical modeling of YSO accretion shocks

    Matsakos T.

    2014-01-01

    Full Text Available The dynamics of YSO accretion shocks is determined by radiative processes as well as the strength and structure of the magnetic field. A quasi-periodic emission signature is theoretically expected to be observed, but observations do not confirm any such pattern. In this work, we assume a uniform background field, in the regime of optically thin energy losses, and we study the multi-dimensional shock evolution in the presence of perturbations, i.e. clumps in the stream and an acoustic energy flux flowing at the base of the chromosphere. We perform 3D MHD simulations using the PLUTO code, modelling locally the impact of the infalling gas onto the chromosphere. We find that the structure and dynamics of the post-shock region is strongly dependent on the plasma-beta (thermal over magnetic pressure, different values of which may give distinguishable emission signatures, relevant for observations. In particular, a strong magnetic field effectively confines the plasma inside its flux tubes and leads to the formation of quasi-independent fibrils. The fibrils may oscillate out of phase and hence the sum of their contributions in the emission results in a smooth overall profile. On the contrary, a weak magnetic field is not found to have any significant effect on the shocked plasma and the turbulent hot slab that forms is found to retain its periodic signature.

  2. Stability of stagnation via an expanding accretion shock wave

    Velikovich, A L; Taylor, B D; Giuliani, J L; Zalesak, S T; Iwamoto, Y

    2016-01-01

    Stagnation of a cold plasma streaming to the center or axis of symmetry via an expanding accretion shock wave is ubiquitous in inertial confinement fusion (ICF) and high-energy-density plasma physics, the examples ranging from plasma flows in x-ray-generating Z pinches [Y. Maron et al., Phys. Rev. Lett. 111, 035001 (2013)] to the experiments in support of the recently suggested concept of impact ignition in ICF [H. Azechi et al., Phys. Rev. Lett. 102, 235002 (2009); M. Murakami et al., Nucl. Fusion 54, 054007 (2014)]. Some experimental evidence indicates that stagnation via an expanding shock wave is stable, but its stability has never been studied theoretically. We present such analysis for the stagnation that does not involve a rarefaction wave behind the expanding shock front and is described by the classic ideal-gas Noh solution in spherical and cylindrical geometry. In either case the stagnated flow has been demonstrated to be stable, initial perturbations exhibiting a power-law, oscillatory or monotonic...

  3. Stability of stagnation via an expanding accretion shock wave

    Velikovich, A. L.; Murakami, M.; Taylor, B. D.; Giuliani, J. L.; Zalesak, S. T.; Iwamoto, Y.

    2016-05-01

    Stagnation of a cold plasma streaming to the center or axis of symmetry via an expanding accretion shock wave is ubiquitous in inertial confinement fusion (ICF) and high-energy-density plasma physics, the examples ranging from plasma flows in x-ray-generating Z pinches [Maron et al., Phys. Rev. Lett. 111, 035001 (2013)] to the experiments in support of the recently suggested concept of impact ignition in ICF [Azechi et al., Phys. Rev. Lett. 102, 235002 (2009); Murakami et al., Nucl. Fusion 54, 054007 (2014)]. Some experimental evidence indicates that stagnation via an expanding shock wave is stable, but its stability has never been studied theoretically. We present such analysis for the stagnation that does not involve a rarefaction wave behind the expanding shock front and is described by the classic ideal-gas Noh solution in spherical and cylindrical geometry. In either case, the stagnated flow has been demonstrated to be stable, initial perturbations exhibiting a power-law, oscillatory or monotonic, decay with time for all the eigenmodes. This conclusion has been supported by our simulations done both on a Cartesian grid and on a curvilinear grid in spherical coordinates. Dispersion equation determining the eigenvalues of the problem and explicit formulas for the eigenfunction profiles corresponding to these eigenvalues are presented, making it possible to use the theory for hydrocode verification in two and three dimensions.

  4. Nature of the Wiggle Instability of Galactic Spiral Shocks

    Kim, Woong-Tae; Kim, Jeong-Gyu

    2014-01-01

    Gas in disk galaxies interacts nonlinearly with an underlying stellar spiral potential to form galactic spiral shocks. While numerical simulations typically show that spiral shocks are unstable to wiggle instability (WI) even in the absence of magnetic fields and self-gravity, its physical nature has remained uncertain. To clarify the mechanism behind the WI, we conduct a normal-mode linear stability analysis as well as nonlinear simulations assuming that the disk is isothermal and infinitesimally thin. We find that the WI is physical, originating from the generation of potential vorticity at a deformed shock front, rather than Kelvin-Helmholtz instabilities as previously thought. Since gas in galaxy rotation periodically passes through the shocks multiple times, the potential vorticity can accumulate successively, setting up a normal mode that grows exponentially with time. Eigenfunctions of the WI decay exponentially downstream from the shock front. Both shock compression of acoustic waves and a discontinui...

  5. Magnetic curtailment of the shock-induced thermal instability

    Efect of magnetic field on the thermal instability is studied in the radiatively cooling region behind an intersteller shock of moderate propagation velocity (approx.10km/sec). It is shown that the presence of intersteller magnetic field of a few micro gauss is very effective in preventing the thermal instability from building-up density concentration. In the absence of magnetic field, the shock-induced thermal instability amplifies preshock density inhomogeneity by more than an order of magnitude. However, in the presence of magnetic field, the amplified density contrast is shown to be only a factor 2. (Author)

  6. Soft X-Ray Excess from Shocked Accreting Plasma in Active Galactic Nuclei

    Fukumura, Keigo; Clark, Peter; Tombesi, Francesco; Takahashi, Masaaki

    2016-01-01

    We propose a novel theoretical model to describe a physical identity of the soft X-ray excess, ubiquitously detected in many Seyfert galaxies, by considering a steady-state, axisymmetric plasma accretion within the innermost stable circular orbit (ISCO) around a black hole (BH) accretion disk. We extend our earlier theoretical investigations on general relativistic magnetohydrodynamic (GRMHD) accretion which has implied that the accreting plasma can develop into a standing shock for suitable physical conditions causing the downstream flow to be sufficiently hot due to shock compression. We numerically calculate to examine, for sets of fiducial plasma parameters, a physical nature of fast MHD shocks under strong gravity for different BH spins. We show that thermal seed photons from the standard accretion disk can be effectively Compton up-scattered by the energized sub-relativistic electrons in the hot downstream plasma to produce the soft excess feature in X-rays. As a case study, we construct a three-paramet...

  7. The subcritical baroclinic instability in local accretion disc models

    Lesur, G

    2009-01-01

    (abridged) Aims: We present new results exhibiting a subcritical baroclinic instability (SBI) in local shearing box models. We describe the 2D and 3D behaviour of this instability using numerical simulations and we present a simple analytical model describing the underlying physical process. Results: A subcritical baroclinic instability is observed in flows stable for the Solberg-Hoiland criterion using local simulations. This instability is found to be a nonlinear (or subcritical) instability, which cannot be described by ordinary linear approaches. It requires a radial entropy gradient weakly unstable for the Schwartzchild criterion and a strong thermal diffusivity (or equivalently a short cooling time). In compressible simulations, the instability produces density waves which transport angular momentum outward with typically alpha<3e-3, the exact value depending on the background temperature profile. Finally, the instability survives in 3D, vortex cores becoming turbulent due to parametric instabilities...

  8. Electrostatic Instabilities at High Frequency in a Plasma Shock Front

    LV Jian-Hong; HE Yong; HU Xi-Wei

    2007-01-01

    New electrostatic instabilities in the plasma shock front are reported.These instabilities are driven by the electrostatic field which is caused by charge separation and the parameter gradients in a plasma shock front.The linear analysis to the high frequency branch of electrostatic instabilities has been carried out and the dispersion relations are obtained numerically.There are unstable disturbing waves in both the parallel and perpendicular directions of shock propagation.The real frequencies of both unstable waves are similar to the electron electrostatic wave,and the unstable growth rate in the parallel direction is much greater than the one in the perpendicular direction.The dependence of growth rates on the electric field and parameter gradients is also presented.

  9. Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars

    Orlando, S; Argiroffi, C; Reale, F; Peres, G; Miceli, M; Matsakos, T; Stehle', C; Ibgui, L; de Sa, L; Chie`ze, J P; Lanz, T

    2013-01-01

    (abridged) AIMS. We investigate the dynamics and stability of post-shock plasma streaming along nonuniform stellar magnetic fields at the impact region of accretion columns. We study how the magnetic field configuration and strength determine the structure, geometry, and location of the shock-heated plasma. METHODS. We model the impact of an accretion stream onto the chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our model takes into account the gravity, the radiative cooling, and the magnetic-field-oriented thermal conduction. RESULTS. The structure, stability, and location of the shocked plasma strongly depend on the configuration and strength of the magnetic field. For weak magnetic fields, a large component of B may develop perpendicular to the stream at the base of the accretion column, limiting the sinking of the shocked plasma into the chromosphere. An envelope of dense and cold chromospheric material may also develop around the shocked column. For strong magnetic fields, th...

  10. Supermassive star formation via episodic accretion: protostellar disc instability and radiative feedback efficiency

    Sakurai, Y.; Vorobyov, E. I.; Hosokawa, T.; Yoshida, N.; Omukai, K.; Yorke, H. W.

    2016-06-01

    The formation of supermassive stars (SMSs) is a potential pathway to seed supermassive black holes in the early universe. A critical issue for forming SMSs is stellar UV feedback, which may limit the stellar mass growth via accretion. In this paper, we study the evolution of an accreting SMS and its UV emissivity with realistic variable accretion from a circumstellar disc. First we conduct a 2D hydrodynamical simulation to follow the protostellar accretion until the stellar mass exceeds 104 M⊙. The disc fragments by gravitational instability, creating many clumps that migrate inward to fall on to the star. The resulting accretion history is highly time-dependent: short episodic accretion bursts are followed by longer quiescent phases. We show that the disc for the direct collapse model is more unstable and generates greater variability than normal Pop III cases. Next, we conduct a stellar evolution calculation using the obtained accretion history. Our results show that, regardless of the variable accretion, the stellar radius monotonically increases with almost constant effective temperature at Teff ≃ 5000 K as the stellar mass increases. The resulting UV feedback is too weak to hinder accretion due to the low flux of stellar UV photons. The insensitivity of stellar evolution to variable accretion is attributed to the fact that time-scales of variability, ≲103 yr, are too short to affect the stellar structure. We argue that this evolution will continue until the SMS collapses to produce a black hole by the general relativistic instability after the mass reaches ≳105 M⊙.

  11. DIFFUSIVE PARTICLE ACCELERATION IN SHOCKED, VISCOUS ACCRETION DISKS: GREEN'S FUNCTION ENERGY DISTRIBUTION

    The acceleration of relativistic particles in a viscous accretion disk containing a standing shock is investigated as a possible explanation for the energetic outflows observed around radio-loud black holes. The energy/space distribution of the accelerated particles is computed by solving a transport equation that includes the effects of first-order Fermi acceleration, bulk advection, spatial diffusion, and particle escape. The velocity profile of the accreting gas is described using a model for shocked viscous disks recently developed by the authors, and the corresponding Green's function distribution for the accelerated particles in the disk and the outflow is obtained using a classical method based on eigenfunction analysis. The accretion-driven, diffusive shock acceleration scenario explored here is conceptually similar to the standard model for the acceleration of cosmic rays at supernova-driven shocks. However, in the disk application, the distribution of the accelerated particles is much harder than would be expected for a plane-parallel shock with the same compression ratio. Hence the disk environment plays a key role in enhancing the efficiency of the shock acceleration process. The presence of the shock helps to stabilize the disk by reducing the Bernoulli parameter, while channeling the excess binding energy into the escaping relativistic particles. In applications to M87 and Sgr A*, we find that the kinetic power in the jet is ∼0.01 M-dot c2, and the outflowing relativistic particles have a mean energy ∼300 times larger than that of the thermal gas in the disk at the shock radius. Our results suggest that a standing shock may be an essential ingredient in accretion onto underfed black holes, helping to resolve the long-standing problem of the stability of advection-dominated accretion disks.

  12. Theoretical, numerical and experimental study of accretion shocks dynamics in magnetic cataclysmic variables

    Magnetic cataclysmic variables are interacting binary Systems containing a highly magnetized white dwarf which accretes material from a companion. Material is led along magnetic field lines and falls onto the magnetic pole(s) supersonically forming an accretion column. As the material hits the surface, a reverse shock is formed and the shocked region is structured by the cooling effect of radiation processes. This work is a multidisciplinary study of the dynamics of the accretion column. Firstly, a numerical study of the accretion column structure at the astrophysical scale is presented. The observational consequences are discussed. This approach is completed by experiments using radiative flows generated by powerful lasers. The relevance of such experiments is based on the establishment of scaling laws. News scaling laws in the frame of radiative ideal or resistive MHD are exposed. The results of the sizing and the interpretation of the POLAR experimental campaign of 2012 on LULI2000 installation are presented. (author)

  13. Collisionless shock experiments with lasers and observation of Weibel instabilities

    Park, H.-S., E-mail: park1@llnl.gov; Huntington, C. M.; Fiuza, F.; Levy, M. C.; Pollock, B. B.; Remington, B. A.; Ross, J. S.; Ryutov, D. D.; Turnbull, D. P.; Weber, S. V. [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States); Drake, R. P.; Kuranz, C. C. [University of Michigan, Ann Arbor, Michigan 48109 (United States); Froula, D. H.; Rosenberg, M. [Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14636 (United States); Gregori, G.; Meinecke, J. [University of Oxford, Parks Road, Oxford OX1 3PU (United Kingdom); Koenig, M. [LULI, Ecole Polytechnique, Palaiseau (France); Kugland, N. L. [Lam Research Corporation, Fremont, California 94538 (United States); Lamb, D. Q.; Tzeferacos, P. [University of Chicago, Chicago, California 94538 (United States); and others

    2015-05-15

    Astrophysical collisionless shocks are common in the universe, occurring in supernova remnants, gamma ray bursts, and protostellar jets. They appear in colliding plasma flows when the mean free path for ion-ion collisions is much larger than the system size. It is believed that such shocks could be mediated via the electromagnetic Weibel instability in astrophysical environments without pre-existing magnetic fields. Here, we present laboratory experiments using high-power lasers and investigate the dynamics of high-Mach-number collisionless shock formation in two interpenetrating plasma streams. Our recent proton-probe experiments on Omega show the characteristic filamentary structures of the Weibel instability that are electromagnetic in nature with an inferred magnetization level as high as ∼1% [C. M. Huntington et al., “Observation of magnetic field generation via the weibel instability in interpenetrating plasma flows,” Nat. Phys. 11, 173–176 (2015)]. These results imply that electromagnetic instabilities are significant in the interaction of astrophysical conditions.

  14. The signature of the magnetorotational instability in the Reynolds and Maxwell stress tensors in accretion discs

    Pessah, Martin Elias; Chan, Chi-kwan; Psaltis, Dimitrios

    2006-01-01

    The magnetorotational instability is thought to be responsible for the generation of magnetohydrodynamic turbulence that leads to enhanced outward angular momentum transport in accretion discs. Here, we present the first formal analytical proof showing that, during the exponential growth of the...

  15. Ultrahigh energy cosmic rays as heavy nuclei from cluster accretion shocks

    Inoue, Susumu; Sigl, Guenter; Miniati, Francesco; Armengaud, Eric

    2007-01-01

    Large-scale accretion shocks around massive clusters of galaxies, generically expected in the cold dark matter scenario of cosmological structure formation, are shown to be plausible sources of the observed ultrahigh energy cosmic rays (UHECRs) by accelerating a mixture of heavy nuclei including the iron group elements. Current observations can be explained if the source composition at injection for the heavier nuclei is somewhat enhanced from simple expectations for the accreting gas. The pr...

  16. Kinetic instabilities in plasmas: from electromagnetic fluctuations to collisionless shocks

    Collisionless shocks play a major role in powerful astrophysical objects (e.g., gamma-ray bursts, supernova remnants, pulsar winds, etc.), where they are thought to be responsible for non-thermal particle acceleration and radiation. Numerical simulations have shown that, in the absence of an external magnetic field, these self-organizing structures originate from electromagnetic instabilities triggered by high-velocity colliding flows. These Weibel-like instabilities are indeed capable of producing the magnetic turbulence required for both efficient scattering and Fermi-type acceleration. Along with rapid advances in their theoretical understanding, intense effort is now underway to generate collisionless shocks in the laboratory using energetic lasers. In a first part we study the (w,k)-resolved electromagnetic thermal spectrum sustained by a drifting relativistic plasma. In particular, we obtain analytical formulae for the fluctuation spectra, the latter serving as seeds for growing magnetic modes in counterstreaming plasmas. Distinguishing between sub-luminal and supra-luminal thermal fluctuations, we derived analytical formulae of their respective spectral contributions. Comparisons with particle-in-cell (PIC) simulations are made, showing close agreement in the sub-luminal regime along with some discrepancy in the supra-luminal regime. Our formulae are then used to estimate the saturation time of the Weibel instability of relativistic pair plasmas. Our predictions are shown to match 2-D particle-in-cell (PIC) simulations over a three-decade range in flow energy. We then develop a predictive kinetic model of the nonlinear phase of the Weibel instability induced by two counter-streaming, symmetric and non-relativistic ion beams. This self consistent, fully analytical model allows us to follow the evolution of the beams' properties up to a stage close to complete isotropization and thus to shock formation. Its predictions are supported by 2D and 3D particle

  17. Electron Weibel Instability Mediated Laser Driven Electromagnetic Collisionless Shock

    Jia, Qing; Mima, Kunioki; Cai, Hong-Bo; Taguchi, Toshihiro; Nagatomo, Hideo; He, X. T.

    2015-11-01

    As a fundamental nonlinear structure, collisionless shock is widely studied in astrophysics. Recently, the rapidly-developing laser technology provides a good test-bed to study such shock physics in laboratory. In addition, the laser driven shock ion acceleration is also interested due to its potential applications. We explore the effect of external parallel magnetic field on the collisionless shock formation and resultant particle acceleration by using the 2D3V PIC simulations. We show that unlike the electrostatic shock generated in the unmagnetized plasma, the shock generated in the weakly-magnetized laser-driven plasma is mostly electromagnetic (EM)-like with higher Mach number. The generation mechanism is due to the stronger transverse magnetic field self-generated at the nonlinear stage of the electron Weibel instability which drastically scatters particles and leads to higher energy dissipation. Simulation results also suggest more ions are reflected by this EM shock and results in larger energy transfer rate from the laser to ions, which is of advantage for applications such as neutron production and ion fast ignition.

  18. MHD instabilities in accretion mounds on neutron star binaries

    Mukherjee, Dipanjan; Mignone, Andrea

    2013-01-01

    We have numerically solved the Grad-Shafranov equation for axisymmetric static MHD equilibria of matter confined at the polar cap of neutron stars. From the equilibrium solutions we explore the stability of the accretion mounds using the PLUTO MHD code. We find that pressure driven modes disrupt the equilibria beyond a threshold mound mass. This results in formation of dynamic structures inside the mound, as matter spreads over the neutron star surface. Our results show that local variation of magnetic field will significantly affect the shape and nature of the cyclotron features observed in the spectra of High Mass X-ray Binaries.

  19. Instability evolution in shock-accelerated inclined heavy gas cylinder

    Olmstead, Dell; Wayne, Patrick; Vorobieff, Peter; Davis, Daniel; Truman, C. Randall

    2014-11-01

    A heavy gas cylinder interacts with a normal or oblique shockwave at Mach numbers M ranging from 1.13 to 2.0. The angle between the shock front and cylinder axis is varied between 0 and 30°, while the Atwood numbers A range from 0.25 (SF6-N2 mix) to 0.67 (pure SF6). The evolution of the column is imaged in two perpendicular planes with Planar Laser Induced Fluorescence (PLIF). For oblique shock interactions, the nature of the flow is fully three-dimensional, with several instabilities developing in separate directions. In the plane that captures a cross-section of the column, Richtmyer-Meshkov instability (RMI) leads to formation of a pair of counter-rotating vortex columns. A uniform scaling appears to govern the primary instability growth in this plane across the M and A ranges, when the length scale is normalized by a product of the minimum streamwise scale after shock compression and M0.5. In the vertical plane through the column, Kelvin-Helmholtz vortices form with regular spacing along the column. The dominant wavelength of the structures in the vertical plane also appears to scale with the minimum compressed streamwise length. This research is supported by the US DOE National Nuclear Security Administration (NNSA) Grant DE-NA0002220.

  20. Occurrence of instability through the protostellar accretion disks by landing of low-mass condensations

    Elyasi, Mahjubeh

    2016-01-01

    Low-mass condensations (LMCs) are observed inside the envelope of the collapsing molecular cloud cores. In this research, we investigate the effects of landing LMCs for occurrence of instability through the protostellar accretion disks. We consider some regions of the disk where duration of infalling and landing of the LMCs are shorter than the orbital period. In this way, we can consider the landing LMCs as density bumps and grooves in the azimuthal direction of an initial thin axisymmetric steady state self-gravitating protostellar accretion disk (nearly Keplerian). Using the linear effects of the bump quantities, we obtain a characteristic equation for growth/decay rate of bumps; we numerically solve it to find occurrence of instability. We also evaluate the minimum-growth-time-scale (MGTS) and the enhanced mass accretion rate. The results show that infalling and landing of the LMCs in the inner regions of the protostellar accretion disks can cause faster unstable modes and less enhanced accretion rates re...

  1. Evolutionary processes in protoplanetary accretion disks: the propagation of axisymmetric shock waves

    Willerding, Eugen

    1998-12-01

    In this paper we investigate both the global and the local hydrodynamics of axisymmetric accretion disks around young stellar objects under the simultaneous action of viscosity, self-gravity and pressure forces. For simplicity, we take for the global model a polytropic equation of state, make the infinitely thin disk approximation and characterize the surface density and temperature profiles in the disk as power laws in the radial distance r from the protostar. We solve the problem of the general density profile of a Keplerian disk showing that self-gravity could not be an important factor for the fast formation of the rocky cores of giant gaseous planets in our solar system. Under the hypothesis that the unperturbed rotation curve of the disk is nearly Keplerian throughout the radial extent, we can estimate with our polytropic model a lower limit for the resulting masses Md( r) of stable disks up to 100 AU. These masses are in the range of the so-called minimum mass solar nebular ( d/M s ≈ 0.01-0.02 ). By adopting a simplified viscosity model, where the height-integrated turbulent dynamical viscosity ν is a function of the surface density σ like η ∝ σΓ, we derive in the local shearing sheet model linearized evolution equations for small density perturbations describing both a diffusion process and the propagation of acoustic density waves. We solve a special initial value problem and calculate the appropriate Green's function. The analytical solutions so obtained describe in the case Γ Γc the density wave equation describes the propagation of an "overstable" ring-shaped acoustic density wavelet to the outer ranges of the accretion disk. Whereas the group velocity of the wave packet is subsonic, the phase velocities of individual wave crests in the wave packet are supersonic. The mode of maximum instability, the growth rate and the number of growing waves in the wavelet are controlled by Γ and α. Our present knowledge concerning turbulent viscosity in

  2. Nonlinear Weibel Instability and Turbulence in Strong Collisionless Shocks

    Medvedev, Mikhail M.

    2008-08-31

    This research project was devoted to studies of collisionless shocks, their properties, microphysics and plasma physics of underlying phenomena, such as Weibel instability and generation of small-scale fields at shocks, particle acceleration and transport in the generated random fields, radiation mechanisms from these fields in application to astrophysical phenomena and laboratory experiments (e.g., laser-plasma and beam-plasma interactions, the fast ignition and inertial confinement, etc.). Thus, this study is highly relevant to astrophysical sciences, the inertial confinement program and, in particular, the Fast Ignition concept, etc. It makes valuable contributions to the shock physics, nonlinear plasma theory, as well as to the basic plasma science, in general.

  3. Nonlinear Weibel Instability and Turbulence in Strong Collisionless Shocks

    This research project was devoted to studies of collisionless shocks, their properties, microphysics and plasma physics of underlying phenomena, such as Weibel instability and generation of small-scale fields at shocks, particle acceleration and transport in the generated random fields, radiation mechanisms from these fields in application to astrophysical phenomena and laboratory experiments (e.g., laser-plasma and beam-plasma interactions, the fast ignition and inertial confinement, etc.). Thus, this study is highly relevant to astrophysical sciences, the inertial confinement program and, in particular, the Fast Ignition concept, etc. It makes valuable contributions to the shock physics, nonlinear plasma theory, as well as to the basic plasma science, in general

  4. VERTICAL STRUCTURE AND CORONAL POWER OF ACCRETION DISKS POWERED BY MAGNETOROTATIONAL-INSTABILITY TURBULENCE

    In this paper, we consider two outstanding intertwined problems in modern high-energy astrophysics: (1) the vertical-thermal structure of an optically thick accretion disk heated by the dissipation of magnetohydrodynamic turbulence driven by the magnetorotational instability (MRI), and (2) determining the fraction of the accretion power released in the corona above the disk. For simplicity, we consider a gas-pressure-dominated disk and assume a constant opacity. We argue that the local turbulent dissipation rate due to the disruption of the MRI channel flows by secondary parasitic instabilities should be uniform across most of the disk, almost up to the disk photosphere. We then obtain a self-consistent analytical solution for the vertical thermal structure of the disk, governed by the balance between the heating by MRI turbulence and the cooling by radiative diffusion. Next, we argue that the coronal power fraction is determined by the competition between the Parker instability, viewed as a parasitic instability feeding off of MRI channel flows, and other parasitic instabilities. We show that the Parker instability inevitably becomes important near the disk surface, leading to a certain lower limit on the coronal power. While most of the analysis in this paper focuses on the case of a disk threaded by an externally imposed vertical magnetic field, we also discuss the zero net flux case, in which the magnetic field is produced by the MRI dynamo itself, and show that most of our arguments and conclusions should be valid in this case as well

  5. ACCRETION SHOCKS IN CLUSTERS OF GALAXIES AND THEIR SZ SIGNATURE FROM COSMOLOGICAL SIMULATIONS

    Cold dark matter (CDM) hierarchical structure formation models predict the existence of large-scale accretion shocks between the virial and turnaround radii of clusters of galaxies. Kocsis et al. suggest that the Sunyaev-Zel'dovich signal associated with such shocks might be observable with the next generation radio interferometer, ALMA (Atacama Large Millimeter Array). We study the three-dimensional distribution of accretion shocks around individual clusters of galaxies drawn from adaptive mesh refinement (AMR) and smoothed particle hydrodynamics simulations of ΛCDM (dark energy dominated CDM) models. In relaxed clusters, we find two distinct sets of shocks. One set ('virial shocks'), with Mach numbers of 2.5-4, is located at radii 0.9-1.3 R vir, where R vir is the spherical infall estimate of the virial radius, covering about 40%-50% of the total surface area around clusters at these radii. Another set of stronger shocks (external shocks) is located farther out, at about 3 R vir, with large Mach numbers (∼100), covering about 40%-60% of the surface area. We simulate SZ surface brightness maps of relaxed massive galaxy clusters drawn from high-resolution AMR runs, and conclude that ALMA should be capable of detecting the virial shocks in massive clusters of galaxies. More simulations are needed to improve estimates of astrophysical noise and to determine optimal observational strategies.

  6. On the observability of T Tauri accretion shocks in the X-ray band

    Sacco, G G; Argiroffi, C; Maggio, A; Peres, G; Reale, F; Curran, R L

    2010-01-01

    Context. High resolution X-ray observations of classical T Tauri stars (CTTSs) show a soft X-ray excess due to high density plasma (n_e=10^11-10^13 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 accreting material in the X-ray band as a function of the accretion stream properties (velocity, density, and metal abundance) in the case of plasma-beta<<1 in the post-shock zone. Methods. We use a 1-D hydrodynamic model describing the impact of an accretion stream onto the chromosphere, including the effects of radiative cooling, gravity and thermal conduction. We explore the space of relevant parameters and synthesize from the model results the X-ray emission in the [0.5-8.0] keV band and in the resonance lines of O VII (21.60 Ang) and Ne IX (13.45 Ang), taking into account the absorption from the chromosphere. Results. The accretion stream properties influence the temperature and the s...

  7. Supermassive star formation via episodic accretion: protostellar disc instability and radiative feedback efficiency

    Sakurai, Yuya; Hosokawa, Takashi; Yoshida, Naoki; Omukai, Kazuyuki; Yorke, Harold W

    2015-01-01

    The formation of SMSs is a potential pathway to seed SMBHs in the early universe. A critical issue for forming SMSs is stellar UV feedback, which may limit the stellar mass growth via accretion. In this paper we study the evolution of an accreting SMS and its UV emissivity under conditions of realistic variable accretion from a self-gravitating circumstellar disc. First we conduct a 2D hydrodynamical simulation to follow the long-term protostellar accretion until the stellar mass exceeds $10^4~M_\\odot$. The disc fragments due to gravitational instability, creating a number of small clumps that rapidly migrate inward to fall onto the star. The resulting accretion history is thus highly time-dependent: short episodic accretion bursts are followed by longer, relative quiescent phases. We show that the circumstellar disc for the so-called direct collapse model is more unstable and generates greater variability over shorter timescales than normal Pop III cases. We conduct a post-process stellar evolution calculati...

  8. Self-sustained asymmetry of lepton-number emission: a new phenomenon during the supernova shock-accretion phase in three dimensions

    During the stalled-shock phase of our three-dimensional, hydrodynamical core-collapse simulations with energy-dependent, three-flavor neutrino transport, the lepton-number flux (ν e minus ν-bar e) emerges predominantly in one hemisphere. This novel, spherical-symmetry breaking neutrino-hydrodynamical instability is termed LESA for 'Lepton-number Emission Self-sustained Asymmetry'. While the individual ν e and ν-bar e fluxes show a pronounced dipole pattern, the heavy-flavor neutrino fluxes and the overall luminosity are almost spherically symmetric. Initially, LESA seems to develop stochastically from convective fluctuations. It exists for hundreds of milliseconds or more and persists during violent shock sloshing associated with the standing accretion shock instability. The ν e minus ν-bar e flux asymmetry originates predominantly below the neutrinosphere in a region of pronounced proto-neutron star (PNS) convection, which is stronger in the hemisphere of enhanced lepton-number flux. On this side of the PNS, the mass accretion rate of lepton-rich matter is larger, amplifying the lepton-emission asymmetry, because the spherical stellar infall deflects on a dipolar deformation of the stalled shock. The increased shock radius in the hemisphere of less mass accretion and minimal lepton-number flux ( ν-bar e flux maximum) is sustained by stronger convection on this side, which is boosted by stronger neutrino heating due to 〈ϵν-bare〉>〈ϵνe〉. Asymmetric heating thus supports the global deformation despite extremely nonstationary convective overturn behind the shock. While these different elements of the LESA phenomenon form a consistent picture, a full understanding remains elusive at present. There may be important implications for neutrino-flavor oscillations, the neutron-to-proton ratio in the neutrino-heated supernova ejecta, and neutron-star kicks, which remain to be explored.

  9. Physical properties of the inner shocks in hot, tilted black hole accretion flows

    Simulations of hot, pressure-supported, tilted black hole accretion flows, in which the angular momentum of the flow is misaligned with the black hole spin axis, can exhibit two nonaxisymmetric shock structures in the inner regions of the flow. We analyze the strength and significance of these shock structures in simulations with tilt angles of 10° and 15°. By integrating fluid trajectories in the simulations through the shocks and tracking the variations of fluid quantities along these trajectories, we show that these shocks are strong, with substantial compression ratios, in contrast to earlier claims. However, they are only moderately relativistic. We also show that the two density enhancements resembling flow streams in their shape are in fact merely post-shock compressions, as fluid trajectories cut across, rather than flow along, them. The dissipation associated with the shocks is a substantial fraction (≅ 3%-12%) of the rest mass energy advected into the hole, and therefore comparable to the dissipation expected from turbulence. The shocks should therefore make order unity changes in the observed properties of black hole accretion flows that are tilted.

  10. Accretion-Ejection Instability and a "Magnetic Flood" scenario for GRS 1915+105

    Tagger, M

    1999-01-01

    We present an instability, occurring in the inner region of magnetized accretion disks, which seems to be a good candidate to explain the low-frequency QPO observed in many X-ray binaries. We then briefly show how, in the remarkable case of the microquasar GRS 1915+105, identifying this QPO with our instability leads to a scenario for the $\\sim$ 30 mn cycles of this source. In this scenario the cycles are controlled by the build-up of magnetic flux in the disk.

  11. X-ray optical depth diagnostics of T Tauri accretion shocks

    Argiroffi, C; Peres, G; Drake, J J; Santiago, J Lopez; Sciortino, S; Stelzer, B

    2009-01-01

    In classical T Tauri stars, X-rays are produced by two plasma components: a hot low-density plasma, with frequent flaring activity, and a high-density lower temperature plasma. The former is coronal plasma related to the stellar magnetic activity. The latter component, never observed in non-accreting stars, could be plasma heated by the shock formed by the accretion process. However its nature is still being debated. Our aim is to probe the soft X-ray emission from the high-density plasma component in classical T Tauri stars to check whether this is plasma heated in the accretion shock or whether it is coronal plasma. High-resolution X-ray spectroscopy allows us to measure individual line fluxes. We analyze X-ray spectra of the classical T Tauri star MP Muscae and TW Hydrae. Our aim is to evaluate line ratios to search for optical depth effects, which are expected in the accretion-driven scenario. We also derive the plasma emission measure distributions EMD, to investigate whether and how the EMD of accreting...

  12. Ultrahigh energy cosmic rays as heavy nuclei from cluster accretion shocks

    Inoue, Susumu; Miniati, Francesco; Armengaud, Eric

    2007-01-01

    Large-scale accretion shocks around massive clusters of galaxies, generically expected in hierarchical scenarios of cosmological structure formation, are shown to be potential sources of the observed ultrahigh energy cosmic rays (UHECRs) by accelerating a mixture of heavy nuclei including the iron group elements. Current observations can be explained if the source composition at injection for the heavier nuclei is somewhat enhanced from simple expectations for the accreting gas. The proposed picture should be testable by current and upcoming facilities in the near future through characteristic features in the UHECR spectrum, composition and anisotropy. The associated X-ray and gamma-ray signatures are also briefly discussed.

  13. Instability of Non-uniform Toroidal Magnetic Fields in Accretion Disks

    Hirabayashi, Kota

    2016-01-01

    A new type of instability that is expected to drive magnetohydrodynamic (MHD) turbulence from a purely toroidal magnetic field in an accretion disk is presented. It is already known that in a differentially rotating system, the uniform toroidal magnetic field is unstable due to a magnetorotational instability (MRI) under a non-axisymmetric and vertical perturbation, while it is stable under a purely vertical perturbation. Contrary to the previous study, this paper proposes an unstable mode completely confined to the equatorial plane, driven by the expansive nature of the magnetic pressure gradient force under a non-uniform toroidal field. The basic nature of this growing eigenmode, to which we give a name "magneto-gradient driven instability", is studied using linear analysis, and the corresponding nonlinear evolution is then investigated using two-dimensional ideal MHD simulations. Although a single localized magnetic field channel alone cannot provide sufficient Maxwell stress to contribute significantly to...

  14. 3D Finite Volume Simulation of Accretion Discs with Spiral Shocks

    Makita, M; Makita, Makoto; Matsuda, Takuya

    1998-01-01

    We perform 2D and 3D numerical simulations of an accretion disc in a close binary system using the Simplified Flux vector Splitting (SFS) finite volume method. In our calculations, gas is assumed to be the ideal one, and we calculate the cases with gamma=1.01, 1.05, 1.1 and 1.2. The mass ratio of the mass losing star to the mass accreting star is unity. Our results show that spiral shocks are formed on the accretion disc in all cases. In 2D calculations we find that the smaller gamma is, the more tightly the spiral winds. We observe this trend in 3D calculations as well in somewhat weaker sense.

  15. Soft X-Ray Excess from Shocked Accreting Plasma in Active Galactic Nuclei

    Fukumura, Keigo; Hendry, Douglas; Clark, Peter; Tombesi, Francesco; Takahashi, Masaaki

    2016-08-01

    We propose a novel theoretical model to describe the physical identity of the soft X-ray excess that is ubiquitously detected in many Seyfert galaxies, by considering a steady-state, axisymmetric plasma accretion within the innermost stable circular orbit around a black hole (BH) accretion disk. We extend our earlier theoretical investigations on general relativistic magnetohydrodynamic accretion, which implied that the accreting plasma can develop into a standing shock under suitable physical conditions, causing the downstream flow to be sufficiently hot due to shock compression. We perform numerical calculations to examine, for sets of fiducial plasma parameters, the physical nature of fast magnetohydrodynamic shocks under strong gravity for different BH spins. We show that thermal seed photons from the standard accretion disk can be effectively Compton up-scattered by the energized sub-relativistic electrons in the hot downstream plasma to produce the soft excess feature in X-rays. As a case study, we construct a three-parameter Comptonization model of inclination angle θ obs, disk photon temperature kT in, and downstream electron energy kT e to calculate the predicted spectra in comparison with a 60 ks XMM-Newton/EPIC-pn spectrum of a typical radio-quiet Seyfert 1 active galactic nucleus, Ark 120. Our χ 2-analyses demonstrate that the model is plausible for successfully describing data for both non-spinning and spinning BHs with derived ranges of 61.3 keV ≲ kT e ≲ 144.3 keV, 21.6 eV ≲ kT in ≲ 34.0 eV, and 17.°5 ≲ θ obs ≲ 42.°6, indicating a compact Comptonizing region of three to four gravitational radii that resembles the putative X-ray coronae.

  16. An Accretion Model for the Growth of the Central Black Holes Associated with Ionization Instability in Quasars

    Lu, Y.; Cheng, K. S.; Zhang, S. N.

    2003-01-01

    A possible accretion model associated with the ionization instability of quasar disks is proposed to address the growth of the central black hole (BH) harbored in the host galaxy. The evolution of quasars in cosmic time is assumed to change from a highly active state to a quiescent state triggered by the S-shaped ionization instability of the quasar accretion disk. For a given external mass transfer rate supplied by the quasar host galaxy, ionization instability can modify the accretion rate in the disk and separate the accretion flows of the disk into three different phases, like an S-shape. We suggest that the bright quasars observed today are those quasars with disks in the upper branch of the S-shaped instability, and the faint or 'dormant' quasars are simply these systems in the lower branch. The middle branch is the transition state, which is unstable. We assume the quasar disk evolves according to the advection-dominated inflow-outflow solution (ADIOS) configuration in the stable lower branch of the S-shaped instability, and the Eddington accretion rate is used to constrain the accretion rate in the highly active phase. The mass ratio between a BH and its host galactic bulge is a natural consequence of an ADIOS. Our model also demonstrates that a seed BH approx. 2 x 10(exp 6) solar masses similar to those found in spiral galaxies today is needed to produce a BH with a final mass of approx. 2 x 10(exp 8) solar masses.

  17. YSO accretion shocks: magnetic, chromospheric or stochastic flow effects can suppress fluctuations of X-ray emission

    Matsakos, T; Stehlé, C; González, M; Ibgui, L; de Sá, L; Lanz, T; Orlando, S; Bonito, R; Argiroffi, C; Reale, F; Peres, G

    2013-01-01

    Context. Theoretical arguments and numerical simulations of radiative shocks produced by the impact of the accreting gas onto young stars predict quasi-periodic oscillations in the emitted radiation. However, observational data do not show evidence of such periodicity. Aims. We investigate whether physically plausible perturbations in the accretion column or in the chromosphere could disrupt the shock structure influencing the observability of the oscillatory behavior. Methods. We performed local 2D magneto-hydrodynamical simulations of an accretion shock impacting a chromosphere, taking optically thin radiation losses and thermal conduction into account. We investigated the effects of several perturbation types, such as clumps in the accretion stream or chromospheric fluctuations, and also explored a wide range of plasma-\\beta values. Results. In the case of a weak magnetic field, the post-shock region shows chaotic motion and mixing, smoothing out the perturbations and retaining a global periodic signature....

  18. The Accretion-Ejection Instability and a "Magnetic Flood" scenario for GRS 1915+105

    Tagger, M

    2000-01-01

    I present a global view of recent results on the Accretion-Ejection Instability (AEI), described in more details in other contributions to this workshop. These results address essentially the characteristics of the AEI as a good candidate to explain the low-frequency QPO of X-ray binaries, in particular (at $\\sim 1-10$ Hz) of micro-quasars. I then discuss how, if the AEI is considered as the source of the QPO, a possible scenario can be considered where the $\\sim 30$ mn. cycles of GRS 1915+105 are controlled by the evolution of magnetic flux in the disk.

  19. A pure hydrodynamic instability in shear flows and its application to astrophysical accretion disks

    Nath, Sujit Kumar

    2016-01-01

    We provide the possible resolution for the century old problem of hydrodynamic shear flows, which are apparently stable in linear analysis but shown to be turbulent in astrophysically observed data and experiments. This mismatch is noticed in a variety of systems, from laboratory to astrophysical flows. There are so many uncountable attempts made so far to resolve this mismatch, beginning with the early work of Kelvin, Rayleigh, and Reynolds towards the end of the nineteenth century. Here we show that the presence of stochastic noise, whose inevitable presence should not be neglected in the stability analysis of shear flows, leads to pure hydrodynamic linear instability therein. This explains the origin of turbulence, which has been observed/interpreted in astrophysical accretion disks, laboratory experiments and direct numerical simulations. This is, to the best of our knowledge, the first solution to the long standing problem of hydrodynamic instability of Rayleigh stable flows.

  20. Ultrahigh energy cosmic rays as heavy nuclei from cluster accretion shocks

    Inoue, S; Miniati, F; Armengaud, E; Inoue, Susumu; Sigl, Guenter; Miniati, Francesco; Armengaud, Eric

    2007-01-01

    Large-scale accretion shocks around massive clusters of galaxies, generically expected in the cold dark matter scenario of cosmological structure formation, are shown to be plausible sources of the observed ultrahigh energy cosmic rays (UHECRs) by accelerating a mixture of heavy nuclei including the iron group elements. Current observations can be explained if the source composition at injection for the heavier nuclei is somewhat enhanced from simple expectations for the accreting gas. The proposed picture should be clearly testable by current and upcoming facilities in the near future through characteristic features in the UHECR spectrum, composition and anisotropy, in particular the rapid increase of the average mass composition with energy from $10^{19}$ to $10^{20}$ eV.

  1. Ultrahigh energy cosmic rays as heavy nuclei from cluster accretion shocks

    Inoue, Susumu; Sigl, Günter; Miniati, Francesco; et al.

    Large-scale accretion shocks around massive clusters of galaxies, generically expected in hierarchical scenarios of cosmological structure formation, are shown to be plausible sources of the observed ultrahigh energy cosmic rays (UHECRs) by accelerating a mixture of heavy nuclei including the iron group elements. Current observations can be explained if the source composition at injection for the heavier nuclei is somewhat enhanced from simple expectations for the accreting gas. The proposed picture should be clearly testable by current and upcoming facilities in the near future through characteristic features in the UHECR spectrum, composition and anisotropy, in particular the rapid increase of the average mass composition with energy from 1019 to 1020 eV. The associated X-ray and gamma-ray signatures are also briefly discussed.

  2. IP Pegasi Investigation of the accretion disk structure. Searching evidences for spiral shocks in the quiescent accretion disk

    Neustroev, V V; Barwig, H; Bobinger, A; Mantel, K H; Simic, D; Wolf, S

    2002-01-01

    We present the results of spectral investigations of the cataclysmic variable IP Peg in quiescence. Optical spectra obtained on the 6-m telescope at the Special Astrophysical Observatory (Russia), and on the 3.5-m telescope at the German-Spanish Astronomical Center (Calar Alto, Spain), have been analysed by means of Doppler tomography and Phase Modelling Technique. From this analysis we conclude that the quiescent accretion disk of IP Peg has a complex structure. There are also explicit indications of spiral shocks. The Doppler maps and the variations of the peak separation of the emission lines confirm this interpretation. We have detected that all the emission lines show a rather considerable asymmetry of their wings varying with time. The wing asymmetry shows quasi-periodic modulations with a period much shorter than the orbital one. This indicates the presence of an emission source in the binary rotating asynchronously with the binary system. We also have found that the brightness of the bright spot chang...

  3. Double Relics in the Outskirts of A3376: Accretion Flows Meet Merger Shocks?

    Ruta Kale; K. S. Dwarakanath; Joydeep Bagchi; Surajit Paul

    2011-12-01

    The case of spectacular ring-like double radio relics in the merging, rich galaxy cluster A3376 is of great interest to study non-thermal phenomena at cluster outskirts.We present the first low frequency (330 and 150 MHz) images of the double relics using the GMRT. With our GMRT 330 MHz map and the VLA 1400 MHz map (Bagchi et al. 2006), we have constructed and analyzed the distribution of spectral indices over the radio relics. We find flat spectral indices at the outer edges of both the relics and a gradual steepening of spectral indices toward the inner regions. This supports the model of outgoing merger shock waves. The eastern relic has a complex morphology and spectral index distribution toward the inner region. This will be discussed in the context of the effect of large-scale accretion flows on the outgoing merger shocks as reported in the recent simulations.

  4. ASTRO-H White Paper - Stars -- Accretion, Shocks, Charge Exchanges and Magnetic Phenomena

    Tsuboi, Y; Audard, M; Hamaguchi, K; Leutenegger, M A; Maeda, Y; Mori, K; H,; Murakami,; Sugawara, Y; Tsujimoto, M

    2014-01-01

    X-ray emission from stars has origins as diverse as the stars themselves: accretion shocks, shocks generated in wind-wind collisions, or release of magnetic energy. Although the scenarios responsible for X-ray emission are thought to be known, the physical mechanisms operating are in many cases not yet fully understood. Full testing of many of these mechanisms requires high energy resolution, large effective area, and coverage of broad energy bands. The loss of the X-ray calorimeter spectrometer on board ASTRO-E2 was a huge blow to the field; it would have provided a large sample of high resolution spectra of stars with high signal-to-noise ratio. Now, with the advent of the ASTRO-H Soft X-ray Spectrometer and Hard X-ray Imager, we will be able to examine some of the hot topics in stellar astrophysics and solve outstanding mysteries.

  5. Electrostatic and electromagnetic instabilities associated with electrostatic shocks: two-dimensional particle-in-cell simulation

    Kato, Tsunehiko N.; Takabe, Hideaki

    2010-01-01

    A two-dimensional electromagnetic particle-in-cell simulation with the realistic ion-to-electron mass ratio of 1836 is carried out to investigate the electrostatic collisionless shocks in relatively high-speed (~3000 km s^-1) plasma flows and also the influence of both electrostatic and electromagnetic instabilities, which can develop around the shocks, on the shock dynamics. It is shown that the electrostatic ion-ion instability can develop in front of the shocks, where the plasma is under c...

  6. Magnetohydrodynamic modeling of the accretion shocks in classical T Tauri stars: the role of local absorption on the X-ray emission

    Bonito, R; Argiroffi, C; Miceli, M; Peres, G; Matsakos, T; Stehle, C; Ibgui, L

    2014-01-01

    We investigate the properties of X-ray emission from accretion shocks in classical T Tauri stars (CTTSs), generated where the infalling material impacts the stellar surface. Both observations and models of the accretion process reveal several aspects that are still unclear: the observed X-ray luminosity in accretion shocks is below the predicted value, and the density versus temperature structure of the shocked plasma, with increasing densities at higher temperature, deduced from the observations, is at odds with that proposed in the current picture of accretion shocks. To address these open issues we investigate whether a correct treatment of the local absorption by the surrounding medium is crucial to explain the observations. To this end, we describe the impact of an accretion stream on a CTTS by considering a magnetohydrodynamic model. From the model results we synthesize the X-ray emission from the accretion shock by producing maps and spectra. We perform density and temperature diagnostics on the synthe...

  7. Experimental study of the Richtmyer-Meshkov instability induced by a Mach 3 shock wave

    OAK-B135 An experimental investigation of a shock-induced interfacial instability (Richtmyer-Meshkov instability) is undertaken in an effort to study temporal evolution of interfacial perturbations in the late stages of development. The experiments are performed in a vertical shock tube with a square cross-section. A membraneless interface is prepared by retracting a sinusoidally shaped metal plate initially separating carbon dioxide from air, with both gases initially at atmospheric pressure. With carbon dioxide above the plate, the Rayleigh-Taylor instability commences as the plate is retracted and the amplitude of the initial sinusoidal perturbation imposed on the interface begins to grow. The interface is accelerated by a strong shock wave (M=3.08) while its shape is still sinusoidal and before the Kelvin-Helmhotz instability distorts it into the well known mushroom-like structures; its initial amplitude to wavelength ratio is large enough that the interface evolution enters its nonlinear stage very shortly after shock acceleration. The pre-shock evolution of the interface due to the Rayleigh-Taylor instability and the post-shock evolution of the interface due to the Richtmyer-Meshkov instability are visualized using planar Mie scattering. The pre-shock evolution of the interface is carried out in an independent set of experiments. The initial conditions for the Richtmyer-Meshkov experiment are determined from the pre-shock Rayleigh-Taylor growth. One image of the post-shock interface is obtained per experiment and image sequences, showing the post-shock evolution of the interface, are constructed from several experiments. The growth rate of the perturbation amplitude is measured and compared with two recent analytical models of the Richtmyer-Meshkov instability

  8. Magnetic viscosity by localized shear flow instability in magnetized accretion disks

    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 confined between two Alfven singularities at ωd = ± ωA, where ωd is the Doppler-shifted wave frequency, and ωA = k parallel vA is the Alfven frequency. The radial width of the unstable eigenfunction is Δx ∼ ωA/(Aky), where A is the Oort's constant, and ky is the azimuthal wave number. The growth rate of the fundamental mode is larger for smaller value of ky/kz. The maximum growth rate when ky/kz ∼ 0.1 is ∼ 0.2Ω for the Keplerian disk with local angular velocity Ω. It is found that the purely growing mode disappears when ky/kz > 0.12. In a perfectly conducting disk, the instability grows even when the seed magnetic field is infinitesimal. Inclusion of the resistivity, however, leads to the appearance of an instability threshold. When the resistivity η depends on the instability-induced turbulent magnetic fields δB as η([δB2]), the marginal stability condition self-consistently determines the α parameter of the angular momentum transport due to the magnetic stress. For fully ionized disks, the magnetic viscosity parameter αB is between 0.001 and 1. The authors' three-dimensional MHD simulation confirms these unstable eigenmodes. It also shows that the α parameter observed in simulation is between 0.01 and 1, in agreement with theory. The observationally required smaller α in the quiescent phase of accretion disks in dwarf novae may be explained by the decreased ionization due to the temperature drop

  9. Detection of the Entropy of the Intergalactic Medium Accretion Shocks in Clusters, Adiabatic Cores in Groups

    Tozzi, P; Schärf, C A; Tozzi, Paolo; Norman, Colin; Scharf, Caleb

    2000-01-01

    The thermodynamics of the diffuse, X-ray emitting gas in clusters of galaxies is linked to the entropy level of the external intergalactic medium at the epoch of accretion. In particular, models that successfully reproduce the properties of local X-ray clusters and groups require the presence of a minimum value for the entropy of the pre-collapse, intergalactic medium. Such an entropy floor can be generated by non-gravitational phenomena, such as SNe heating, stellar winds and radiative processes. However, there is no consensus on the level, the source or the time evolution of this entropy. In this paper we propose that the best way to investigate the entropy distribution of the local universe is via the detection of the shock fronts expected at the virial boundary of rich clusters, and the extended isentropic gas distribution in the center of low mass clusters and groups. In particular we describe a strategy to look for accretion shocks with the next generation of X-ray facilities (in this case XMM). The mea...

  10. Interaction of the magnetorotational instability with hydrodynamic turbulence in accretion disks

    Workman, Jared C

    2008-01-01

    Accretion disks in which angular momentum transport is dominated by the magnetorotational instability (MRI) can also possess additional, purely hydrodynamic, drivers of turbulence. Even when the hydrodynamic processes, on their own, generate negligible levels of transport, they may still affect the evolution of the disk via their influence on the MRI. Here, we study the interaction between the MRI and hydrodynamic turbulence using local MRI simulations that include hydrodynamic forcing. As expected, we find that hydrodynamic forcing is generally negligible if it yields a saturated kinetic energy density that is small compared to the value generated by the MRI. For stronger hydrodynamic forcing levels, we find that hydrodynamic turbulence modifies transport, with the effect varying depending upon the spatial scale of hydrodynamic driving. Large scale forcing boosts transport by an amount that is approximately linear in the forcing strength, and leaves the character of the MRI (for example the ratio between Max...

  11. Role of local absorption on the X-ray emission from MHD accretion shocks in classical T Tauri stars

    Bonito

    2014-01-01

    Full Text Available Accretion processes onto classical T Tauri stars (CTTSs are believed to generate shocks at the stellar surface due to the impact of supersonic downflowing plasma. Although current models of accretion streams provide a plausible global picture of this process, several aspects are still unclear. For example, the observed X-ray luminosity in accretion shocks is, in general, well below the predicted value. A possible explanation discussed in the literature is in terms of significant absorption of the emission due to the thick surrounding medium. Here we consider a 2D MHD model describing an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere. The model includes all the relevant physics, namely the gravity, the thermal conduction, and the radiative cooling, and a realistic description of the unperturbed stellar atmosphere (from the chromosphere to the corona. From the model results, we synthesize the X-ray emission emerging from the hot slab produced by the accretion shock, exploring different configurations and strengths of the stellar magnetic field. The synthesis includes the local absorption by the thick surrounding medium and the Doppler shift of lines due to the component of plasma velocity along the line-of-sight. We explore the effects of absorption on the emerging X-ray spectrum, considering different inclinations of the accretion stream with respect to the observer. Finally we compare our results with the observations.

  12. Locations of accretion shocks around galaxy clusters and the ICM properties: insights from self-similar spherical collapse with arbitrary mass accretion rates

    Shi, Xun

    2016-09-01

    Accretion shocks around galaxy clusters mark the position where the infalling diffuse gas is significantly slowed down, heated up, and becomes a part of the intracluster medium (ICM). They play an important role in setting the ICM properties. Hydrodynamical simulations have found an intriguing result that the radial position of this accretion shock tracks closely the position of the `splashback radius' of the dark matter, despite the very different physical processes that gas and dark matter experience. Using the self-similar spherical collapse model for dark matter and gas, we find that an alignment between the two radii happens only for a gas with an adiabatic index of γ ≈ 5/3 and for clusters with moderate mass accretion rates. In addition, we find that some observed ICM properties, such as the entropy slope and the effective polytropic index lying around ˜1.1-1.2, are captured by the self-similar spherical collapse model, and are insensitive to the mass accretion history.

  13. Supermassive black hole formation by the cold accretion shocks in the first galaxies

    Inayoshi, Kohei

    2012-01-01

    We propose a new scenario for supermassive star (SMS;>10^5Msun) formation in shocked regions of colliding cold accretion flows near the centers of first galaxies. Recent numerical simulations indicate that assembly of a typical first galaxy with virial temperature (~10^4K) proceeds via cold and dense flows penetrating deep to the center, where the supersonic streams collide each other to develop a hot and dense (~10^4K, ~10^3/cc) shocked gas. The post-shock layer first cools by efficient Ly alpha emission and contracts isobarically until 8000K. Whether the layer continues the isobaric contraction depends on the density at this moment: if the density is high enough for collisionally exciting H2 rovibrational levels (>10^4/cc), enhanced H2 collisional dissociation suppresses the gas to cool further. In this case, the layer fragments into massive (>10^5Msun) clouds, which collapse isothermally (~8000K) by the Ly alpha cooling without subsequent fragmentation. As an outcome, SMSs are expected to form and evolve e...

  14. Magnetic viscosity by localized shear flow instability in magnetized accretion disks

    Matsumoto, R.; Tajima, T.

    1995-01-01

    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 confined between two Alfven singularities at {omega}{sub d} = {+-} {omega}{sub A}, where {omega}{sub d} is the Doppler-shifted wave frequency, and {omega}{sub A} = k{parallel}v{sub A} is the Alfven frequency. The radial width of the unstable eigenfunction is {Delta}x {approximately} {omega}{sub A}/(Ak{sub y}), where A is the Oort`s constant, and k{sub y} is the azimuthal wave number. The growth rate of the fundamental mode is larger for smaller value of k{sub y}/k{sub z}. The maximum growth rate when k{sub y}/k{sub z} {approximately} 0.1 is {approximately} 0.2{Omega} for the Keplerian disk with local angular velocity {Omega}. It is found that the purely growing mode disappears when k{sub y}/k{sub z} > 0.12. In a perfectly conducting disk, the instability grows even when the seed magnetic field is infinitesimal. Inclusion of the resistivity, however, leads to the appearance of an instability threshold. When the resistivity {eta} depends on the instability-induced turbulent magnetic fields {delta}B as {eta}([{delta}B{sup 2}]), the marginal stability condition self-consistently determines the {alpha} parameter of the angular momentum transport due to the magnetic stress. For fully ionized disks, the magnetic viscosity parameter {alpha}{sub B} is between 0.001 and 1. The authors` three-dimensional MHD simulation confirms these unstable eigenmodes. It also shows that the {alpha} parameter observed in simulation is between 0.01 and 1, in agreement with theory. The observationally required smaller {alpha} in the quiescent phase of accretion disks in dwarf novae may be explained by the decreased ionization due to the temperature drop.

  15. X-ray Signatures of Non-Equilibrium Ionization Effects in Galaxy Cluster Accretion Shock Regions

    Wong, Ka-Wah; Ji, Li

    2010-01-01

    The densities in the outer regions of clusters of galaxies are very low, and the collisional timescales are very long. As a result, heavy elements will be under-ionized after they have passed through the accretion shock. We have studied systematically the effects of non-equilibrium ionization for relaxed clusters in the LambdaCDM cosmology using one-dimensional hydrodynamic simulations. We found that non-equilibrium ionization effects do not depend on cluster mass but depend strongly on redshift which can be understood by self-similar scaling arguments. The effects are stronger for clusters at lower redshifts. We present X-ray signatures such as surface brightness profiles and emission lines in detail for a massive cluster at low redshift. In general, soft emission (0.3-1.0 keV) is enhanced significantly by under-ionization, and the enhancement can be nearly an order of magnitude near the shock radius. The most prominent non-equilibrium ionization signature we found is the O VII and O VIII line ratio. The rat...

  16. Radio Observations as a Tool to Investigate Shocks and Asymmetries in Accreting White Dwarf Binaries

    Weston, Jennifer H. S.

    2016-07-01

    This dissertation uses radio observations with the Karl G. Jansky Very Large Array (VLA) to investigate the mechanisms that power and shape accreting white dwarfs (WD) and their ejecta. We test the predictions of both simple spherical and steady-state radio emission models by examining nova V1723 Aql, nova V5589 Sgr, symbiotic CH Cyg, and two small surveys of symbiotic binaries. First, we highlight classical nova V1723 Aql with three years of radio observations alongside optical and X-ray observations. We use these observations to show that multiple outflows from the system collided to create early non-thermal shocks with a brightness temperature of ≥106 K. While the late-time radio light curve is roughly consistent an expanding thermal shell of mass 2x10-4 M⊙ solar masses, resolved images of V1723 Aql show elongated material that apparently rotates its major axis over the course of 15 months, much like what is seen in gamma-ray producing nova V959 Mon, suggesting similar structures in the two systems. Next, we examine nova V5589 Sgr, where we find that the early radio emission is dominated by a shock-powered non-thermal flare that produces strong (kTx > 33 keV) X-rays. We additionally find roughly 10-5 M⊙ solar masses of thermal bremsstrahlung emitting material, all at a distance of ~4 kpc. The similarities in the evolution of both V1723 Aql and V5589 Sgr to that of nova V959 Mon suggest that these systems may all have dense equatorial tori shaping faster flows at their poles. Turning our focus to symbiotic binaries, we first use our radio observations of CH Cyg to link the ejection of a collimated jet to a change of state in the accretion disk. We additionally estimate the amount of mass ejected during this period (10-7 M⊙ masses), and improve measurements of the period of jet precession (P=12013 ± 74 days). We then use our survey of eleven accretion-driven symbiotic systems to determine that the radio brightness of a symbiotic system could potentially

  17. Cosmic-ray-induced filamentation instability in collisionless shocks

    Caprioli, D

    2012-01-01

    We used unprecedentedly large 2D and 3D hybrid (kinetic ions - fluid electrons) simulations of non-relativistic collisionless strong shocks in order to investigate the effects of self-consistently accelerated ions on the overall shock dynamics. The current driven by suprathermal particles streaming ahead of the shock excites modes transverse to the background magnetic field. The Lorentz force induced by these self-amplified fields tends to excavate tubular, underdense, magnetic-field-depleted cavities that are advected with the fluid and perturb the shock surface, triggering downstream turbulent motions. These motions further amplify the magnetic field, up to factors of 50-100 in knot-like structures. Once downstream, the cavities tend to be filled by hot plasma plumes that compress and stretch the magnetic fields in elongated filaments; this effect is particularly evident if the shock propagates parallel to the background field. Highly-magnetized knots and filaments may provide explanations for the rapid X-r...

  18. Origin of nonlinearity and plausible turbulence by hydromagnetic transient growth in accretion disks: faster growth rate than magnetorotational instability

    Nath, Sujit Kumar

    2015-01-01

    We investigate the evolution of hydromagnetic perturbations in a small section of accretion disks. It is known that molecular viscosity is negligible in accretion disks. Hence, it has been argued that a mechanism, known as Magnetorotational Instability (MRI), is responsible for transporting matter in the presence of weak magnetic field. However, there are some shortcomings, which question effectiveness of MRI. Now the question arises, whether other hydromagnetic effects, e.g. transient growth (TG), can play important role to bring nonlinearity in the system, even at weak magnetic fields. Otherwise, whether MRI or TG, which is primarily responsible to reveal nonlinearity to make the flow turbulent? Our results prove explicitly that the flows with high Reynolds number (Re ), which is the case of realistic astrophysical accretion disks, exhibit nonlinearity by TG of perturbation modes faster than that by modes producing MRI. For a fixed wave vector, MRI dominates over transient effects, only at low Re , lower th...

  19. Emergence of nonlinearity and plausible turbulence in accretion disks via hydromagnetic transient growth faster than magnetorotational instability

    Nath, Sujit K

    2016-01-01

    We investigate the evolution of hydromagnetic perturbations in a small section of accretion disks. It is known that molecular viscosity is negligible in accretion disks. Hence, it has been argued that Magnetorotational Instability (MRI) is responsible for transporting matter in the presence of weak magnetic field. However, there are some shortcomings, which question effectiveness of MRI. Now the question arises, whether other hydromagnetic effects, e.g. transient growth (TG), can play an important role to bring nonlinearity in the system, even at weak magnetic fields. Otherwise, whether MRI or TG, which is primarily responsible to reveal nonlinearity to make the flow turbulent? Our results prove explicitly that the flows with high Reynolds number (Re), which is the case of realistic astrophysical accretion disks, exhibit nonlinearity by best TG of perturbation modes faster than that by best modes producing MRI. For a fixed wavevector, MRI dominates over transient effects, only at low Re, lower than its value ...

  20. Hybrid viscosity and the magnetoviscous instability in hot, collisionless accretion disks

    Subramanian, Prasad; Kafatos, Menas

    2008-01-01

    We aim to illustrate the role of hot protons in enhancing the magnetorotational instability (MRI) via the ``hybrid'' viscosity, which is due to the redirection of protons interacting with static magnetic field perturbations, and to establish that it is the only relevant mechanism in this situation. It has recently been shown by Balbus \\cite{PBM1} and Islam & Balbus \\cite{PBM11} using a fluid approach that viscous momentum transport is key to the development of the MRI in accretion disks for a wide range of parameters. However, their results do not apply in hot, advection-dominated disks, which are collisionless. We develop a fluid picture using the hybrid viscosity mechanism, that applies in the collisionless limit. We demonstrate that viscous effects arising from this mechanism can significantly enhance the growth of the MRI as long as the plasma $\\beta \\gapprox 80$. Our results facilitate for the first time a direct comparison between the MHD and quasi-kinetic treatments of the magnetoviscous instabilit...

  1. Bow-shock instability induced by Helmholtz resonator-like feedback in slipstream

    Ohnishi, Naofumi; Sato, Yosuke; Kikuchi, Yuta; Ohtani, Kiyonobu; Yasue, Kanako

    2015-06-01

    Bow-shock instability has been experimentally observed in a low-γ flow. To clarify its mechanism, a parametric study was conducted with three-dimensional numerical simulations for specific heat ratio γ and Mach number M. A critical boundary of the instability was found in the γ-M parametric space. The bow shock tends to be unstable with low γ and high M, and the experimental demonstration was designed based on this result. The experiments were conducted with the ballistic range of the single-stage powder gun mode using HFC-134a of γ = 1.12 at Mach 9.6. Because the deformation of the shock front was observed in a shadowgraph image, the numerical prediction was validated to some extent. The theoretical estimation of vortex formation in a curved shock wave indicates that the generated vorticity is proportional to the density ratio across the shock front and that the critical density ratio can be predicted as ˜10. A strong slipstream from the surface edge generates noticeable acoustic waves because it can be deviated by the upstream flow. The acoustic waves emitted by synchronizing the vortex formation can propagate upstream and may trigger bow-shock instability. This effect should be emphasized in terms of unstable shock formation around an edged flat body.

  2. Hydrodynamic Modeling of Accretion Impacts in Classical T Tauri Stars: Radiative Heating of the Pre-shock Plasma

    Costa, G; Peres, G; Argiroffi, C; Bonito, R

    2016-01-01

    Context. It is generally accepted that, in Classical T Tauri Stars, the plasma from the circumstellar disc accretes onto the stellar surface with free fall velocity, and the impact generates a shock. The impact region is expected to contribute to emission in different spectral bands; many studies have confirmed that the X-rays arise from the post-shock plasma but, otherwise, there are no studies in the literature investigating the origin of the observed UV emission which is apparently correlated to accretion. Aims. We investigated the effect of radiative heating of the infalling material by the post-shock plasma at the base of the accretion stream with the aim to identify in which region a significant part of the UV emission originates. Methods. We developed a 1D hydrodynamic model describing the impact of an accretion stream onto the stellar surface; the model takes into account the gravity, the radiative cooling of an optically thin plasma, the thermal conduction, and the heating due to absorption of X-ray ...

  3. Experimental demonstration of bow-shock instability and its numerical analysis

    Kikuchi, Y.; Ohnishi, N.; Ohtani, K.

    2016-07-01

    An experimental demonstration was carried out in a ballistic range at high Mach numbers with the low specific heat ratio gas hydrofluorocarbon HFC-134a to observe the unstable bow-shock wave generated in front of supersonic blunt objects. The shadowgraph images obtained from the experiments showed instability characteristics, in which the disturbances grow and flow downstream and the wake flow appears wavy because of the shock oscillation. Moreover, the influence of the body shape and specific heat ratio on the instability was investigated for various experimental conditions. Furthermore, the observed features, such as wave structure and disturbance amplitude, were captured by numerical simulations, and it was demonstrated that computational fluid dynamics could effectively simulate the physical instability. In addition, it was deduced that the shock instability is induced by sound emissions from the edge of the object. This inference supports the dependence of the instability on the specific heat ratio and Mach number because the shock stand-off distance is affected by these parameters and limits the sound wave propagation.

  4. MAGNETOHYDRODYNAMIC MODELING OF THE ACCRETION SHOCKS IN CLASSICAL T TAURI STARS: THE ROLE OF LOCAL ABSORPTION IN THE X-RAY EMISSION

    We investigate the properties of X-ray emission from accretion shocks in classical T Tauri stars (CTTSs), generated where the infalling material impacts the stellar surface. Both observations and models of the accretion process reveal several aspects that are still unclear: the observed X-ray luminosity in accretion shocks is below the predicted value, and the density versus temperature structure of the shocked plasma, with increasing densities at higher temperature, deduced from the observations, is at odds with that proposed in the current picture of accretion shocks. To address these open issues, we investigate whether a correct treatment of the local absorption by the surrounding medium is crucial to explain the observations. To this end, we describe the impact of an accretion stream on a CTTS by considering a magnetohydrodynamic model. From the model results, we synthesize the X-ray emission from the accretion shock by producing maps and spectra. We perform density and temperature diagnostics on the synthetic spectra, and we directly compare the results with observations. Our model shows that the X-ray fluxes inferred from the emerging spectra are lower than expected because of the complex local absorption by the optically thick material of the chromosphere and of the unperturbed stream. Moreover, our model, including the effects of local absorption, explains in a natural way the apparently puzzling pattern of density versus temperature observed in the X-ray emission from accretion shocks

  5. COSMIC-RAY-INDUCED FILAMENTATION INSTABILITY IN COLLISIONLESS SHOCKS

    We used unprecedentedly large two-dimensional and three-dimensional hybrid (kinetic ions—fluid electrons) simulations of non-relativistic collisionless strong shocks in order to investigate the effects of self-consistently accelerated ions on the overall shock dynamics. The current driven by suprathermal particles streaming ahead of the shock excites modes transverse to the background magnetic field. The Lorentz force induced by these self-amplified fields tends to excavate tubular, underdense, magnetic-field-depleted cavities that are advected with the fluid and perturb the shock surface, triggering downstream turbulent motions. These motions further amplify the magnetic field, up to factors of 50-100 in knot-like structures. Once downstream, the cavities tend to be filled by hot plasma plumes that compress and stretch the magnetic fields in elongated filaments; this effect is particularly evident if the shock propagates parallel to the background field. Highly magnetized knots and filaments may provide explanations for the rapid X-ray variability observed in RX J1713.7–3946 and for the regular pattern of X-ray bright stripes detected in Tycho's supernova remnant.

  6. On physical and numerical instabilities arising in simulations of non-stationary radiatively cooling shocks

    Badjin, D. A.; Glazyrin, S. I.; Manukovskiy, K. V.; Blinnikov, S. I.

    2016-06-01

    We describe our modelling of the radiatively cooling shocks and their thin shells with various numerical tools in different physical and calculational setups. We inspect structure of the dense shell, its formation and evolution, pointing out physical and numerical factors that sustain its shape and also may lead to instabilities. We have found that under certain physical conditions, the circular shaped shells show a strong bending instability and successive fragmentation on Cartesian grids soon after their formation, while remain almost unperturbed when simulated on polar meshes. We explain this by physical Rayleigh-Taylor-like instabilities triggered by corrugation of the dense shell surfaces by numerical noise. Conditions for these instabilities follow from both the shell structure itself and from episodes of transient acceleration during re-establishing of dynamical pressure balance after sudden radiative cooling onset. They are also easily excited by physical perturbations of the ambient medium. The widely mentioned non-linear thin shell instability, in contrast, in tests with physical perturbations is shown to have only limited chances to develop in real radiative shocks, as it seems to require a special spatial arrangement of fluctuations to be excited efficiently. The described phenomena also set new requirements on further simulations of the radiatively cooling shocks in order to be physically correct and free of numerical artefacts.

  7. Long-term quasi-periodicity of 4U 1636-536 resulting from accretion disc instability

    Wisniewicz, Mateusz; Gondek-Rosinska, Dorota; Zdziarski, Andrzej A; Janiuk, Agnieszka

    2015-01-01

    We present the results of a study of the low-mass X-ray binary 4U 1636-536. We have performed temporal analysis of all available RXTE/ASM, Swift/BAT and MAXI data. We have confirmed the previously discovered quasi-periodicity of ~45 d present during ~2004, however we found it continued to 2006. At other epochs, the quasi-periodicity is only transient, and the quasi-period, if present, drifts. We have then applied a time-dependent accretion disc model to the interval with the significant X-ray quasi-periodicity. For our best model, the period and the amplitude of the theoretical light curve agree well with that observed. The modelled quasi-periodicity is due to the hydrogen thermal-ionization instability occurring in outer regions of the accretion disc. The model parameters are the average mass accretion rate (estimated from the light curves), and the accretion disc viscosity parameters, for the hot and cold phases. Our best model gives relatively low values of viscosity parameter for cold phase 0.01 and for h...

  8. Instabilities in large economies: aggregate volatility without idiosyncratic shocks

    Bonart, Julius; Bouchaud, Jean-Philippe; Landier, Augustin; Thesmar, David

    2014-10-01

    We study a dynamical model of interconnected firms which allows for certain market imperfections and frictions, restricted here to be myopic price forecasts and slow adjustment of production. Whereas the standard rational equilibrium is still formally a stationary solution of the dynamics, we show that this equilibrium becomes linearly unstable in a whole region of parameter space. When agents attempt to reach the optimal production target too quickly, coordination breaks down and the dynamics becomes chaotic. In the unstable, ‘turbulent’ phase, the aggregate volatility of the total output remains substantial even when the amplitude of idiosyncratic shocks goes to zero or when the size of the economy becomes large. In other words, crises become endogenous. This suggests an interesting resolution of the ‘small shocks, large business cycles’ puzzle.

  9. Instabilities in large economies: aggregate volatility without idiosyncratic shocks

    We study a dynamical model of interconnected firms which allows for certain market imperfections and frictions, restricted here to be myopic price forecasts and slow adjustment of production. Whereas the standard rational equilibrium is still formally a stationary solution of the dynamics, we show that this equilibrium becomes linearly unstable in a whole region of parameter space. When agents attempt to reach the optimal production target too quickly, coordination breaks down and the dynamics becomes chaotic. In the unstable, ‘turbulent’ phase, the aggregate volatility of the total output remains substantial even when the amplitude of idiosyncratic shocks goes to zero or when the size of the economy becomes large. In other words, crises become endogenous. This suggests an interesting resolution of the ‘small shocks, large business cycles’ puzzle. (paper)

  10. Bow shock fragmentation driven by a thermal instability in laboratory-astrophysics experiments

    Suzuki-Vidal, F; Ciardi, A; Pickworth, L A; Rodriguez, R; Gil, J M; Espinosa, G; Hartigan, P; Swadling, G F; Skidmore, J; Hall, G N; Bennett, M; Bland, S N; Burdiak, G; de Grouchy, P; Music, J; Suttle, L; Hansen, E; Frank, A

    2015-01-01

    The role of radiative cooling during the evolution of a bow shock was studied in laboratory-astrophysics experiments that are scalable to bow shocks present in jets from young stellar objects. The laboratory bow shock is formed during the collision of two counter-streaming, supersonic plasma jets produced by an opposing pair of radial foil Z-pinches driven by the current pulse from the MAGPIE pulsed-power generator. The jets have different flow velocities in the laboratory frame and the experiments are driven over many times the characteristic cooling time-scale. The initially smooth bow shock rapidly develops small-scale non-uniformities over temporal and spatial scales that are consistent with a thermal instability triggered by strong radiative cooling in the shock. The growth of these perturbations eventually results in a global fragmentation of the bow shock front. The formation of a thermal instability is supported by analysis of the plasma cooling function calculated for the experimental conditions with...

  11. Detection of a Cool, Accretion-Shock-Generated X-Ray Plasma in EX Lupi During the 2008 Optical Eruption

    Teets, William K.; Weintraub, David A.; Kastner, Joel H.; Grosso, Nicholas; Hamaguchi, Kenji; Richmond, Michael

    2012-01-01

    EX Lupi is the prototype for a class of young, pre-main-sequence stars which are observed to undergo irregular, presumably accretion-generated, optical outbursts that result in a several magnitude rise of the optical flux. EX Lupi was observed to optically erupt in 2008 January, triggering Chandra ACIS Target of Opportunity observations shortly thereafter. We find very strong evidence that most of the X-ray emission in the first few months after the optical outburst is generated by accretion of circumstellar material onto the stellar photosphere. Specifically, we find a strong correlation between the decreasing optical and X-ray fluxes following the peak of the outburst in the optical, which suggests that these observed declines in both the optical and X-ray fluxes are the result of declining accretion rate. In addition, in our models of the X-ray spectrum, we find strong evidence for an approx 0.4 keV plasma component, as expected for accretion shocks on low-mass, pre-main-sequence stars. From 2008 March through October, this cool plasma component appeared to fade as EX Lupi returned to its quiescent level in the optical, consistent with a decrease in the overall emission measure of accretion-shock-generated plasma. The overall small increase of the X-ray flux during the optical outburst of EX Lupi is similar to what was observed in previous X-ray observations of the 2005 optical outburst of the EX Lupi-type star V1118 Ori but contrasts with the large increase of the X-ray flux from the erupting young star V1647 Ori during its 2003 and 2008 optical outbursts.

  12. The fate or organic matter during planetary accretion - Preliminary studies of the organic chemistry of experimentally shocked Murchison meteorite

    Tingle, Tracy N.; Tyburczy, James A.; Ahrens, Thomas J.; Becker, Christopher H.

    1992-01-01

    The fate of organic matter in carbonaceous meteorites during hypervelocity (1-2 km/sec) impacts is investigated using results of experiments in which three samples of the Murchison (CM2) carbonaceous chondrite were shocked to 19, 20, and 36 GPa and analyzed by highly sensitive thermal-desorption photoionization mass spectrometry (SALI). The thermal-desorptive SALI mass spectra of unshocked CM2 material revealed presence of indigenous aliphatic, aromatic, sulfur, and organosulfur compounds, and samples shocked to about 20 GPa showed little or no loss of organic matter. On the other hand, samples shocked to 36 GPa exhibited about 70 percent loss of organic material and a lower alkene/alkane ratio than did the starting material. The results suggest that it is unlikely that the indigenous organic matter in carbonaceous chondritelike planetesimals could have survived the impact on the earth in the later stages of earth's accretion.

  13. The multipolar magnetic fields of accreting pre-main-sequence stars: B at the inner disk, B along the accretion flow, and B at the accretion shock

    Gregory, Scott G; Hussain, Gaitee A J

    2016-01-01

    Zeeman-Doppler imaging studies have revealed the complexity of the large-scale magnetic fields of accreting pre-main-sequence stars. All have multipolar magnetic fields with the octupole component being the dominant field mode for many of the stars studied thusfar. Young accreting stars with fully convective interiors often feature simple axisymmetric magnetic fields with dipole components of order a kilo-Gauss (at least those of mass $\\gtrsim0.5\\,{\\rm M}_\\odot$), while those with substantially radiative interiors host more complex non-axisymmetric magnetic fields with dipole components of order a few 0.1 kilo-Gauss. Here, via several simple examples, we demonstrate that i). in most cases, the dipole component alone can be used to estimate the disk truncation radius (but little else); ii) due the presence of higher order magnetic field components, the field strength in the accretion spots is far in excess of that expected if a pure dipole magnetic field is assumed. (Fields of $\\sim$6$\\,{\\rm kG}$ have been mea...

  14. ELECTRON HEATING BY THE ION CYCLOTRON INSTABILITY IN COLLISIONLESS ACCRETION FLOWS. I. COMPRESSION-DRIVEN INSTABILITIES AND THE ELECTRON HEATING MECHANISM

    Sironi, Lorenzo [NASA Einstein Postdoctoral Fellow. (United States); Narayan, Ramesh, E-mail: lsironi@cfa.harvard.edu, E-mail: rnarayan@cfa.harvard.edu [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

    2015-02-20

    In systems accreting well below the Eddington rate, such as the central black hole in the Milky Way (Sgr A*), the plasma in the innermost regions of the disk is believed to be collisionless and have two temperatures, with the ions substantially hotter than the electrons. However, whether a collisionless faster-than-Coulomb energy transfer mechanism exists in two-temperature accretion flows is still an open question. We study the physics of electron heating during the growth of ion velocity-space instabilities by means of multidimensional, fully kinetic, particle-in-cell (PIC) simulations. A background large-scale compression—embedded in a novel form of the PIC equations—continuously amplifies the field. This constantly drives a pressure anisotropy P > P {sub ∥} because of the adiabatic invariance of the particle magnetic moments. We find that, for ion plasma beta values β{sub 0i} ∼ 5-30 appropriate for the midplane of low-luminosity accretion flows (here, β{sub 0i} is the ratio of ion thermal pressure to magnetic pressure), mirror modes dominate if the electron-to-proton temperature ratio is T {sub 0e}/T {sub 0i} ≳ 0.2, whereas for T {sub 0e}/T {sub 0i} ≲ 0.2 the ion cyclotron instability triggers the growth of strong Alfvén-like waves, which pitch-angle scatter the ions to maintain marginal stability. We develop an analytical model of electron heating during the growth of the ion cyclotron instability, which we validate with PIC simulations. We find that for cold electrons (β{sub 0e} ≲ 2 m{sub e} /m{sub i} , where β{sub 0e} is the ratio of electron thermal pressure to magnetic pressure), the electron energy gain is controlled by the magnitude of the E-cross-B velocity induced by the ion cyclotron waves. This term is independent of the initial electron temperature, so it provides a solid energy floor even for electrons starting with extremely low temperatures. On the other hand, the electron energy gain for β{sub 0e} ≳ 2 m{sub e} /m{sub i}

  15. The numerical study of shock-induced hydrodynamic instability and mixing

    Wang Tao; Bai Jing-Song; Li Ping; Zhong Min

    2009-01-01

    Based on multi-fluid volume fraction and piecewise parabolic method (PPM), a multi-viscosity-fluid hydrodynamic code MVPPM (Multi-Viscosity-Fluid Piecewise Parabolic Method) is developed and applied to the problems of shock-induced hydrodynamic interfacial instability and mixing. Simulations of gas/liquid interface instability show that the influences of initial perturbations on the fluid mixing zone (FMZ) growth are significant, especially at the late stages, while grids have only a slight effect on the FMZ width, when the interface is impulsively accelerated by a shock wave passing through it. A numerical study of the hydrodynamic interfacial instability and mixing of gaseous flows impacted by re-shocks is presented. It reveals that the numerical results are in good agreement with the experimental results and the mixing growth rate strongly depends on initial conditions. Ultimately, the jelly layer experiment relevant to the instability impacted by exploding is simulated. The shape of jelly interface, position of front face of jelly layer, crest and trough of perturbation versus time are given; their simulated results are in good agreement with experimental results.

  16. On physical and numerical instabilities arising in simulations of non-stationary radiatively cooling shocks

    Badjin, D A; Manukovskiy, K V; Blinnikov, S I

    2015-01-01

    We describe our experience of modelling of the radiatively cooling shocks and their thin shells with various numerical tools in different physical and calculational setups. We have found that under certain physical conditions, the circular shaped shells show a strong bending instability and successive fragmentation on Cartesian grids soon after their formation, while remain almost unperturbed when simulated on polar meshes. We explain these results as an interplay of numerical perturbations superimposed by grids not aligned to the flow lines, and a physical Rayleigh--Taylor like instability of the thin shell inner boundary being accelerated during re-estabilshing of pressure balance within and behind the shell after preceding sudden temperature loss. This phenomenon also sets new requirements on further radiatively cooling shocks simulations in order to be physically correct and free of numerical artefacts.

  17. A Local Model for Angular Momentum Transport in Accretion Disks Driven by the Magnetorotational Instability

    Pessah, Martin Elias; Chan, Chi-kwan; Psaltis, Dimitrios

    2006-01-01

    We develop a local model for the exponential growth and saturation of the Reynolds and Maxwell stresses in turbulent flows driven by the magnetorotational instability. We first derive equations that describe the effects of the instability on the growth and pumping of the stresses. We highlight th...

  18. Suppression of transverse instabilities of dark solitons and their dispersive shock waves

    Armaroli, Andrea

    2009-11-03

    We investigate the impact of nonlocality, owing to diffusive behavior, on transverse instabilities of a dark stripe propagating in a defocusing cubic medium. The nonlocal response turns out to have a strongly stabilizing effect both in the case of a single soliton input and in the regime where dispersive shock waves develop (multisoliton regime). Such conclusions are supported by the linear stability analysis and numerical simulation of the propagation. © 2009 The American Physical Society.

  19. Magnetic field generation by Biermann battery and Weibel instability in laboratory shock waves

    Gregori, G.; Miniati, F.; Reville, B.; Drake, R. P.

    2012-02-01

    Magnetic field generation in the Universe is still an open problem. Possible mechanisms involve the Weibel instability, due to anisotropic phase-space distributions, as well as the Biermann battery, due to misaligned density and temperature gradients. These mechanisms can be reproduced in scaled laboratory experiments. In this contribution we estimate the relative importance of these two processes and explore the laser-energy requirements for producing Weibel dominated shocks.

  20. Pulsational instability of supergiant protostars: Do they grow supermassive by accretion?

    Inayoshi, Kohei; Omukai, Kazuyuki

    2013-01-01

    Supermassive stars (SMSs; M>10^5 Msun) and their remnant black holes are promising progenitors for supermassive black holes (SMBHs) observed in the early universe at z>7. It has been postulated that SMSs forms through very rapid mass accretion onto a protostar at a high rate exceeding 0.01 Msun/yr. According to recent studies, such rapidly accreting protostars evolve into "supergiant protostars", i.e. protostars consisting of a bloated envelope and a contracting core, similar to giant star. However, like massive stars as well as giant stars, both of which are known to be pulsationally unstable, supergiant protostars may also be also unstable to launch strong pulsation-driven outflows. If this is the case, the stellar growth via accretion will be hindered by the mass loss. We here study the pulsational stability of the supergiant protostars in the mass range M600 Msun and very high accretion rate Mdot>1.0 Msun/yr are unstable due to the kappa mechanism. The pulsation is excited in the He^+ ionization layer in ...

  1. Effects of Toroidal Magnetic Fields on the Thermal Instability of Thin Accretion Disks

    Sheng-Ming Zheng; Feng Yuan; Wei-Min Gu; Ju-Fu Lu

    2011-03-01

    The standard thin disk model predicts that when the accretion rate is moderately high, the disk is radiation–pressure-dominated and thermally unstable. However, observations indicate the opposite, namely the disk is quite stable. We present an explanation in this work by taking into account the role of the magnetic field which was ignored in the previous analysis.

  2. A high-order Godunov scheme for global 3D MHD accretion disks simulations. I. The linear growth regime of the magneto-rotational instability

    Flock, M; Klahr, H; Mignone, A

    2009-01-01

    We employ the PLUTO code for computational astrophysics to assess and compare the validity of different numerical algorithms on simulations of the magneto-rotational instability in 3D accretion disks. In particular we stress on the importance of using a consistent upwind reconstruction of the electro-motive force (EMF) when using the constrained transport (CT) method to avoid the onset of numerical instabilities. We show that the electro-motive force (EMF) reconstruction in the classical constrained transport (CT) method for Godunov schemes drives a numerical instability. The well-studied linear growth of magneto-rotational instability (MRI) is used as a benchmark for an inter-code comparison of PLUTO and ZeusMP. We reproduce the analytical results for linear MRI growth in 3D global MHD simulations and present a robust and accurate Godunov code which can be used for 3D accretion disk simulations in curvilinear coordinate systems.

  3. Three dimensional simulations of Richtmyer-Meshkov instabilities in gas-curtain shock-tube experiments

    It is not feasible to compute high Reynolds-number (Re) turbulent flows by directly resolving all scales of motion and material interfaces; instead, macroscale portions of the unsteady turbulent motion are computed while the rest of the flow physics including molecular diffusion and other micro scale physics (e.g., combustion) remains unresolved. In large eddy simulation (LES), the large energy containing structures are resolved whereas the smaller, presumably more isotropic, structures are filtered out and their unresolved subgrid scale (SGS) effects are modeled. The construction of SGS models for LES is pragmatic and based primarily on empirical information. Adding to the physics based difficulties in developing and validating SGS models, truncation terms due to discretization are comparable to SGS models in typical LES strategies, and LES resolution requirements become prohibitively expensive for practical flows and regimes. Implicit LES (ILES) - and monotone integrated LES (MILES) introduced earlier, effectively address the seemingly insurmountable issues posed to LES by underresolution, by relying on the use of SGS modeling and filtering provided implicitly by physics capturing numerics. Extensive work has demonstrated that predictive unresolved simulations of turbulent velocity fields are possible using any of the class of nonoscillatory finite-volume (NFV) numerical algorithms. Popular NFV methods such as flux-corrected transport (FCT), the piecewise parabolic method (PPM), total variation diminishing (TVD), and hybrid algorithms are being used for ILES. In many applications of interest, turbulence is generated by shock waves via Richtmyer-Meshkov instabilities (RMI). The instability results in vorticity being introduced at material interfaces by the impulsive loading of the shock wave. A critical feature of this impulsive driving is that the turbulence decays as dissipation removes kinetic energy from the system. RMI add the complexity of shock waves and

  4. Internal shocks driven by accretion flow variability in the compact jet of the black hole binary GX 339-4

    Drappeau, S.; Malzac, J.; Belmont, R.; Gandhi, P.; Corbel, S.

    2015-03-01

    In recent years, compact jets have been playing a growing role in the understanding of accreting black hole engines. In the case of X-ray binary systems, compact jets are usually associated with the hard state phase of a source outburst. Recent observations of GX 339-4 have demonstrated the presence of a variable synchrotron spectral break in the mid-infrared band that was associated with its compact jet. In the model used in this study, we assume that the jet emission is produced by electrons accelerated in internal shocks driven by rapid fluctuations of the jet velocity. The resulting spectral energy distribution (SED) and variability properties are very sensitive to the Fourier power spectrum density (PSD) of the assumed fluctuations of the jet Lorentz factor. These fluctuations are likely to be triggered by the variability of the accretion flow which is best traced by the X-ray emission. Taking the PSD of the jet Lorentz factor fluctuations to be identical to the observed X-ray PSD, our study finds that the internal shock model successfully reproduces the radio to infrared SED of the source at the time of the observations as well as the reported strong mid-infrared spectral variability.

  5. Normal velocity freeze-out of the Richtmyer-Meshkov instability when a shock is reflected

    It is known that for some values of the initial parameters that define the Richtmyer-Meshkov instability, the normal velocity at the contact surface vanishes asymptotically in time. This phenomenon, called freeze-out, is studied here with an exact analytic model. The instability freeze-out, already considered by previous authors [K. O. Mikaelian, Phys. Fluids 6, 356 (1994), Y. Yang, Q. Zhang, and D. H. Sharp, Phys. Fluids 6, 1856 (1994), and A. L. Velikovich, Phys. Fluids 8, 1666 (1996)], is the result of a subtle interaction between the unstable surface and the corrugated shock fronts. In particular, it is seen that the transmitted shock at the contact surface plays a key role in determining the asymptotic behavior of the normal velocity at the contact surface. By properly tuning the fluids compressibilities, the density jump, and the incident shock Mach number, the value of the initial circulation deposited by the reflected and transmitted shocks at the material interface can be adjusted in such a way that the normal growth at the contact surface will vanish for large times. The conditions for this to happen are calculated exactly, by expressing the initial density ratio as a function of the other parameters of the problem: fluids compressibilities and incident shock Mach number. This is done by means of a linear theory model developed in a previous work [J. G. Wouchuk, Phys. Rev. E. 63, 056303 (2001)]. A general and qualitative criterion to decide the conditions for freezing-out is derived, and the evolution of different cases (freeze-out and non-freeze-out) are studied with some detail. A comparison with previous works is also presented

  6. Shear-driven instabilities and shocks in the atmospheres of hot Jupiters

    Fromang, Sébastien; Leconte, Jeremy; Heng, Kevin

    2016-07-01

    Context. General circulation models of the atmosphere of hot Jupiters have shown the existence of a supersonic eastward equatorial jet. These results have been obtained using numerical schemes that filter out vertically propagating sound waves and assume vertical hydrostatic equilibrium, or were acquired with fully compressive codes that use large dissipative coefficients. Aims: We remove these two limitations and investigate the effects of compressibility on the atmospheric dynamics by solving the standard Euler equations. Methods: This was done by means of a series of simulations performed in the framework of the equatorial β-plane approximation using the finite-volume shock-capturing code RAMSES. Results: At low resolution, we recover the classical results described in the literature: we find a strong and steady supersonic equatorial jet of a few km s-1 that displays no signature of shocks. We next show that the jet zonal velocity depends significantly on the grid meridional resolution. When this resolution is fine enough to properly resolve the jet, the latter is subject to a Kelvin-Helmholtz instability. The jet zonal mean velocity displays regular oscillations with a typical timescale of a few days and a significant amplitude of about 15% of the jet velocity. We also find compelling evidence for the development of a vertical shear instability at pressure levels of a few bars. It seems to be responsible for an increased downward kinetic energy flux that significantly affects the temperature of the deep atmosphere and appears to act as a form of drag on the equatorial jet. This instability also creates velocity fluctuations that propagate upward and steepen into weak shocks at pressure levels of a few mbars. Conclusions: We conclude that hot-Jupiter equatorial jets are potentially unstable to both a barotropic Kelvin-Helmholtz instability and a vertical shear instability. Upon confirmation using more realistic models, these two instabilities could result in

  7. Suppression of the multi-azimuthal-angle instability in dense neutrino gas during supernova accretion phase

    Chakraborty, Sovan; Saviano, Ninetta; Seixas, David de Sousa

    2014-01-01

    It has been recently pointed out that removing the axial symmetry in the ``multi-angle effects'' associated with the neutrino-neutrino interactions for supernova (SN) neutrinos, a new multi-azimuthal-angle (MAA) instability would arise. In particular, for a flux ordering $F_{\

  8. Suppression of the multi-azimuthal-angle instability in dense neutrino gas during supernova accretion phase

    Chakraborty, Sovan; Mirizzi, Alessandro; Saviano, Ninetta; Seixas, David de Sousa

    2014-01-01

    It has been recently pointed out that removing the axial symmetry in the "multi-angle effects" associated with the neutrino-neutrino interactions for supernova (SN) neutrinos, a new multi-azimuthal-angle (MAA) instability would arise. In particular, for a flux ordering $F_{\\\

  9. Self-Destructing Spiral Waves: Global Simulations of a Spiral Wave Instability in Accretion Disks

    Bae, Jaehan; Hartmann, Lee; Richard, Samuel

    2016-01-01

    We present results from a suite of three-dimensional global hydrodynamic simulations which show that spiral density waves propagating in circumstellar disks are unstable to the growth of a parametric instability that leads to break-down of the flow into turbulence. This spiral wave instability (SWI) arises from a resonant interaction between pairs of inertial waves, or inertial-gravity waves, and the background spiral wave. The development of the instability in the linear regime involves the growth of a broad spectrum of inertial modes, with growth rates on the order of the orbital time, and results in a nonlinear saturated state in which turbulent velocity perturbations are of a similar magnitude to those induced by the spiral wave. The turbulence induces angular momentum transport, and vertical mixing, at a rate that depends locally on the amplitude of the spiral wave (we obtain a stress parameter $\\alpha \\sim 5 \\times 10^{-4}$ in our reference model). The instability is found to operate in a wide-range of ...

  10. A Double Outburst from IGR J00291+5934: Implications for Accretion Disk Instability Theory

    Hartman, Jacob M; Chakrabarty, Deepto

    2010-01-01

    The accretion-powered millisecond pulsar IGR J00291+5934 underwent two ~10 d long outbursts during 2008, separated by 30 d in quiescence. Such a short quiescent period between outbursts has never been seen before from a neutron star X-ray transient. X-ray pulsations at the 599 Hz spin frequency are detected throughout both outbursts. For the first time, we derive a pulse phase model that connects two outbursts, providing a long baseline for spin frequency measurement. Comparison with the frequency measured during the 2004 outburst of this source gives a spin-down during quiescence of -4(1)x10^-15 Hz/s, approximately an order of magnitude larger than the long-term spin-down observed in the 401 Hz accretion-powered pulsar SAX J1808.4-3658. If this spin-down is due to magnetic dipole radiation, it requires a 2x10^8 G field strength, and its high spin-down luminosity may be detectable with the Fermi Large Area Telescope. Alternatively, this large spin-down could be produced by gravitational wave emission from a f...

  11. Gas Evolution Dynamics in Godunov-Type Schemes and Analysis of Numerical Shock Instability

    Xu, Kun

    1999-01-01

    In this paper we are going to study the gas evolution dynamics of the exact and approximate Riemann solvers, e.g., the Flux Vector Splitting (FVS) and the Flux Difference Splitting (FDS) schemes. Since the FVS scheme and the Kinetic Flux Vector Splitting (KFVS) scheme have the same physical mechanism and similar flux function, based on the analysis of the discretized KFVS scheme the weakness and advantage of the FVS scheme are closely observed. The subtle dissipative mechanism of the Godunov method in the 2D case is also analyzed, and the physical reason for shock instability, i.e., carbuncle phenomena and odd-even decoupling, is presented.

  12. Pulsar spins from an instability in the accretion shock of supernovae

    Blondin, John M.; Mezzacappa, Anthony

    2006-01-01

    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 progenitor star. So far, however, stellar theory has not been able to explain the distribution of ...

  13. Internal shocks driven by accretion flow variability in the compact jet of the black hole binary GX 339-4

    Drappeau, S; Belmont, J; Gandhi, P; Corbel, S

    2014-01-01

    In recent years, compact jets have been playing a growing role in the understanding of accreting black hole engines. In the case of X-ray binary systems, compact jets are usually associated with the hard state phase of a source outburst. Recent observations of GX 339-4 have demonstrated the presence of a variable synchrotron spectral break in the mid-infrared band that was associated with its compact jet. In the model used in this study, we assume that the jet emission is produced by electrons accelerated in internal shocks driven by rapid fluctuations of the jet velocity. The resulting spectral energy distribution (SED) and variability properties are very sensitive to the Fourier power spectrum density (PSD) of the assumed fluctuations of the jet Lorentz factor. These fluctuations are likely to be triggered by the variability of the accretion flow which is best traced by the X-ray emission. Taking the PSD of the jet Lorentz factor fluctuations to be identical to the observed X-ray PSD, our study finds that t...

  14. Impact of MHD shock physics on magnetosheath asymmetry and Kelvin-Helmholtz instability

    Nykyri, K.

    2013-08-01

    We have performed 13 three-dimensional global magnetohydrodynamic (MHD) simulations of the magnetosheath plasma and magnetic field properties for Parker spiral (PS) and ortho-Parker spiral interplanetary magnetic field (IMF) orientations corresponding to a wide range of solar wind plasma conditions. To study the growth of the Kelvin-Helmholtz instability on the dawn and dusk flank magnetopause, we have performed 26 local two-dimensional MHD simulations, with the initial conditions taken from global simulations on both sides of the velocity shear layer at the dawn-dusk terminator. These simulations indicate that while the MHD physics of the fast shocks does not directly lead to strong asymmetry of the magnetosheath temperature for typical solar wind conditions, the magnetosheath on the quasi-parallel shock side has a smaller tangential magnetic field along the magnetosheath flow which enables faster growth of the Kelvin-Helmholtz instability (KHI). Because the IMF is statistically mostly in the PS orientation, the KHI formation may statistically favor the dawnside flank. For all the 26 simulations, the growth rates of the KHI correlated well with the ratio of the velocity shear and Alfvén speed along the wave vector, k. Dynamics of the KHI may subsequently lead to formation of kinetic Alfvén waves and reconnection in the Kelvin-Helmholtz vortices which can lead to particle energization. This may partly help to explain the observed plasma sheet asymmetry of cold-component ions, which are heated more on the dawnside plasma sheet.

  15. Analytical scalings of the linear Richtmyer-Meshkov instability when a shock is reflected

    Campos, F. Cobos; Wouchuk, J. G.

    2016-05-01

    When a planar shock hits a corrugated contact surface between two fluids, hydrodynamic perturbations are generated in both fluids that result in asymptotic normal and tangential velocity perturbations in the linear stage, the so called Richtmyer-Meshkov instability. In this work, explicit and exact analytical expansions of the asymptotic normal velocity (δ vi∞ ) are presented for the general case in which a shock is reflected back. The expansions are derived from the conservation equations and take into account the whole perturbation history between the transmitted and reflected fronts. The important physical limits of weak and strong shocks and the high/low preshock density ratio at the contact surface are shown. An approximate expression for the normal velocity, valid even for high compression regimes, is given. A comparison with recent experimental data is done. The contact surface ripple growth is studied during the linear phase showing good agreement between theory and experiments done in a wide range of incident shock Mach numbers and preshock density ratios, for the cases in which the initial ripple amplitude is small enough. In particular, it is shown that in the linear asymptotic phase, the contact surface ripple (ψi) grows as ψ∞+δ vi∞t , where ψ∞ is an asymptotic ordinate different from the postshock ripple amplitude at t =0 + . This work is a continuation of the calculations of F. Cobos Campos and J. G. Wouchuk, [Phys. Rev. E 90, 053007 (2014), 10.1103/PhysRevE.90.053007] for a single shock moving into one fluid.

  16. An accretion disc instability induced by a temperature sensitive {\\alpha} parameter

    Potter, William J

    2014-01-01

    In the standard thin disc formalism the dimensionless {\\alpha} parameter is usually assumed to be constant. However, there are good theoretical reasons for believing, as well as evidence from simulations, that {\\alpha} is dependent on intrinsic disc properties. In this paper we analyse the conditions for the stability of a thin accretion disc in which {\\alpha} is a function of the magnetic Prandtl number, the ratio of collisional viscosity to resistivity. In the inner disc, where the free electron opacity and radiation viscosity dominate, the disc is unstable if {\\alpha} is proportional to the magnetic Prandtl number with an exponent > 0.5. This is within the range of values for the power-law index found in MHD simulations with simple energetics. We calculate the evolution of the unstable disc within the {\\alpha} formalism and show that the physically accessible solutions form a limit cycle, analogous to the behaviour seen in recurrent dwarf novae. It is noteworthy that the time-dependent global behaviour of ...

  17. Linear and nonlinear evolution of the vertical shear instability in accretion discs

    Nelson, Richard P; Umurhan, Orkan M

    2012-01-01

    (Abridged) We analyse the stability and evolution of power-law accretion disc models. These have midplane densities that follow radial power-laws, and have either temperature or entropy distributions that are power-law functions of cylindrical radius. We employ two different hydrodynamic codes to perform 2D-axisymmetric and 3D simulations that examine the long-term evolution of the disc models as a function of the power-law indices of the temperature or entropy, the thermal relaxation time of the fluid, and the viscosity. We present a stability analysis of the problem that we use to interpret the simulation results. We find that disc models whose temperature or entropy profiles cause the equilibrium angular velocity to vary with height are unstable to the growth of modes with wavenumber ratios |k_R/k_Z| >> 1 when the thermodynamic response of the fluid is isothermal, or the thermal evolution time is comparable to or shorter than the local dynamical time scale. These discs are subject to the Goldreich-Schubert...

  18. Can the magnetic field in the Orion arm inhibit the growth of instabilities in the bow shock of Betelgeuse?

    van Marle, Allard Jan; Meliani, Zakaria

    2013-01-01

    Many evolved stars travel through space at supersonic velocities, which leads to the formation of bow shocks ahead of the star where the stellar wind collides with the interstellar medium (ISM). Herschel observations of the bow shock of $\\alpha$-Orionis show that the shock is almost free of instabilities, despite being, at least in theory, subject to both Kelvin-Helmholtz and Rayleigh-Taylor instabilities. A possible explanation for the lack of instabilities lies in the presence of an interstellar magnetic field. We wish to investigate whether the magnetic field of the interstellar medium (ISM) in the Orion arm can inhibit the growth of instabilities in the bow shock of $\\alpha$-Orionis. We used the code MPI-AMRVAC to make magneto-hydrodynamic simulations of a circumstellar bow shock, using the wind parameters derived for $\\alpha$-Orionis and interstellar magnetic field strengths of $B\\,=\\,1.4,\\, 3.0$, and $5.0\\, \\mu$G, which fall within the boundaries of the observed magnetic field strength in the Orion arm ...

  19. High-order Godunov schemes for global 3D MHD simulations of accretion disks. I. Testing the linear growth of the magneto-rotational instability

    Flock, M.; Dzyurkevich, N.; Klahr, H.; Mignone, A.

    2010-06-01

    We assess the suitability of various numerical MHD algorithms for astrophysical accretion disk simulations with the PLUTO code. The well-studied linear growth of the magneto-rotational instability is used as the benchmark test for a comparison between the implementations within PLUTO and against the ZeusMP code. The results demonstrate the importance of using an upwind reconstruction of the electro-motive force (EMF) in the context of a constrained transport scheme, which is consistent with plane-parallel, grid-aligned flows. In contrast, constructing the EMF from the simple average of the Godunov fluxes leads to a numerical instability and the unphysical growth of the magnetic energy. We compare the results from 3D global calculations using different MHD methods against the analytical solution for the linear growth of the MRI, and discuss the effect of numerical dissipation. The comparison identifies a robust and accurate code configuration that is vital for realistic modeling of accretion disk processes.

  20. Perturbed disks get shocked. Binary black hole merger effects on accretion disks

    Megevand, Miguel; Frank, Juhan; Hirschmann, Eric W; Lehner, Luis; Liebling, Steven L; Motl, Patrick M; Neilsen, David

    2009-01-01

    The merger process of a binary black hole system can have a strong impact on a circumbinary disk. In the present work we study the effect of both central mass reduction (due to the energy loss through gravitational waves) and a possible black hole recoil (due to asymmetric emission of gravitational radiation). For the mass reduction case and recoil directed along the disk's angular momentum, oscillations are induced in the disk which then modulate the internal energy and bremsstrahlung luminosities. On the other hand, when the recoil direction has a component orthogonal to the disk's angular momentum, the disk's dynamics are strongly impacted, giving rise to relativistic shocks. The shock heating leaves its signature in our proxies for radiation, the total internal energy and bremsstrahlung luminosity. Interestingly, for cases where the kick velocity is below the smallest orbital velocity in the disk (a likely scenario in real AGN), we observe a common, characteristic pattern in the internal energy of the dis...

  1. ELECTRON HEATING BY THE ION CYCLOTRON INSTABILITY IN COLLISIONLESS ACCRETION FLOWS. II. ELECTRON HEATING EFFICIENCY AS A FUNCTION OF FLOW CONDITIONS

    In the innermost regions of low-luminosity accretion flows, including Sgr A* at the center of our Galaxy, the frequency of Coulomb collisions is so low that the plasma has two temperatures, with the ions substantially hotter than the electrons. This paradigm assumes that Coulomb collisions are the only channel for transferring the ion energy to the electrons. In this work, the second of a series, we assess the efficiency of electron heating by ion velocity-space instabilities in collisionless accretion flows. The instabilities are seeded by the pressure anisotropy induced by magnetic field amplification, coupled to the adiabatic invariance of the particle magnetic moments. Using two-dimensional particle-in-cell (PIC) simulations, we showed in Paper I that if the electron-to-ion temperature ratio is T 0e/T 0i ≲ 0.2, the ion cyclotron instability is the dominant mode for ion betas β0i ∼ 5-30 (here, β0i is the ratio of ion thermal pressure to magnetic pressure), as appropriate for the midplane of low-luminosity accretion flows. In this work, we employ analytical theory and one-dimensional PIC simulations (with the box aligned with the fastest-growing wave vector of the ion cyclotron mode) to fully characterize how the electron heating efficiency during the growth of the ion cyclotron instability depends on the electron-to-proton temperature ratio, the plasma beta, the Alfvén speed, the amplification rate of the mean field (in units of the ion Larmor frequency), and the proton-to-electron mass ratio. Our findings can be incorporated as a physically grounded subgrid model into global fluid simulations of low-luminosity accretion flows, thus helping to assess the validity of the two-temperature assumption

  2. ELECTRON HEATING BY THE ION CYCLOTRON INSTABILITY IN COLLISIONLESS ACCRETION FLOWS. II. ELECTRON HEATING EFFICIENCY AS A FUNCTION OF FLOW CONDITIONS

    Sironi, Lorenzo, E-mail: lsironi@cfa.harvard.edu [NASA Einstein Postdoctoral Fellow. (United States)

    2015-02-20

    In the innermost regions of low-luminosity accretion flows, including Sgr A* at the center of our Galaxy, the frequency of Coulomb collisions is so low that the plasma has two temperatures, with the ions substantially hotter than the electrons. This paradigm assumes that Coulomb collisions are the only channel for transferring the ion energy to the electrons. In this work, the second of a series, we assess the efficiency of electron heating by ion velocity-space instabilities in collisionless accretion flows. The instabilities are seeded by the pressure anisotropy induced by magnetic field amplification, coupled to the adiabatic invariance of the particle magnetic moments. Using two-dimensional particle-in-cell (PIC) simulations, we showed in Paper I that if the electron-to-ion temperature ratio is T {sub 0e}/T {sub 0i} ≲ 0.2, the ion cyclotron instability is the dominant mode for ion betas β{sub 0i} ∼ 5-30 (here, β{sub 0i} is the ratio of ion thermal pressure to magnetic pressure), as appropriate for the midplane of low-luminosity accretion flows. In this work, we employ analytical theory and one-dimensional PIC simulations (with the box aligned with the fastest-growing wave vector of the ion cyclotron mode) to fully characterize how the electron heating efficiency during the growth of the ion cyclotron instability depends on the electron-to-proton temperature ratio, the plasma beta, the Alfvén speed, the amplification rate of the mean field (in units of the ion Larmor frequency), and the proton-to-electron mass ratio. Our findings can be incorporated as a physically grounded subgrid model into global fluid simulations of low-luminosity accretion flows, thus helping to assess the validity of the two-temperature assumption.

  3. Modelling Shock Heating in Cluster Mergers: I. Moving Beyond the Spherical Accretion Model

    McCarthy, Ian G; Balogh, Michael L; Voit, G Mark; Pearce, Frazer R; Theuns, Tom; Babul, Arif; Lacey, Cedric G; Frenk, Carlos S; 10.1111/j.1365-2966.2007.11465.x

    2008-01-01

    (Abridged) The thermal history of the intracluster medium (ICM) is complex. Heat input from cluster mergers, AGN, and galaxy winds offsets and may even halt the cooling of the ICM. Consequently, the processes that set the properties of the ICM play a key role in determining how galaxies form. In this paper we focus on the shock heating of the ICM during cluster mergers, with the eventual aim of incorporating this mechanism into semi-analytic models of galaxy formation. We use a suite of hydrodynamic simulations to track the evolution of the ICM in idealised two-body mergers. We find the heating of the ICM can be understood relatively simply by considering the evolution of the gas entropy during the mergers. We examine the processes that generate the entropy in order to understand why previous analytic shock heating models failed. We find that: (1) The energy that is thermalised in the collision greatly exceeds the kinetic energy available when the systems first touch. The smaller system penetrates deep into t...

  4. Shear-driven instabilities and shocks in the atmospheres of hot Jupiters

    Fromang, Sébastien; Heng, Keving

    2016-01-01

    General circulation models of the atmosphere of hot Jupiter have shown the existence of a supersonic eastward equatorial jet. In this paper, we investigate the effects of compressibility on the atmospheric dynamics by solving the standard Euler equations. This is done by means of a series of simulations performed in the framework of the equatorial beta-plane approximation using the finite volume shock-capturing code RAMSES. At low resolution, we recover the classical results described in the literature: we find a strong and steady supersonic equatorial jet of a few km/s that displays no signature of shocks. We next show that the jet zonal velocity depends significantly on the grid meridional resolution. When that resolution is fine enough to properly resolve the jet, the latter is subject to a Kelvin-Helmholtz instability. The jet zonal mean velocity displays regular oscillations with a typical timescale of few days and a significant amplitude of about 15% of the jet velocity. We also find compelling evidence...

  5. Accretion on to Magnetic White Dwarfs

    Wickramasinghe Dayal

    2014-01-01

    The polars have no counterparts in neutron star systems and their study provides unique insights into the complex nature of the magnetospheric boundary. The observed properties of accretion shocks at the white dwarf surface such as the anomalous soft-X-ray excess and its time variability provide strong support for the hypothesis that under certain circumstances the field channelled funnel flow is “blobby”. This has been attributed to interchange instabilities such as the Magnetic Rayleigh-Taylor instability in the shocked gas at the stream-magnetosphere boundary where the stream fragments into discrete clumps of gas. As the clumps penetrate into the magnetosphere, they are shredded into smaller mass blobs via the Kelvin-Helmholtz instability that then couple on to field lines over an extended inner transition region in the orbital plane. The more massive blobs penetrate deep into the photosphere of the white dwarf releasing their energy as a reprocessed soft-X-ray black body component. Although similar instabilities are expected in the inner transition region in disced accretion albeit on a different scale there has been no direct observational evidence for blobby accretion in the generally lower field and disced IPs.

  6. Development of nonlinear two fluid interfacial structures by combined action of Rayleigh-Taylor, Kelvin-Helmholtz and Richtmyer-Meshkov instabilities:Oblique shock

    Gupta, M R; Roy, Sourav; Banerjee, Rahul; Khan, Manoranjan

    2010-01-01

    The nonlinear evolution of two fluid interfacial structures like bubbles and spikes arising due to the combined action of Rayleigh-Taylor and Kelvin-Helmholtz instability or due to that of Richtmyer-Meshkov and Kelvin-Helmholtz instability resulting from oblique shock is investigated. Using Layzer's model analytic expressions for the asymptotic value of the combined growth rate are obtained in both cases for spikes and bubbles. However, if the overlying fluid is of lower density the interface perturbation behaves in different ways. Depending on the magnitude of the velocity shear associated with Kelvin-Helmholtz instability both the bubble and spike amplitude may simultaneously grow monotonically (instability) or oscillate with time or it may so happen that while this spike steepens the bubble tends to undulate. In case of an oblique shock which causes combined action of Richtmyer-Meshkov instability arising due to the normal component of the shock and Kelvin Helmholtz instability through creation of velocity...

  7. Electron Heating by the Ion Cyclotron Instability in Collisionless Accretion Flows. II. Electron Heating Efficiency as a Function of Flow Conditions

    Sironi, Lorenzo

    2014-01-01

    In the innermost regions of low-luminosity accretion flows, including Sgr A* at the center of our Galaxy, the frequency of Coulomb collisions is so low that the plasma is two-temperature, with the ions substantially hotter than the electrons. This paradigm assumes that Coulomb collisions are the only channel for transferring the ion energy to the electrons. In this work, the second of a series, we assess the efficiency of electron heating by ion velocity-space instabilities in collisionless accretion flows. The instabilities are seeded by the pressure anisotropy induced by magnetic field amplification, coupled to the adiabatic invariance of the particle magnetic moments. Using two-dimensional (2D) particle-in-cell (PIC) simulations, we showed in Paper I that if the electron-to-ion temperature ratio is < 0.2, the ion cyclotron instability is the dominant mode for values of ion beta_i ~ 5-30 (here, beta_i is the ratio of ion thermal pressure to magnetic pressure), as appropriate for the midplane of low-lumin...

  8. Multi-shocks generation and collapsing instabilities induced by competing nonlinearities

    Crosta, Matteo

    2012-01-01

    We investigate dispersive shock dynamics in materials with competing cubic-quintic nonlinearities. Whitham theory of modulation, hydrodynamic analysis and numerics demonstrate a rich physical scenario, ranging from multi-shock generation to collapse.

  9. Photon Bubbles and the Vertical Structure of Accretion Disks

    Begelman, M C

    2006-01-01

    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 instability. They increase the rate at which radiation escapes from the disk, and may allow disks to exceed the Eddington limit by a substantial factor. We first generalize the theory of photon bubbles to include the effects of finite optical depths and radiation damping. Modifications to the diffusion law at low optical depth tend to fill in the low-density regions of photon bubbles, while radiation damping inhibits the formation of photon bubbles at large radii, small accretion rates, and small heights above the equatorial plane. Accretion disks dominated by photon bubble transport may reach luminosities of 10 to >100 times the Eddington limit (L_E), depending on the mass of the central ...

  10. Temperature dependence of parametric instabilities in the context of the shock-ignition approach to inertial confinement fusion

    Weber, Stefan A.; Riconda, C.

    2015-01-01

    Roč. 3, Feb (2015), e6. ISSN 2095-4719 R&D Projects: GA MŠk ED1.1.00/02.0061; GA MŠk EE2.3.20.0279 Grant ostatní: ELI Beamlines(XE) CZ.1.05/1.1.00/02.0061; LaserZdroj (OP VK 3)(XE) CZ.1.07/2.3.00/20.0279 Institutional support: RVO:68378271 Keywords : inertial confinement fusion * shock ignition * laser-plasma interaction * parametric instabilities Subject RIV: BL - Plasma and Gas Discharge Physics

  11. Effects of magnetic field on the runaway instability of relativistic accretion tori near a rotating black hole

    Karas, Vladimír; Hamerský, Jaroslav

    Cambridge: Cambridge University Press, 2014 - (Sjouwerman, L.), s. 424-426. (IAU Symposium Proceedings Series. 303). ISBN 9781107044616. ISSN 1743-9213. [Symposium of the International Astronomical Union /303./. Santa Fe (US), 30.09.2013-04.10.2013] Grant ostatní: UK(CZ) SVV-26089 Institutional support: RVO:67985815 Keywords : accretion discs * gravitation * black hole physics Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics

  12. Neutrino signature of supernova hydrodynamical instabilities in three dimensions

    Tamborra, Irene; Hanke, Florian; Mueller, Bernhard; Janka, Hans-Thomas; Raffelt, Georg

    2013-01-01

    The first full-scale three-dimensional (3D) core-collapse supernova (SN) simulations with sophisticated neutrino transport show pronounced effects of the standing accretion shock instability (SASI) for two high-mass progenitors (20 and 27 M_sun). In a low-mass progenitor (11.2 M_sun), large-scale convection is the dominant nonradial hydrodynamic instability in the postshock accretion layer. The SASI-associated modulation of the neutrino signal (80 Hz in our two examples) will be clearly detec...

  13. Bow shock fragmentation driven by a thermal instability in laboratory-astrophysics experiments

    Suzuki-Vidal, F.; Lebedev, S. V.; Ciardi, A.; Pickworth, L. A.; Rodriguez, R.; Gil, J. M.; Espinosa, G. (Gaudencio); Hartigan, P.; Swadling, G. F.; Skidmore, J.; Hall, G. N.; Bennett, M; Bland, S. N.; Burdiak, G.; de Grouchy, P.

    2015-01-01

    The role of radiative cooling during the evolution of a bow shock was studied in laboratory-astrophysics experiments that are scalable to bow shocks present in jets from young stellar objects. The laboratory bow shock is formed during the collision of two counter-streaming, supersonic plasma jets produced by an opposing pair of radial foil Z-pinches driven by the current pulse from the MAGPIE pulsed-power generator. The jets have different flow velocities in the laboratory frame and the exper...

  14. Modification of the formation of high-Mach number electrostatic shock-like structures by the ion acoustic instability

    Dieckmann, Mark E; Doria, Domenico; Pohl, Martin; Borghesi, Marco

    2013-01-01

    The formation of unmagnetized electrostatic shock-like structures with a high Mach number is examined with one- and two-dimensional particle-in-cell (PIC) simulations. The structures are generated through the collision of two identical plasma clouds, which consist of equally hot electrons and ions with a mass ratio of 250. The Mach number of the collision speed with respect to the initial ion acoustic speed of the plasma is set to 4.6. This high Mach number delays the formation of such structures by tens of inverse ion plasma frequencies. A pair of stable shock-like structures is observed after this time in the 1D simulation, which gradually evolve into electrostatic shocks. The ion acoustic instability, which can develop in the 2D simulation but not in the 1D one, competes with the nonlinear process that gives rise to these structures. The oblique ion acoustic waves fragment their electric field. The transition layer, across which the bulk of the ions change their speed, widens and their speed change is redu...

  15. Effect of the dynamics of the impurity distribution over the ionization states on the radiative plasma instabilities and shock wave structure

    We show that conventional quasistationary (coronal, or improved coronal) approximation for the energy loss due to the impurity radiation in practice can never be applied to the investigations of the impurity radiation driven instabilities in the edge plasmas due to relatively slow evolution of the impurity population over ionization states. We show that taking into account the effect of the evolution of the impurity population over ionization states results in a very significant change of the growth rates of the radiative driven instabilities and the structure of the shock wave in the radiative plasmas and leads to the strict conditions for the existence of the shock wave. (orig.)

  16. Corotational Instability of Inertial-Acoustic Modes in Black Hole Accretion Discs and Quasi-Periodic Oscillations

    Lai, Dong

    2008-01-01

    We study the global stability of non-axisymmetric p-modes (also called inertial-acoustic modes) trapped in the inner-most regions of accretion discs around black holes. We show that the lowest-order (highest-frequency) p-modes, with frequencies $\\omega=(0.5-0.7) m\\Omega_{\\rm ISCO}$, can be overstable due to general relativistic effects, according to which the radial epicyclic frequency is a non-monotonic function of radius near the black hole. The mode is trapped inside the corotation resonance radius and carries a negative energy. The mode growth arises primarily from wave absorption at the corotation resonance, and the sign of the wave absorption depends on the gradient of the disc vortensity. When the mode frequency is sufficiently high, such that the slope of the vortensity is positive at corotation positive wave energy is absorbed at the resonance, leading to the growth of mode amplitude. We also study how the rapid radial inflow at the inner edge of the disc affects the mode trapping and growth. Our ana...

  17. Multi-dimensional PIC-simulations of parametric instabilities for shock-ignition conditions

    Riconda C.; Weber S.; Klimo O.; Héron A.; Tikhonchuk V.T.

    2013-01-01

    Laser-plasma interaction is investigated for conditions relevant for the shock-ignition (SI) scheme of inertial confinement fusion using two-dimensional particle-in-cell (PIC) simulations of an intense laser beam propagating in a hot, large-scale, non-uniform plasma. The temporal evolution and interdependence of Raman- (SRS), and Brillouin- (SBS), side/backscattering as well as Two-Plasmon-Decay (TPD) are studied. TPD is developing in concomitance with SRS creating a broad spectrum of plasma ...

  18. Oscillating shocks in the low angular momentum flows as a source of variability of accreting black holes

    ,

    2014-01-01

    We derive the conditions for shock formation in a quasi-spherical, slightly rotating flows. We verify the results of semi-analytical, stationary calculations with the time evolution studied by numerical hydro-simulations, and we study the oscillations of the shock position. We also study the behaviour of flows with varying specific angular momentum, where the 'hysteresis' type of loop is found when passing through the multiple sonic points region. Our results are in agreement with the timescales and shapes of the luminosity flares observed in Sgr A*. These models may also be applicable for the Galactic stellar mass black holes, like GX 339-4 or GRS 1915+105, where periodic oscillations of X-ray luminosity are detected.

  19. Properties of the propagating oscillatory shock wave in the accretion flows around few transient black hole candidates during their outbursts

    Debnath, Dipak

    2013-01-01

    In our study of the timing properties of few Galactic black hole candidates evolutions of the low and intermediate frequency quasi-periodic oscillations (LIFQPOs) are observed. In 2005, for explaining evolution of QPO frequency during rising phase of 2005 GRO J1655-40 outburst, Chakrabarti and his students introduced a new model, namely propagating oscillatory shock (POS) model. Here we present the results obtained from the same POS model fitted QPO evolutions during both the rising and declining phases of the outbursts of 2005 GRO J165540, 2010-11 GX 339-4, and 2010 & 2011 H 1743-322.

  20. Multi-dimensional PIC-simulations of parametric instabilities for shock-ignition conditions

    Riconda C.

    2013-11-01

    Full Text Available Laser-plasma interaction is investigated for conditions relevant for the shock-ignition (SI scheme of inertial confinement fusion using two-dimensional particle-in-cell (PIC simulations of an intense laser beam propagating in a hot, large-scale, non-uniform plasma. The temporal evolution and interdependence of Raman- (SRS, and Brillouin- (SBS, side/backscattering as well as Two-Plasmon-Decay (TPD are studied. TPD is developing in concomitance with SRS creating a broad spectrum of plasma waves near the quarter-critical density. They are rapidly saturated due to plasma cavitation within a few picoseconds. The hot electron spectrum created by SRS and TPD is relatively soft, limited to energies below one hundred keV.

  1. Spherical Accretion

    Sari, Re'em; Goldreich, Peter

    2006-01-01

    We compare different examples of spherical accretion onto a gravitating mass. Limiting cases include the accretion of a collisionally dominated fluid and the accretion of collisionless particles. We derive expressions for the accretion rate and density profile for semi-collisional accretion which bridges the gap between these limiting cases. Particle crossing of the Hill sphere during the formation of the outer planets is likely to have taken place in the semi-collisional regime.

  2. Neutrino signature of supernova hydrodynamical instabilities in three dimensions.

    Tamborra, Irene; Hanke, Florian; Müller, Bernhard; Janka, Hans-Thomas; Raffelt, Georg

    2013-09-20

    The first full-scale three-dimensional core-collapse supernova (SN) simulations with sophisticated neutrino transport show pronounced effects of the standing accretion shock instability (SASI) for two high-mass progenitors (20 and 27 M([Symbol: see text])). In a low-mass progenitor (11.2 M([Symbol: see text])), large-scale convection is the dominant nonradial hydrodynamic instability in the postshock accretion layer. The SASI-associated modulation of the neutrino signal (80 Hz in our two examples) will be clearly detectable in IceCube or the future Hyper-Kamiokande detector, depending on progenitor properties, distance, and observer location relative to the main SASI sloshing direction. The neutrino signal from the next galactic SN can, therefore, diagnose the nature of the hydrodynamic instability. PMID:24093243

  3. Theory of wind accretion

    Shakura N.I.

    2014-01-01

    Full Text Available A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the magnetospheric boundary layer where the angular momentum between the rotating magnetosphere and the base of the differentially rotating quasi-spherical shell takes place. We show how observations of equilibrium X-ray pulsars Vela X-1 and GX 301-2 can be used to estimate dimensionless parameters of the subsonic settling accretion theory, and obtain the width of the magnetospheric boundary layer for these pulsars.

  4. Theory of wind accretion

    Shakura, N. I.; Postnov, K. A.; Kochetkova, A. Yu.; Hjalmarsdotter, L.

    2014-01-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the magnetospheric boundary layer where the angular momentum between the rotating magnetosphere and the base of the differentially rotating quasi-spherical shell takes place. We show how observations of equilibrium X-ray pulsars Vela X-1 and GX 301-2 can be used to estimate dimensionless parameters of the subsonic settling accretion theory, and obtain the width of the magnetospheric boundary layer for these pulsars.

  5. Theory of wind accretion

    Shakura, N I; Kochetkova, A Yu; Hjalmarsdotter, L

    2013-01-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about $4\\times10^{36}$ erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the ...

  6. Cold Mode Accretion in Galaxy Formation

    Benson, Andrew J.; Bower, Richard

    2010-01-01

    A generic expectation for gas accreted by high mass haloes is that it is shock heated to the virial temperature of the halo. In low mass haloes, or at high redshift, however, the gas cooling rate is sufficiently rapid that an accretion shock is unlikely to form. Instead, gas can accrete directly into the centre of the halo in a `cold mode' of accretion. Although semi-analytic models have always made a clear distinction between hydrostatic and rapid cooling they have not made a distinction bet...

  7. An Investigation into the Character of Pre-Explosion Core-Collapse Supernova Shock Motion

    Burrows, Adam; Dolence, Joshua C.; Murphy, Jeremiah W.

    2012-01-01

    We investigate the structure of the stalled supernova shock in both 2D and 3D and explore the differences in the effects of neutrino heating and the standing accretion shock instability (SASI). We find that early on the amplitude of the dipolar mode of the shock is factors of 2 to 3 smaller in 3D than in 2D. However, later in both 3D and 2D the monopole and dipole modes start to grow until explosion. Whereas in 2D the (l,m) = (1,0) mode changes sign quasi-periodically, producing the "up-and-d...

  8. The Linear Instability of Astrophysical Flames in Magnetic Fields

    Dursi, L J

    2004-01-01

    Supernovae of Type Ia are used as standard candles for cosmological observations despite the as yet incomplete understanding of their explosion mechanism. In one model, these events are thought to result from subsonic burning in the core of an accreting Carbon/Oxygen white dwarf that is accelerated through flame wrinkling and flame instabilities. Many such white dwarfs have significant magnetic fields. Here we derive the linear effects of such magnetic fields on one flame instability, the well-known Landau-Darrieus instability. When the magnetic field is strong enough that the flame is everywhere sub-Alfvenic, the instability can be greatly suppressed. Super-Alfvenic flames are much less affected by the field, with flames propagating parallel to the field somewh at destabilized, and flames propagating perpendicular to the field somewhat stabili zed. Trans-Alfvenic parallel flames, however, like trans-Alfvenic parallel shocks, are seen to be non-evolutionary; understanding the behavior of these flames will req...

  9. Modelling Accretion Disk and Stellar Wind Interactions: the Case of Sgr A*

    Christie, I M; Mimica, P; Giannios, D

    2016-01-01

    Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disk, the ram and thermal pressures of the disk terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin-Helmholtz instabilities. Because the characteristic wind and stellar velocities are in $\\sim10^{8}$ cm s$^{-1}$ range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericenter passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly ...

  10. Linear analysis on the growth of non-spherical perturbations in supersonic accretion flows

    Takahashi, Kazuya; Yamada, Shoichi, E-mail: ktakahashi@heap.phys.waseda.ac.jp [Advanced Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku 169-8555 (Japan)

    2014-10-20

    We analyzed the growth of non-spherical perturbations in supersonic accretion flows. We have in mind an application to the post-bounce phase of core-collapse supernovae (CCSNe). Such non-spherical perturbations have been suggested by a series of papers by Arnett, who has numerically investigated violent convections in the outer layers of pre-collapse stars. Moreover, Couch and Ott demonstrated in their numerical simulations that such perturbations may lead to a successful supernova even for a progenitor that fails to explode without fluctuations. This study investigated the linear growth of perturbations during the infall onto a stalled shock wave. The linearized equations are solved as an initial and boundary value problem with the use of a Laplace transform. The background is a Bondi accretion flow whose parameters are chosen to mimic the 15 M {sub ☉} progenitor model by Woosley and Heger, which is supposed to be a typical progenitor of CCSNe. We found that the perturbations that are given at a large radius grow as they flow down to the shock radius; the density perturbations can be amplified by a factor of 30, for example. We analytically show that the growth rate is proportional to l, the index of the spherical harmonics. We also found that the perturbations oscillate in time with frequencies that are similar to those of the standing accretion shock instability. This may have an implication for shock revival in CCSNe, which will be investigated in our forthcoming paper in more detail.

  11. Linear analysis on the growth of non-spherical perturbations in supersonic accretion flows

    We analyzed the growth of non-spherical perturbations in supersonic accretion flows. We have in mind an application to the post-bounce phase of core-collapse supernovae (CCSNe). Such non-spherical perturbations have been suggested by a series of papers by Arnett, who has numerically investigated violent convections in the outer layers of pre-collapse stars. Moreover, Couch and Ott demonstrated in their numerical simulations that such perturbations may lead to a successful supernova even for a progenitor that fails to explode without fluctuations. This study investigated the linear growth of perturbations during the infall onto a stalled shock wave. The linearized equations are solved as an initial and boundary value problem with the use of a Laplace transform. The background is a Bondi accretion flow whose parameters are chosen to mimic the 15 M ☉ progenitor model by Woosley and Heger, which is supposed to be a typical progenitor of CCSNe. We found that the perturbations that are given at a large radius grow as they flow down to the shock radius; the density perturbations can be amplified by a factor of 30, for example. We analytically show that the growth rate is proportional to l, the index of the spherical harmonics. We also found that the perturbations oscillate in time with frequencies that are similar to those of the standing accretion shock instability. This may have an implication for shock revival in CCSNe, which will be investigated in our forthcoming paper in more detail.

  12. Evolution of Massive Protostars Via Disk Accretion

    Hosokawa, Takashi; Yorke, Harold W.; Omukai, Kazuyuki

    2010-09-01

    Mass accretion onto (proto-)stars at high accretion rates \\dot{M}_* > 10^{-4} M_{⊙} 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 limiting case of "cold" disk accretion, whereby most of the stellar photosphere can radiate freely with negligible backwarming from the accretion flow, and the accreting material settles onto the star with the same specific entropy as the photosphere. We compare our results to the calculated evolution via spherically symmetric accretion, the opposite limit, whereby the material accreting onto the star contains the entropy produced in the accretion shock front. We examine how different accretion geometries affect the evolution of massive protostars. For cold disk accretion at 10-3 M sun yr-1, the radius of a protostar is initially small, R *sime a few R sun. After several solar masses have accreted, the protostar begins to bloat up and for M * ~= 10 M sun the stellar radius attains its maximum of 30-400 R sun. The large radius ~100 R sun is also a feature of spherically symmetric accretion at the same accreted mass and accretion rate. Hence, expansion to a large radius is a robust feature of accreting massive protostars. At later times, the protostar eventually begins to contract and reaches the zero-age main sequence (ZAMS) for M * ~= 30 M sun, independent of the accretion geometry. For accretion rates exceeding several 10-3 M sun yr-1, the protostar never contracts to the ZAMS. The very large radius of several hundreds R sun results in the low effective temperature and low UV luminosity of the protostar. Such bloated protostars could well explain the existence of bright high-mass protostellar objects, which lack detectable H II regions.

  13. EVOLUTION OF MASSIVE PROTOSTARS VIA DISK ACCRETION

    Mass accretion onto (proto-)stars at high accretion rates M-dot*> 10-4 Msun 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 limiting case of 'cold' disk accretion, whereby most of the stellar photosphere can radiate freely with negligible backwarming from the accretion flow, and the accreting material settles onto the star with the same specific entropy as the photosphere. We compare our results to the calculated evolution via spherically symmetric accretion, the opposite limit, whereby the material accreting onto the star contains the entropy produced in the accretion shock front. We examine how different accretion geometries affect the evolution of massive protostars. For cold disk accretion at 10-3 Msun yr-1, the radius of a protostar is initially small, R*≅ a few Rsun. After several solar masses have accreted, the protostar begins to bloat up and for M* ≅ 10 Msun the stellar radius attains its maximum of 30-400 Rsun. The large radius ∼100 Rsun is also a feature of spherically symmetric accretion at the same accreted mass and accretion rate. Hence, expansion to a large radius is a robust feature of accreting massive protostars. At later times, the protostar eventually begins to contract and reaches the zero-age main sequence (ZAMS) for M* ≅ 30 Msun, independent of the accretion geometry. For accretion rates exceeding several 10-3 Msun yr-1, the protostar never contracts to the ZAMS. The very large radius of several hundreds Rsun results in the low effective temperature and low UV luminosity of the protostar. Such bloated protostars could well explain the existence of bright high-mass protostellar objects, which lack detectable H II regions.

  14. Cosmological shock waves

    Bykov, A M; Durret, F

    2008-01-01

    Large-scale structure formation, accretion and merging processes, AGN activity produce cosmological gas shocks. The shocks convert a fraction of the energy of gravitationally accelerated flows to internal energy of the gas. Being the main gas-heating agent, cosmological shocks could amplify magnetic fields and accelerate energetic particles via the multi-fluid plasma relaxation processes. We first discuss the basic properties of standard single-fluid shocks. Cosmological plasma shocks are expected to be collisionless. We then review the plasma processes responsible for the microscopic structure of collisionless shocks. A tiny fraction of the particles crossing the shock is injected into the non-thermal energetic component that could get a substantial part of the ram pressure power dissipated at the shock. The energetic particles penetrate deep into the shock upstream producing an extended shock precursor. Scaling relations for postshock ion temperature and entropy as functions of shock velocity in strong coll...

  15. The Collisionless Magnetothermal Instability

    Islam, Tanim

    2013-01-01

    It is likely that nearly all central galactic massive and supermassive black holes are nonradiative: their accretion luminosities are orders of magnitude below what can be explained by efficient black hole accretion within their ambient environments. These objects, of which Sagittarius A* is the best-known example, are also dilute (mildly collisional to highly collisionless) and optically thin. In order for accretion to occur, magnetohydrodynamic instabilities must develop that not only transport angular momentum, but also gravitational energy generated through matter infall, outwards. A class of new magnetohydrodynamical fluid instabilities -- the magnetoviscous-thermal instability (MVTI) (Islam12) -- was found to transport angular momentum and energy along magnetic field lines through large (fluid) viscosities and thermal conductivities. This paper describes the collisionless and mildly collisional analogue to the MVTI, the collisional magnetothermal instability (CMTI), that similarly transports energy and ...

  16. Evolution of Massive Protostars via Disk Accretion

    Hosokawa, Takashi; Omukai, Kazuyuki

    2010-01-01

    Mass accretion onto (proto-)stars at high accretion rates > 10^-4 M_sun/yr 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 limiting case of "cold" disk accretion, whereby most of the stellar photosphere can radiate freely with negligible backwarming from the accretion flow, and the accreting material settles onto the star with the same specific entropy as the photosphere. We compare our results to the calculated evolution via spherically symmetric accretion, the opposite limit, whereby the material accreting onto the star contains the entropy produced in the accretion shock front. We examine how different accretion geometries affect the evolution of massive protostars. For cold disk accretion at 10^-3 M_sun/yr the radius of a protostar is initially small, about a few R_sun. After several solar masses have accreted, the protostar...

  17. Hoyle-Lyttleton Accretion onto Accretion Disks

    Fukue, Jun; Ioroi, Masayuki

    1999-01-01

    We investigate Hoyle-Lyttleton accretion for the case where the central source is a luminous accretion disk. %In classical Hoyle-Lyttleton accretion onto a ``spherical'' source, accretion takes place in an axially symmetric manner around a so-called accretion axis. The accretion rate of the classical Hoyle-Lyttleton accretion onto a non-luminous object and $\\Gamma$ the luminosity of the central object normalized by the Eddington luminosity. %If the central object is a compact star with a lumi...

  18. Wind accretion: Theory and Observations

    Shakura, N I; Kochetkova, A Yu; Hjalmarsdotter, L; Sidoli, L; Paizis, A

    2014-01-01

    A review of wind accretion in HMXB is presented. We focus on different regimes of quasi-spherical accretion onto a NS: supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the NS magnetosphere, and subsonic (settling) accretion which occurs when the plasma remains hot until it meets the magnetospheric boundary. The two regimes are separated by a limit in X-ray luminosity at about 4 10^{36} erg/s. In subsonic accretion, which works a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto the NS is determined by the ability of the plasma to enter the magnetosphere due to the Rayleigh-Taylor instability. Two regimes of subsonic accretion are possible, depending on the plasma cooling mechanism (Compton or radiative) near the magnetopshere. The transition from the high-luminosity regime with Compton cooling to the low-luminosity (L_x < 3\\times 10^35 erg/s) regime with radiative cooling can be respon...

  19. Conservative GRMHD simulations of moderately thin, tilted accretion disks

    This paper presents our latest numerical simulations of accretion disks that are misaligned with respect to the rotation axis of a Kerr black hole. In this work, we use a new, fully conservative version of the Cosmos++ general relativistic magnetohydrodynamics (GRMHD) code, coupled with an ad hoc cooling function designed to control the thickness of the disk. Together these allow us to simulate the thinnest tilted accretion disks ever using a GRMHD code. In this way, we are able to probe the regime where the dimensionless stress and scale height of the disk become comparable. We present results for both prograde and retrograde cases. The simulated prograde tilted disk shows no sign of Bardeen-Petterson alignment even in the innermost parts of the disk. The simulated retrograde tilted disk, however, does show modest alignment. The implication of these results is that the parameter space associated with Bardeen-Petterson alignment for prograde disks may be rather small, only including very thin disks. Unlike our previous work, we find no evidence for standing shocks in our simulated tilted disks. We ascribe this to the black hole spin, tilt angle, and disk scale height all being small in these simulations. We also add to the growing body of literature pointing out that the turbulence driven by the magnetorotational instability in global simulations of accretion disks is not isotropic. Finally, we provide a comparison between our moderately thin, untilted reference simulation and other numerical simulations of thin disks in the literature.

  20. A kinetic approach to non resonant modes and growth rates of streaming instability: consequences for shock acceleration

    Pasquale BlasiINAF/Osservatorio Astrofisico di Arcetri; Elena Amato(INAF/Osservatorio Astrofisico di Arcetri)

    2015-01-01

    We show here that a purely kinetic approach to the excitation of waves by cosmic rays in the vicinity of a shock front leads to predict the appearance of a non-alfvenic fastly growing mode which is the same that was found by Bell (2004) by treating the plasma in the MHD approximation. The kinetic approach we present is more powerful in that it allows us to investigate different models for the compensation of the cosmic ray current in the background upstream plasma.

  1. Black Hole Accretion in Low States: Electron Heating

    Liu, Siming; Fryer, Christopher L.; Li, Hui

    2007-01-01

    Plasmas in an accretion flow are heated by MHD turbulence generated through the magneto-rotational instability. The viscous stress driving the accretion is intimately connected to the microscopic processes of turbulence dissipation. We show that, in a few well-observed black hole accretion systems, there is compelling observational evidence of efficient electron heating by turbulence or collective plasma effects in low accretion states, when Coulomb collisions are not efficient enough to esta...

  2. Dislocation mechanics based analysis of material dynamics behavior: enhanced ductility, deformation twinning, shock deformation, shear instability, dynamic recovery

    Further developments are described for the dislocation mechanics based constitutive equation analysis previously used to describe the separate dynamic stress-strain behavior of fcc and bcc metal polycrystals. An enhanced hardening and ductility in copper and certain tantalum materials at higher strain rates in split Hopkinson pressure bar tests and in shock loading are attributed to enhanced dislocation generation rather than to dislocation drag. Added material strengthening is accounted for also by deformation twinning in ARMCO iron and titanium and in shocked copper and tantalum. The separate equations are applied to calculate the critical strain for shear banding in copper, iron, and the titanium alloy, Ti-6Al-4V. In the two latter cases, the results are very sensitive to the details of the strain-hardening behavior and the need is demonstrated for a dynamic recovery factor to account for the onset of shear banding. Consideration is given also to the possibility that shear band behavior requires explanation on a more fundamental Hall-Petch dislocation pile-up basis. (orig.)

  3. Accretion Disks

    Spruit, H.C.

    1995-01-01

    This is an introduction to accretion disk theory, with emphasis on aspects relevant for X-ray Binaries and Cataclysmic Variables. The text corrects some mistakes in an earlier version, which appeared in 'Lives of Neutron Stars', A. Alpar, \\"U. Kizilo\\u glu and J. van Paradijs (eds.), Kluwer, Dordrecht (NATO ASI series, 1994).

  4. Accretion rates and accretion efficiency in AGNs

    Weihao, Bian; Yongheng, Zhao

    2003-01-01

    We used the standard geometrical thin accretion theory to obtain the accretion rates in Seyfert 1 galaxies and quasars. Combining accretion rates with the bolometric luminosity, we obtained the accretion efficiency. We found most of Seyfert 1 galaxies and radio quiet quasars have lower accretion efficiencies while most of the radio loud quasars possess higher accretion efficiencies. This finding further implies most of radio loud quasars possess Kerr black holes while Seyfert 1 galaxies and r...

  5. How do accretion discs break?

    Dogan, Suzan

    2016-07-01

    Accretion discs are common in binary systems, and they are often found to be misaligned with respect to the binary orbit. The gravitational torque from a companion induces nodal precession in misaligned disc orbits. In this study, we first calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. We compare the disc precession torque with the disc viscous torque to determine whether the disc should warp or break. For typical parameters precession wins: the disc breaks into distinct planes that precess effectively independently. To check our analytical findings, we perform 3D hydrodynamical numerical simulations using the PHANTOM smoothed particle hydrodynamics code, and confirm that disc breaking is widespread and enhances accretion on to the central object. For some inclinations, the disc goes through strong Kozai cycles. Disc breaking promotes markedly enhanced and variable accretion and potentially produces high-energy particles or radiation through shocks. This would have significant implications for all binary systems: e.g. accretion outbursts in X-ray binaries and fuelling supermassive black hole (SMBH) binaries. The behaviour we have discussed in this work is relevant to a variety of astrophysical systems, for example X-ray binaries, where the disc plane may be tilted by radiation warping, SMBH binaries, where accretion of misaligned gas can create effectively random inclinations and protostellar binaries, where a disc may be misaligned by a variety of effects such as binary capture/exchange, accretion after binary formation.

  6. Wind accretion: Theory and observations

    Shakura, N. I.; Postnov, K. A.; Kochetkova, A. Yu.; Hjalmarsdotter, L.; Sidoli, L.; Paizis, A.

    2015-07-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus on different regimes of quasi-spherical accretion onto the neutron star (NS): the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the NS magnetosphere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. These two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg s-1. In the subsonic case, which sets in at lower luminosities, a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto NS is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. In turn, two regimes of subsonic accretion are possible, depending on plasma cooling mechanism (Compton or radiative) near the magnetopshere. The transition from the high-luminosity with Compton cooling to the lowluminosity (Lx ≲ 3 × 1035 erg s-1) with radiative cooling can be responsible for the onset of the off states repeatedly observed in several low-luminosity slowly accreting pulsars, such as Vela X-1, GX 301-2, and 4U 1907+09. The triggering of the transitionmay be due to a switch in the X-ray beam pattern in response to a change in the optical depth in the accretion column with changing luminosity. We also show that in the settling accretion theory, bright X-ray flares (~1038-1040 erg) observed in supergiant fast X-ray transients (SFXT) can be produced by sporadic capture of magnetized stellar wind plasma. At sufficiently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of X-ray photons, strong Compton cooling and ultimately in unstable accretion of the entire shell. A bright flare develops on the free-fall time scale in the shell, and the typical energy released in an SFXT bright flare corresponds to the mass

  7. ACCRETING CIRCUMPLANETARY DISKS: OBSERVATIONAL SIGNATURES

    Zhu, Zhaohuan, E-mail: zhzhu@astro.princeton.edu [Department of Astrophysical Sciences, 4 Ivy Lane, Peyton Hall, Princeton University, Princeton, NJ 08544 (United States)

    2015-01-20

    I calculate the spectral energy distributions of accreting circumplanetary disks using atmospheric radiative transfer models. Circumplanetary disks only accreting at 10{sup –10} M {sub ☉} yr{sup –1} around a 1 M{sub J} planet can be brighter than the planet itself. A moderately accreting circumplanetary disk ( M-dot ∼10{sup −8} M{sub ⊙} yr{sup −1}; enough to form a 10 M{sub J} planet within 1 Myr) around a 1 M{sub J} planet has a maximum temperature of ∼2000 K, and at near-infrared wavelengths (J, H, K bands), this disk is as bright as a late-M-type brown dwarf or a 10 M{sub J} planet with a ''hot start''. To use direct imaging to find the accretion disks around low-mass planets (e.g., 1 M{sub J} ) and distinguish them from brown dwarfs or hot high-mass planets, it is crucial to obtain photometry at mid-infrared bands (L', M, N bands) because the emission from circumplanetary disks falls off more slowly toward longer wavelengths than those of brown dwarfs or planets. If young planets have strong magnetic fields (≳100 G), fields may truncate slowly accreting circumplanetary disks ( M-dot ≲10{sup −9} M{sub ⊙} yr{sup −1}) and lead to magnetospheric accretion, which can provide additional accretion signatures, such as UV/optical excess from the accretion shock and line emission.

  8. Dynamical structure of magnetized dissipative accretion flow around black holes

    Sarkar, Biplob

    2016-01-01

    We study the global structure of optically thin, advection dominated, magnetized accretion flow around black holes. We consider the magnetic field to be turbulent in nature and dominated by the toroidal component. With this, we obtain the complete set of accretion solutions for dissipative flows where bremsstrahlung process is regarded as the dominant cooling mechanism. We show that rotating magnetized accretion flow experiences virtual barrier around black hole due to centrifugal repulsion that can trigger the discontinuous transition of the flow variables in the form of shock waves. We examine the properties of the shock waves and find that the dynamics of the post-shock corona (PSC) is controlled by the flow parameters, namely viscosity, cooling rate and strength of the magnetic field, respectively. We separate the effective region of the parameter space for standing shock and observe that shock can form for wide range of flow parameters. We obtain the critical viscosity parameter that allows global accret...

  9. Diffusive Shock Acceleration at Cosmological Shock Waves

    Kang, Hyesung; Ryu, Dongsu

    2012-01-01

    We reexamine nonlinear diffusive shock acceleration (DSA) at cosmological shocks in the large scale structure of the Universe, incorporating wave-particle interactions that are expected to operate in collisionless shocks. Adopting simple phenomenological models for magnetic field amplification (MFA) by cosmic-ray (CR) streaming instabilities and Alfv'enic drift, we perform kinetic DSA simulations for a wide range of sonic and Alfv'enic Mach numbers and evaluate the CR injection fraction and a...

  10. High energy gamma rays from old accreting neutron stars

    P. Blasi(INAF Arcetri)

    1996-01-01

    We consider a magnetized neutron star with accretion from a companion star or a gas cloud around it, as a possible source of gamma rays with energy between $100$ $MeV$ and $10^{14}-10^{16}~eV$. The flow of the accreting plasma is terminated by a shock at the Alfv\\'en surface. Such a shock is the site for the acceleration of particles up to energies of $\\sim 10^{15}-10^{17}~eV$; gamma photons are produced in the inelastic $pp$ collisions between shock-accelerated particles and accreting matter...

  11. Time dependent white dwarf radiative shocks

    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 conduction are not negligible. The results of our calculations allow us to delineate stability regimes as a function of the dwarf mass, M/sub d/, and the accretion rate, M0. We parameterize M0 in terms of the optical depth to electron scattering through the preshock flow, tau/sub es/. In the Compton cooling and bremsstrahlung case, the shocks are unstable to low order oscillation modes if M/sub d/ less than or equal to (0.7 +- 0.1) M/sub solar/ for tau/sub es/ = 14, and if M/sub d/ less than or equal to (0.9 +- 0.1) M/sub solar/ for tau/sub es/ = 1. When electron thermal conduction is added, low order oscillation modes are unstable only if M/sub d/ less than or equal to (0.3 +- 0.1) M/sub sun mass/. The unstable modes have approximate oscillation periods of 1.1 tau/sub br/ and 0.63 tau/sub br/, where tau/sub br/ is the bremsstrahlung cooling time scale of the postshock plasma. Our results can be scaled to magnetically funneled accretion flows as long as cyclotron emission contributes less than about 10% of the postshock cooling. 14 refs., 1 fig

  12. Accretion, winds and outflows in young stars

    Günther, Hans Moritz

    2012-01-01

    Young stars and planetary systems form in molecular clouds. For classical T Tauri stars (CTTS, F-K type precursors) the accretion disk does not reach down to the central star, but it is truncated near the co-rotation radius. The inner edge of the disk is ionized by the stellar radiation, so that the accretion stream is funneled along the magnetic field lines. On the stellar surface an accretion shock develops, which is observed over a wide wavelength range as X-ray emission, UV excess, optical veiling and optical and IR emission lines. Some of the accretion tracers, e.g. H\\alpha, can be calibrated to measure the accretion rate. This accretion process is variable on time scales of hours to years due to changing accretion rates, stellar rotation and reconfiguration of the magnetic field. Furthermore, many accreting systems also drive strong outflows which are ultimately powered by accretion. Several components could contribute to the outflows: slow, wide-angle disk winds, X-winds launched close to the inner dis...

  13. Theory of disk accretion onto supermassive black holes

    Armitage, P J

    2004-01-01

    Accretion onto supermassive black holes produces both the dramatic phenomena associated with active galactic nuclei and the underwhelming displays seen in the Galactic Center and most other nearby galaxies. I review selected aspects of the current theoretical understanding of black hole accretion, emphasizing the role of magnetohydrodynamic turbulence and gravitational instabilities in driving the actual accretion and the importance of the efficacy of cooling in determining the structure and observational appearance of the accretion flow. Ongoing investigations into the dynamics of the plunging region, the origin of variability in the accretion process, and the evolution of warped, twisted, or eccentric disks are summarized.

  14. Modelling accretion disc and stellar wind interactions: the case of Sgr A*

    Christie, I. M.; Petropoulou, M.; Mimica, P.; Giannios, D.

    2016-07-01

    Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disc, the ram and thermal pressures of the disc terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin-Helmholtz instabilities. Because the characteristic wind and stellar velocities are in ˜108 cm s-1 range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericentre passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly a month long X-ray flare with a peak luminosity of L ≈ 4 × 1033 erg s-1 for a stellar mass-loss rate, disc number density, and thermal pressure strength of dot{M}_w= 10^{-7} M_{⊙} yr^{-1}, nd = 105 cm-3, and α = 0.1, respectively. This peak luminosity is comparable to the quiescent X-ray emission detected from Sgr A* and is within the detection capabilities of current X-ray observatories. Its detection could constrain the density and thickness of the disc at a distance of ˜3000 gravitational radii from the supermassive black hole.

  15. Accreting Matter around Clusters of Galaxies One-Dimensional Considerations

    Ryu, D; Ryu, Dongsu; Kang, Hyesung

    1996-01-01

    During the formation of the large scale structure of the Universe, matter accretes onto high density peaks. Accreting collisionless dark matter (DM) forms caustics around them, while accreting collisional baryonic matter (BM) forms accretion shocks. The properties of the accreting matter depend upon the power spectrum of the initial perturbations on a given scale as well as the background expansion in a given cosmological model. In this paper, we have calculated the accretion of DM particles in one-dimensional spherical geometry under various cosmological models including the Einstein-de Sitter universe, the open universe with $\\Omega_o<1$, and the flat universe with a point mass at the origin has been considered. Since the accretion shock of BM is expected to form close to the first caustic of DM, the properties of the accreting BM are common with those of the DM. Hence, the accretion calculations with DM particles have been used to find the position and velocity of the accretion shock and the cluster mas...

  16. Numerical Simulations of Viscous Accretion Flow around Black Holes

    Lee, Seong-Jae; Chattopadhyay, Indranil; Kumar, Rajiv; Hyung, Siek; Ryu, Dongsu

    2016-06-01

    We present shocked viscous accretion flow onto a black hole in a two dimensional cylindrical geometry, where initial conditions were chosen from analytical solutions. The simulation code used the Lagrangian Total Variation Diminishing (LTVD) and remap routine, which enabled us to attain high accuracy in capturing shocks and to handle the angular momentum distribution correctly. The steady state shocked solution in the inviscid, as well as in the viscous regime, matched theoretical predictions well, but increasing viscosity renders the accretion shock unstable. Large amplitude shock oscillation is accompanied by intermittent, transient inner multiple shocks. Such oscillation of the inner part of disk is interpreted as the source of QPO in hard X-rays observed in microquasars; and strong shock oscillation induces strong episodic jet emission. The periodicity of jets and shock oscillation are similar. Our simulation shows that the jets for higher viscosity parameter are evidently stronger and faster than that for lower viscosity.

  17. Accretion Discs Around Black Holes: Developement of Theory

    Bisnovatyi-Kogan, G. S.

    1999-01-01

    Standard accretion disk theory is formulated which is based on the local heat balance. The energy produced by a turbulent viscous heating is supposed to be emitted to the sides of the disc. Sources of turbulence in the accretion disc are connected with nonlinear hydrodynamic instability, convection, and magnetic field. In standard theory there are two branches of solution, optically thick, and optically thin. Advection in accretion disks is described by the differential equations what makes t...

  18. Theory of disk accretion onto supermassive black holes

    Armitage, Philip J.

    2004-01-01

    Accretion onto supermassive black holes produces both the dramatic phenomena associated with active galactic nuclei and the underwhelming displays seen in the Galactic Center and most other nearby galaxies. I review selected aspects of the current theoretical understanding of black hole accretion, emphasizing the role of magnetohydrodynamic turbulence and gravitational instabilities in driving the actual accretion and the importance of the efficacy of cooling in determining the structure and ...

  19. 2-D MHD Configurations for Accretion Disks Around Magnetized Stars

    Benini, Riccardo; Montani, Giovanni

    2009-01-01

    We discuss basic features of steady accretion disk morphology around magnetized compact astrophysical objects. A comparison between the standard model of accretion based on visco-resistive MHD and the plasma instabilities, like ballooning modes, triggered by very low value of resistivity, is proposed.

  20. Time-dependent Hypercritical Accretion onto Black Holes

    Zampieri, Luca

    1996-01-01

    Results are presented from a time-dependent, numerical investigation of super-Eddington spherical accretion onto black holes with different initial conditions. We have studied the stability of stationary solutions, the non-linear evolution of shocked models and the time-dependent accretion from an expanding medium.

  1. Spreading Layers in Accreting Objects: Role of Acoustic Waves for Angular Momentum Transport, Mixing, and Thermodynamics

    Philippov, Alexander A.; Rafikov, Roman R.; Stone, James M.

    2016-01-01

    Disk accretion at a high rate onto a white dwarf (WD) or a neutron star has been suggested to result in the formation of a spreading layer (SL)—a belt-like structure on the object's surface, in which the accreted matter steadily spreads in the poleward (meridional) direction while spinning down. To assess its basic characteristics, we perform two-dimensional hydrodynamic simulations of supersonic SLs in the relevant morphology with a simple prescription for cooling. We demonstrate that supersonic shear naturally present at the base of the SL inevitably drives sonic instability that gives rise to large-scale acoustic modes governing the evolution of the SL. These modes dominate the transport of momentum and energy, which is intrinsically global and cannot be characterized via some form of local effective viscosity (e.g., α-viscosity). The global nature of the wave-driven transport should have important implications for triggering Type I X-ray bursts in low-mass X-ray binaries. The nonlinear evolution of waves into a system of shocks drives effective rearrangement (sensitively depending on thermodynamical properties of the flow) and deceleration of the SL, which ultimately becomes transonic and susceptible to regular Kelvin-Helmholtz instability. We interpret this evolution in terms of the global structure of the SL and suggest that mixing of the SL material with the underlying stellar fluid should become effective only at intermediate latitudes on the accreting object's surface, where the flow has decelerated appreciably. In the near-equatorial regions the transport is dominated by acoustic waves and mixing is less efficient. We speculate that this latitudinal nonuniformity of mixing in accreting WDs may be linked to the observed bipolar morphology of classical nova ejecta.

  2. Simulations of Viscous Accretion Flow around Black Holes in Two-Dimensional Cylindrical Geometry

    Lee, Seong-Jae; Kumar, Rajiv; Hyung, Siek; Ryu, Dongsu

    2016-01-01

    We simulate shock-free and shocked viscous accretion flow onto a black hole in a two dimensional cylindrical geometry, where initial conditions were chosen from analytical solutions. The simulation code used the Lagrangian Total Variation Diminishing (LTVD) and remap routine, which enabled us to attain high accuracy in capturing shocks and to handle the angular momentum distribution correctly. Inviscid shock-free accretion disk solution produced a thick disk structure, while the viscous shock-free solution attained a Bondi-like structure, but in either case, no jet activity nor any QPO-like activity developed. The steady state shocked solution in the inviscid, as well as, in the viscous regime, matched theoretical predictions well. However, increasing viscosity renders the accretion shock unstable. Large amplitude shock oscillation is accompanied by intermittent, transient inner multiple shocks. Such oscillation of the inner part of disk is interpreted as the source of QPO in hard X-rays observed in micro-qua...

  3. Theory of wind accretion

    Shakura N.I.; Postnov K.A.; Kochetkova A.Yu.; Hjalmarsdotter L.

    2013-01-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about $4\\...

  4. Evolution of Disk Accretion

    Calvet, Nuria; Hartmann, Lee; Strom, Stephen E.

    1999-01-01

    We review the present knowledge of disk accretion in young low mass stars, and in particular, the mass accretion rate and its evolution with time. The methods used to obtain mass accretion rates from ultraviolet excesses and emission lines are described, and the current best estimates of mass accretion rate for Classical T Tauri stars and for objects still surrounded by infalling envelopes are given. We argue that the low mass accretion rates of the latter objects require episodes of high mas...

  5. Discovery of an Accretion-Fed Corona in an Accreting Young Star

    Wolk, Scott J.; Brickhouse, N.; Cranmer, S.; Dupree, A.; Luna, G. J. M.

    2010-01-01

    A deep (489 ks) Chandra High Energy Transmission Grating spectrum of the classical T Tauri star TW Hydrae shows a new type of coronal structure that is produced by the accretion process. In the standard model for a stellar dipole, the magnetic field truncates the disk and channels the accreting material onto the star. The He-like diagnostic lines of Ne IX provide excellent agreement with the shock conditions predicted by this model, with an electron temperature of 2.5 MK and electron density of 3 times 1012 cm-3 (see also Kastner et al. 2002). However, the standard model completely fails to predict the post-shock conditions, significantly overpredicting both the density and absorption observed at O VII. Instead the observations require a second "post-shock" component with 30 times more mass and 1000 times larger volume than found at the shock itself. We note that in the standard model, the shocked plasma is conveniently located near both closed (coronal) and open (stellar wind) magnetic structures, as the magnetic field connecting the star and disk also separates the open and closed field regions on the stellar surface. The shocked plasma thus can provide the energy to heat not only the post-shock plasma, but also adjacent regions (i.e. an "accretion-fed corona") and drive stellar material into surrounding coronal structures. These observations provide new clues to the puzzling soft X-ray excess found in accreting systems, which depends on both the presence of accretion and the level of coronal activity (Guedel and Telleschi 2007). This work is partially supported by CXO grant G07-8018X.

  6. Cross-correlation Aided Transport in Stochastically Driven Accretion Flows

    Nath, Sujit Kumar

    2014-01-01

    Origin of linear instability resulting in rotating sheared accretion flows has remained a controversial subject for long. While some explanations of such non-normal transient growth of disturbances in the Rayleigh stable limit were available for magnetized accretion flows, similar instabilities in absence of magnetic perturbations remained unexplained. This dichotomy was resolved in two recent publications by Chattopadhyay, {\\it et al} where it was shown that such instabilities, especially for non-magnetized accretion flows, were introduced through interaction of the inherent stochastic noise in the system (even a \\enquote{cold} accretion flow at 3000K is too \\enquote{hot} in the statistical parlance and is capable of inducing strong thermal modes) with the underlying Taylor-Couette flow profiles. Both studies, however, excluded the additional energy influx (or efflux) that could result from nonzero cross-correlation of a noise perturbing the velocity flow, say, with the noise that is driving the vorticity fl...

  7. Estimation of mass outflow rates from viscous relativistic accretion discs around black holes

    Chattopadhyay, Indranil; Kumar, Rajiv

    2016-01-01

    We investigated flow in Schwarzschild metric, around a non-rotating black hole and obtained self-consistent accretion - ejection solution in full general relativity. We covered the whole of parameter space in the advective regime to obtain shocked, as well as, shock-free accretion solution. We computed the jet streamline using von - Zeipel surfaces and projected the jet equations of motion on to the streamline and solved them simultaneously with the accretion disc equations of motion. We foun...

  8. The Magnetoviscous-thermal Instability

    Islam, Tanim

    2011-01-01

    Accretion flows onto underluminous black holes, such as Sagittarius A* at the center of our galaxy, are dilute (mildly collisional to highly collisionless), optically thin, and radiatively inefficient. Therefore, the accretion properties of such dilute flows are expected to be modified by their large viscosities and thermal conductivities. Second, turbulence within these systems needs to transport angular momentum as well as thermal energy generated through gravitational infall outwards in order to allow accretion to occur. This is in contrast to classical accretion flows, in which the energy generated through accretion down a gravitational well is locally radiated. In this paper, using an incompressible fluid treatment of an ionized gas, we expand on previous research by considering the stability properties of a magnetized rotating plasma wherein the thermal conductivity and viscosity are not negligible and may be dynamically important. We find a class of MHD instabilities that can transport angular momentum...

  9. Simulations of relativistic collisionless shocks: shock structure and particle acceleration

    Spitkovsky, A

    2006-01-01

    We discuss 3D simulations of relativistic collisionless shocks in electron-positron pair plasmas using the particle-in-cell (PIC) method. The shock structure is mainly controlled by the shock's magnetization ("sigma" parameter). We demonstrate how the structure of the shock varies as a function of sigma for perpendicular shocks. At low magnetizations the shock is mediated mainly by the Weibel instability which generates transient magnetic fields that can exceed the initial field. At larger magnetizations the shock is dominated by magnetic reflections. We demonstrate where the transition occurs and argue that it is impossible to have very low magnetization collisionless shocks in nature (in more than one spatial dimension). We further discuss the acceleration properties of these shocks, and show that higher magnetization perpendicular shocks do not efficiently accelerate nonthermal particles in 3D. Among other astrophysical applications, this may pose a restriction on the structure and composition of gamma-ray...

  10. 一种用于RM不稳定性研究的竖直环形激波管的设计与验证%Design and validation of a vertical annular shock tube for RM instability study

    龙桐; 翟志刚; 司廷; 罗喜胜

    2014-01-01

    设计并加工了一套竖直环形同轴无膜激波管,可用于环形汇聚激波诱导下的Richtmyer-Meshkov 不稳定性实验研究。与前人工作相比,本文在流体界面的形成以及流场的观测方法上做了较大的改进。通过实验和数值方法,对该竖直激波管产生的环形柱状汇聚激波的参数进行测量和分析,验证了同轴激波管形成柱状汇聚激波方法的可行性和可靠性。在界面形成方面,采用细丝约束肥皂膜技术形成正八边形气体界面,并利用数值方法考察了细丝对界面发展的影响。结果表明在界面发展的前期,细丝的影响几乎可以忽略。利用连续激光片光结合高速摄影相机对流场进行观测,获得了正八边形air/SF6气体界面在环形汇聚激波及其反射激波冲击下的演化过程,并与数值结果进行了对比,获得了较好的一致性,进一步验证了汇聚激波的对称性以及细丝约束肥皂膜技术用于形成多边形气体界面的可靠性。%A vertical annular coaxial diaphragm-less shock tube is designed based on the prin-cipal proposed by Hosseini and Takayama and modified in order to conveniently install the initial interface in the test section and visualize the flow field for the investigation of the Richtmyer-Meshkov (RM)instability.Parametric study is carried out both experimentally and numerically to explore the characteristics of the annular coaxial cylindrical converging shock wave.The varia-tion of pressure behind the shock shows the feasibility and reliability of this shock tube to generate the annular coaxial cylindrical converging shock wave.The pressure variations with time at differ-ent positions in the test section are acquired from the experiment and numerical simulation,and the converging effect of the shock wave is emphasized.After the validation of the converging shock wave,the experiment of RM instability induced by this converging shock wave is con

  11. Dynamical structure of magnetized dissipative accretion flow around black holes

    Sarkar, Biplob; Das, Santabrata

    2016-06-01

    We study the global structure of optically thin, advection dominated, magnetized accretion flow around black holes. We consider the magnetic field to be turbulent in nature and dominated by the toroidal component. With this, we obtain the complete set of accretion solutions for dissipative flows where bremsstrahlung process is regarded as the dominant cooling mechanism. We show that rotating magnetized accretion flow experiences virtual barrier around black hole due to centrifugal repulsion that can trigger the discontinuous transition of the flow variables in the form of shock waves. We examine the properties of the shock waves and find that the dynamics of the post-shock corona (PSC) is controlled by the flow parameters, namely viscosity, cooling rate and strength of the magnetic field, respectively. We separate the effective region of the parameter space for standing shock and observe that shock can form for wide range of flow parameters. We obtain the critical viscosity parameter that allows global accretion solutions including shocks. We estimate the energy dissipation at the PSC from where a part of the accreting matter can deflect as outflows and jets. We compare the maximum energy that could be extracted from the PSC and the observed radio luminosity values for several super-massive black hole sources and the observational implications of our present analysis are discussed.

  12. Dynamical structure of magnetized dissipative accretion flow around black holes

    Sarkar, Biplob; Das, Santabrata

    2016-09-01

    We study the global structure of optically thin, advection dominated, magnetized accretion flow around black holes. We consider the magnetic field to be turbulent in nature and dominated by the toroidal component. With this, we obtain the complete set of accretion solutions for dissipative flows where bremsstrahlung process is regarded as the dominant cooling mechanism. We show that rotating magnetized accretion flow experiences virtual barrier around black hole due to centrifugal repulsion that can trigger the discontinuous transition of the flow variables in the form of shock waves. We examine the properties of the shock waves and find that the dynamics of the post-shock corona (PSC) is controlled by the flow parameters, namely viscosity, cooling rate and strength of the magnetic field, respectively. We separate the effective region of the parameter space for standing shock and observe that shock can form for wide range of flow parameters. We obtain the critical viscosity parameter that allows global accretion solutions including shocks. We estimate the energy dissipation at the PSC from where a part of the accreting matter can deflect as outflows and jets. We compare the maximum energy that could be extracted from the PSC and the observed radio luminosity values for several supermassive black hole sources and the observational implications of our present analysis are discussed.

  13. An analytical model of accretion onto white dwarfs

    Ospina, N.; Hernanz, M.

    2013-05-01

    The analytical model of Frank et al. (2002) has been used to investigate the structure of the accretion stream onto white dwarfs (WD). In particular, the post-shock region (temperature, density and gas velocity distributions) and X-ray spectrum emitted by this region. We have obtained the temperature, density and gas velocity distributions of the emission region for different masses of white dwarfs and at different positions in the shock coordinate. Also, we calculated the emitted spectrum for different WD masses and at different positions of the shock with the principal objective of study the accretion at different points of the emission region.

  14. Time-dependent corona models: coronae with accretion

    Models of stationary extended coronae are presented for various values of the interstellar density. These calculations have been performed with the implicit time-dependent numerical method developed by Korevaar and Van Leer (1988). If the interstellar density is sufficiently low, the coronal gas expands through the Parker critical point to supersonic velocities. An increase in the interstellar density moves the interstellar shock closer to the star. When it comes closer than the critical point, the flow changes to a breeze solution that is subsonic everywhere. A further increase in the interstellar density reverses the flow. First an accretion breeze solution is found and then an inflow with a stationary accretion shock. This is the first numerical calculation of the complete set of stationary stellar wind solutions in spherical symmetry with boundary conditions specified at the stellar surface and at infinity, including the solutions with an interstellar shock or an accretion shock

  15. Dynamics of continental accretion.

    Moresi, L; Betts, P G; Miller, M S; Cayley, R A

    2014-04-10

    Subduction zones become congested when they try to consume buoyant, exotic crust. The accretionary mountain belts (orogens) that form at these convergent plate margins have been the principal sites of lateral continental growth through Earth's history. Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides, along the eastern margin of the supercontinent Gondwana, and the Altaïdes, which formed on the southern margin of Laurasia. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback and transient episodes of orogenesis and crustal shortening, coincident with accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region. The complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon. PMID:24670638

  16. Review: Accretion Disk Theory

    Montesinos, Matias

    2012-01-01

    In this paper I review and discuss the basic concepts of accretion disks, focused especially on the case of accretion disks around black holes. The well known alpha-model is revisited, showing the strengths and weaknesses of the model. Other turbulent viscosity prescription, based on the Reynolds number, that may improve our understanding of the accretion paradigm is discussed. A simple but efficient mathematical model of a self-gravitating accretion disk, as well as observational evidence of...

  17. Infall and accretion

    Combes, F.

    2007-01-01

    Gas infall and accretion play a fundamental role in galaxy formation, and several processes of accretion are reviewed. In particular the cold accretion may solve to some extent the angular momentum problem in disk formation, while it is aggravated by mergers. Gas accretion is one of the main actor in secular evolution: it is required to account for recurrent bar formation, and to explain the feedback cycles of formation of bulges and black holes, with correlated masses. Infall is also require...

  18. Rayleigh-Taylor instability in compressible fluids

    Sturtevant, B. (California Inst. of Tech., Pasadena, CA (USA). Graduate Aeronautical Labs.)

    1990-11-05

    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 shock wave interaction with gas-gas interfaces. In particular, the nonlinear stage of shock-initiated Rayleigh-Taylor instability, the secondary instabilities (e.g., Kelvin-Helmholtz instability) arising therefrom and the concomitant mixing of the two fluids are of interest.''

  19. Massive Star Formation: Accreting from Companion

    X. Chen; J. S. Zhang

    2014-09-01

    We report the possible accretion from companion in the massive star forming region (G350.69–0.49). This region seems to be a binary system composed of a diffuse object (possible nebulae or UC HII region) and a Massive Young Stellar Object (MYSO) seen in Spitzer IRAC image. The diffuse object and MYSO are connected by the shock-excited 4.5 m emission, suggesting that the massive star may form through accreting material from the companion in this system.

  20. Magnetically controlled accretion onto a black hole

    Ikhsanov, N R; Beskrovnaya, N G; 10.1088/1742-6596/372/1/012062

    2012-01-01

    An accretion scenario in which the material captured by a black hole from its environment is assumed to be magnetized (\\beta ~ 1) is discussed. We show that the accretion picture in this case is strongly affected by the magnetic field of the flow itself. The accretion power within this Magnetically Controlled Accretion (MCA) scenario is converted predominantly into the magnetic energy of the accretion flow. The rapidly amplified field prevents the accretion flow from forming a homogeneous Keplerian disk. Instead, the flow is decelerated by its own magnetic field at a large distance (Shvartsman radius) from the black hole and switches into a non-Keplerian dense magnetized slab. The material in the slab is confined by the magnetic field and moves towards the black hole on the time scale of the magnetic field annihilation. The basic parameters of the slab are evaluated. Interchange instabilities in the slab may lead to a formation of Z-pinch type configuration of the magnetic field over the slab in which the acc...

  1. Radiative Shock Waves In Emerging Shocks

    Drake, R. Paul; Doss, F.; Visco, A.

    2011-05-01

    In laboratory experiments we produce radiative shock waves having dense, thin shells. These shocks are similar to shocks emerging from optically thick environments in astrophysics in that they are strongly radiative with optically thick shocked layers and optically thin or intermediate downstream layers through which radiation readily escapes. Examples include shocks breaking out of a Type II supernova (SN) and the radiative reverse shock during the early phases of the SN remnant produced by a red supergiant star. We produce these shocks by driving a low-Z plasma piston (Be) at > 100 km/s into Xe gas at 1.1 atm. pressure. The shocked Xe collapses to > 20 times its initial density. Measurements of structure by radiography and temperature by several methods confirm that the shock wave is strongly radiative. We observe small-scale perturbations in the post-shock layer, modulating the shock and material interfaces. We describe a variation of the Vishniac instability theory of decelerating shocks and an analysis of associated scaling relations to account for the growth of these perturbations, identify how they scale to astrophysical systems such as SN 1993J, and consider possible future experiments. Collaborators in this work have included H.F. Robey, J.P. Hughes, C.C. Kuranz, C.M. Huntington, S.H. Glenzer, T. Doeppner, D.H. Froula, M.J. Grosskopf, and D.C. Marion ________________________________ * Supported by the US DOE NNSA under the Predictive Sci. Academic Alliance Program by grant DE-FC52-08NA28616, the Stewardship Sci. Academic Alliances program by grant DE-FG52-04NA00064, and the Nat. Laser User Facility by grant DE-FG03-00SF22021.

  2. Estimation of mass outflow rates from viscous relativistic accretion discs around black holes

    Chattopadhyay, Indranil

    2016-01-01

    We investigated flow in Schwarzschild metric, around a non-rotating black hole and obtained self-consistent accretion - ejection solution in full general relativity. We covered the whole of parameter space in the advective regime to obtain shocked, as well as, shock-free accretion solution. We computed the jet streamline using von - Zeipel surfaces and projected the jet equations of motion on to the streamline and solved them simultaneously with the accretion disc equations of motion. We found that steady shock cannot exist {for $\\alpha \\gsim0.06$} in the general relativistic prescription, but is lower if mass - loss is considered too. We showed that for fixed outer boundary, the shock moves closer to the horizon with increasing viscosity parameter. The mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, maximizing at some intermediate value of shock {location}. The jet terminal speed increases with stronger shocks, quantitatively speaking, the terminal speed of ...

  3. Variable protostellar accretion with episodic bursts

    Vorobyov, Eduard I

    2015-01-01

    We present the latest development of the disk gravitational instability and fragmentation model, originally introduced by us to explain episodic accretion bursts in the early stages of star formation. Using our numerical hydrodynamics model with improved disk thermal balance and star-disk interaction, we computed the evolution of protostellar disks formed from the gravitational collapse of prestellar cores. In agreement with our previous studies, we find that cores of higher initial mass and angular momentum produce disks that are more favorable to gravitational instability and fragmentation, while a higher background irradiation and magnetic fields moderate the disk tendency to fragment. The protostellar accretion in our models is time-variable, thanks to the nonlinear interaction between different spiral modes in the gravitationally unstable disk, and can undergo episodic bursts when fragments migrate onto the star owing to the gravitational interaction with other fragments or spiral arms. Most bursts occur...

  4. Acoustic horizons in axially symmetric relativistic accretion

    Abraham, H; Das, T K; Abraham, Hrvoje; Bilic, Neven; Das, Tapas K.

    2006-01-01

    Transonic accretion onto astrophysical objects is a unique example of analogue black hole realized in nature. In the framework of acoustic geometry we study axially symmetric accretion and wind of a rotating astrophysical black hole or of a neutron star assuming isentropic flow of a fluid described by a polytropic equation of state. In particular we analyze the causal structure of multitransonic configurations with two sonic points and a shock. Retarded and advanced null curves clearly demonstrate the presence of the acoustic black hole at regular sonic points and of the white hole at the shock. We calculate the analogue surface gravity and the Hawking temperature for the inner and the outer acoustic horizons.

  5. Fermi bubbles inflated by winds launched from the hot accretion flow in Sgr A*

    A pair of giant gamma-ray Bubbles has been revealed by Fermi-LAT. In this paper we investigate their formation mechanism. Observations have indicated that the activity of the supermassive black hole located at the Galactic center, Sgr A*, was much stronger than at the present time. Specifically, one possibility is that while Sgr A* was also in the hot accretion regime, the accretion rate should be 103-104 times higher during the past ∼107 yr. On the other hand, recent magnetohydrodynamic numerical simulations of hot accretion flows have unambiguously shown the existence and obtained the properties of strong winds. Based on this knowledge, by performing three-dimensional hydrodynamical simulations, we show in this paper that the Fermi Bubbles could be inflated by winds launched from the 'past' hot accretion flow in Sgr A*. In our model, the active phase of Sgr A* is required to last for about 10 million years and it was quenched no more than 0.2 million years ago. The central molecular zone (CMZ) is included and it collimates the wind orientation toward the Galactic poles. Viscosity suppresses the Rayleigh-Taylor and Kelvin-Helmholtz instabilities and results in the smoothness of the Bubbles edge. The main observational features of the Bubbles can be well explained. Specifically, the ROSAT X-ray features are interpreted by the shocked interstellar medium and the interaction region between the wind and CMZ gas. The thermal pressure and temperature obtained in our model are consistent with recent Suzaku observations.

  6. Local Magnetohydrodynamical Models of Layered Accretion Disks

    Fleming, Timothy; Stone, James M.

    2002-01-01

    Using numerical MHD simulations, we have studied the evolution of the magnetorotational instability in stratified accretion disks in which the ionization fraction (and therefore resistivity) varies substantially with height. This model is appropriate to dense, cold disks around protostars or dwarf nova systems which are ionized by external irradiation of cosmic rays or high-energy photons. We find the growth and saturation of the MRI occurs only in the upper layers of the disk where the magne...

  7. Stability of black hole accretion disks

    Czerny B.

    2012-12-01

    Full Text Available We discuss the issues of stability of accretion disks that may undergo the limit-cycle oscillations due to the two main types of thermal-viscous instabilities. These are induced either by the domination of radiation pressure in the innermost regions close to the central black hole, or by the partial ionization of hydrogen in the zone of appropriate temperatures. These physical processes may lead to the intermittent activity in AGN on timescales between hundreds and millions of years. We list a number of observational facts that support the idea of the cyclic activity in high accretion rate sources. We conclude however that the observed features of quasars may provide only indirect signatures of the underlying instabilities. Also, the support from the sources with stellar mass black holes, whose variability timescales are observationally feasible, is limited to a few cases of the microquasars. Therefore we consider a number of plausible mechanisms of stabilization of the limit cycle oscillations in high accretion rate accretion disks. The newly found is the stabilizing effect of the stochastic viscosity fluctuations.

  8. Virial theorem for radiating accretion discs

    Mach, Patryk

    2011-01-01

    A continuum version of the virial theorem is derived for a radiating self-gravitating accretion disc around a compact object. The central object is point-like, but we can avoid the regularization of its gravitational potential. This is achieved by applying a modified Pohozaev-Rellich identity to the gravitational potential of the disk only. The theorem holds for general stationary configurations, including discontinuous flows (shock waves, contact discontinuities). It is used to test numerica...

  9. Accretion, winds and outflows in young stars

    Günther, H. M.

    2013-02-01

    Young stars and planetary systems form in molecular clouds. After the initial radial infall an accretion disk develops. For classical T Tauri stars (CTTS, F-K type precursors) the accretion disk does not reach down to the central star, but it is truncated near the co-rotation radius by the stellar magnetic field. The inner edge of the disk is ionized by the stellar radiation, so that the accretion stream is funneled along the magnetic field lines. On the stellar surface an accretion shock develops, which is observed over a wide wavelength range as X-ray emission, UV excess, optical veiling and optical and IR emission lines. Some of the accretion tracers, e.g. Hα, can be calibrated to measure the accretion rate. This accretion process is variable on time scales of hours to years due to changing accretion rates, stellar rotation and reconfiguration of the magnetic field. Furthermore, many (if not all) accreting systems also drive strong outflows which are ultimately powered by accretion. However, the exact driving mechanism is still unclear. Several components could contribute to the outflows: slow, wide-angle disk winds, X-winds launched close to the inner disk rim, and thermally driven stellar winds. In any case, the outflows contain material of very different temperatures and speeds. The disk wind is cool and can have a molecular component with just a few tens of km s-1, while the central component of the outflow can reach a few 100 km s-1. In some cases the inner part of the outflow is collimated to a small-angle jet. These jets have an onion-like structure, where the inner components are consecutively hotter and faster. The jets can contain working surfaces, which show up as Herbig-Haro knots. Accretion and outflows in the CTTS phase do not only determine stellar parameters like the rotation rate on the main-sequence, they also can have a profound impact on the environment of young stars. This review concentrates on CTTS in near-by star forming regions where

  10. Observational constraints on viscosity in AGN accretion discs

    The optical/UV/soft X-ray big bump can be modelled as thermal emission from an accretion disc. The observed UV variability in AGN spectra may be caused by accretion-disc instabilities, and can be used to constrain the viscosity. The comparison of thermal time-scales with the observed time-scales of variability in 10 Seyfert galaxies and 16 QSOs indicates values for the parameter α of the order of 0.01 for most cases. (author)

  11. Mineral accretion in seawater

    Bozak, Ronald Richard

    2000-10-01

    By performing electrolysis in seawater a concrete-like accretion of precipitating aragonite (one crystalline form of CaCO3) and brucite (Mg(OH) 2) slowly develops onto the cathode. The accretion forms by high pH conditions caused by the reduction reactions occurring at the cathode. A solid casing of accretions over a preformed cathodic mesh has the potential for many engineering applications such as artificial reefs, sub-surface breakwaters and pipe construction. To investigate using mineral accretion as an alternative means of construction, experiments in the open coast, laboratory and ocean harbor have resulted in tables that can projected into a feasibility study. Inevitable current density variations over the cathodic framework and sensitivity to seawater hydrodynamics make accretion thickness difficult to predict and control in practice. Ideal conditions for growing a large-scale mineral accretion structure are still, clean ocean waters where low DCV power can be delivered on the order of years.

  12. Large Cosmic Shock Waves as Sites for Particle Acceleration

    Miniati, Francesco; Ryu, Dongsu; Kang, Hyesung; Jones, T. W.

    1999-01-01

    The properties of cosmic shock waves are studied through numerical simulations in two cosmological scenarios (SCDM and LCDM). The scaling relations for the average radius and velocity associated with the accretion shocks are somewhat different, yet qualitatively similar to the self similar solutions for a flat Omega_M=1 universe. The energy supplied by infalling gas at accretion shock waves is large enough to sustain production of abundant cosmic ray populations if a viable acceleration mecha...

  13. The Final Fates of Accreting Supermassive Stars

    Umeda, Hideyuki; Omukai, Kazuyuki; Yoshida, Naoki

    2016-01-01

    The formation of supermassive stars (SMSs) via rapid mass accretion and their direct collapse into black holes (BHs) is a promising pathway for sowing seeds of supermassive BHs in the early universe. We calculate the evolution of rapidly accreting SMSs by solving the stellar structure equations including nuclear burning as well as general relativistic (GR) effects up to the onset of the collapse. We find that such SMSs have less concentrated structure than fully-convective counterpart, which is often postulated for non-accreting ones. This effect stabilizes the stars against GR instability even above the classical upper mass limit $\\gtrsim 10^5~M_\\odot$ derived for the fully-convective stars. The accreting SMS begins to collapse at the higher mass with the higher accretion rate. The collapse occurs when the nuclear fuel is exhausted only for cases with $\\dot M \\lesssim 0.1~M_\\odot~{\\rm yr}^{-1}$. With $\\dot{M} \\simeq 0.3 - 1~M_\\odot~{\\rm yr}^{-1}$, the star becomes GR-unstable during the helium-burning stage ...

  14. Modelling accretion disc and stellar wind interactions: the case of Sgr A*

    Christie, I. M.; Petropoulou, M.; Mimica, P.; Giannios, D.

    2016-01-01

    Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disc, the ram and thermal pressures of the disc terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin–Helmholtz instabilities. Because the characteristic wind and stellar velocities are in ∼108 cm s−1 range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericentre passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly a month long X-ray flare with a peak luminosity of L ≈ 4 × 1033 erg s−1 for a stellar mass-loss rate, disc number density, and thermal pressure strength of \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{upgreek} \\usepackage{mathrsfs} \\setlength{\\oddsidemargin}{-69pt} \\begin{document} }{}$\\dot{M}_{\\rm w}= 10^{-7} \\,\\mathrm{M}_{\\odot }\\, {\\rm yr}^{-1}$\\end{document}, nd = 105 cm−3, and α = 0.1, respectively. This peak luminosity is comparable to the quiescent X-ray emission detected from Sgr A* and is within the detection capabilities of current X-ray observatories. Its detection could constrain the density and thickness of the disc at a distance of ∼3000 gravitational radii from the supermassive black hole. PMID:27279781

  15. SUPERNOVA LIGHT CURVES POWERED BY FALLBACK ACCRETION

    Dexter, Jason; Kasen, Daniel, E-mail: jdexter@berkeley.edu [Departments of Physics and Astronomy, University of California, Berkeley, CA 94720 (United States)

    2013-07-20

    Some fraction of the material ejected in a core collapse supernova explosion may remain bound to the compact remnant, and eventually turn around and fall back. We show that the late time ({approx}>days) power potentially associated with the accretion of this 'fallback' material could significantly affect the optical light curve, in some cases producing super-luminous or otherwise peculiar supernovae. We use spherically symmetric hydrodynamical models to estimate the accretion rate at late times for a range of progenitor masses and radii and explosion energies. The accretion rate onto the proto-neutron star or black hole decreases as M-dot {proportional_to}t{sup -5/3} at late times, but its normalization can be significantly enhanced at low explosion energies, in very massive stars, or if a strong reverse shock wave forms at the helium/hydrogen interface in the progenitor. If the resulting super-Eddington accretion drives an outflow which thermalizes in the outgoing ejecta, the supernova debris will be re-energized at a time when photons can diffuse out efficiently. The resulting light curves are different and more diverse than previous fallback supernova models which ignored the input of accretion power and produced short-lived, dim transients. The possible outcomes when fallback accretion power is significant include super-luminous ({approx}> 10{sup 44} erg s{sup -1}) Type II events of both short and long durations, as well as luminous Type I events from compact stars that may have experienced significant mass loss. Accretion power may unbind the remaining infalling material, causing a sudden decrease in the brightness of some long duration Type II events. This scenario may be relevant for explaining some of the recently discovered classes of peculiar and rare supernovae.

  16. Supernova Light Curves Powered by Fallback Accretion

    Dexter, Jason; Kasen, Daniel

    2013-07-01

    Some fraction of the material ejected in a core collapse supernova explosion may remain bound to the compact remnant, and eventually turn around and fall back. We show that the late time (gsimdays) power potentially associated with the accretion of this "fallback" material could significantly affect the optical light curve, in some cases producing super-luminous or otherwise peculiar supernovae. We use spherically symmetric hydrodynamical models to estimate the accretion rate at late times for a range of progenitor masses and radii and explosion energies. The accretion rate onto the proto-neutron star or black hole decreases as \\dot{M} \\propto t^{-5/3} at late times, but its normalization can be significantly enhanced at low explosion energies, in very massive stars, or if a strong reverse shock wave forms at the helium/hydrogen interface in the progenitor. If the resulting super-Eddington accretion drives an outflow which thermalizes in the outgoing ejecta, the supernova debris will be re-energized at a time when photons can diffuse out efficiently. The resulting light curves are different and more diverse than previous fallback supernova models which ignored the input of accretion power and produced short-lived, dim transients. The possible outcomes when fallback accretion power is significant include super-luminous (gsim 1044 erg s-1) Type II events of both short and long durations, as well as luminous Type I events from compact stars that may have experienced significant mass loss. Accretion power may unbind the remaining infalling material, causing a sudden decrease in the brightness of some long duration Type II events. This scenario may be relevant for explaining some of the recently discovered classes of peculiar and rare supernovae.

  17. NEW TWO-DIMENSIONAL MODELS OF SUPERNOVA EXPLOSIONS BY THE NEUTRINO-HEATING MECHANISM: EVIDENCE FOR DIFFERENT INSTABILITY REGIMES IN COLLAPSING STELLAR CORES

    The neutrino-driven explosion mechanism for core-collapse supernovae in its modern flavor relies on the additional support of hydrodynamical instabilities in achieving shock revival. Two possible candidates, convection and the so-called standing accretion shock instability (SASI), have been proposed for this role. In this paper, we discuss new successful simulations of supernova explosions that shed light on the relative importance of these two instabilities. While convection has so far been observed to grow first in self-consistent hydrodynamical models with multi-group neutrino transport, we here present the first such simulation in which the SASI grows faster while the development of convection is initially inhibited. We illustrate the features of this SASI-dominated regime using an explosion model of a 27 M☉ progenitor, which is contrasted with a convectively dominated model of an 8.1 M☉ progenitor with subsolar metallicity, whose early post-bounce behavior is more in line with previous 11.2 M☉ and 15 M☉ explosion models. We analyze the conditions discriminating between the two different regimes, showing that a high mass-accretion rate and a short advection timescale are conducive for strong SASI activity. We also briefly discuss some important factors for capturing the SASI-driven regime, such as general relativity, the progenitor structure, a nuclear equation of state leading to a compact proto-neutron star, and the neutrino treatment. Finally, we evaluate possible implications of our findings for two-dimensional and three-dimensional supernova simulations.

  18. Accretion Flows in Magnetic White Dwarf Systems

    Imamura, James N.

    2005-01-01

    We received Type A and B funding under the NASA Astrophysics Data Program for the analysis and interpretation of hard x-ray data obtained by the Rossi X-ray Timing Explorer and other NASA sponsored missions for Intermediate Polars (IPS) and Polars. For some targets, optical data was available. We reduced and analyzed the X-ray spectra and the X-ray and optical (obtained at the Cerro Tololo Inter-American Observatory) timing data using detailed shock models (which we constructed) to place constraints on the properties of the accreting white dwarfs, the high energy emission mechanisms of white dwarfs, and the large-scale accretion flows of Polars and IPS. IPS and Polars are white dwarf mass-transfer binaries, members of the larger class of cata,clysmic variables. They differ from the bulk of the cataclysmic variables in that they contain strongly magnetic white dwarfs; the white dwarfs in Polars have B, = 7 to 230 MG and those in IPS have B, less than 10 MG. The IPS and Polars are both examples of funneled accretion flows in strong magnetic field systems. The IPS are similar to x-ray pulsars in that accretion disks form in the systems which are disrupted by the strong stellar magnetic fields of the white dwarfs near the stellar surface from where the plasma is funneled to the surface of the white dwarf. The localized hot spots formed at the footpoints of the funnels coupled with the rotation of the white dwarf leads to coherent pulsed x-ray emission. The Polars offer an example of a different accretion topology; the magnetic field of the white dwarf controls the accretion flow from near the inner Lagrangian point of the system directly to the stellar surface. Accretion disks do not form. The strong magnetic coupling generally leads to synchronous orbital/rotational motion in the Polars. The physical system in this sense resembles the Io/Jupiter system. In both IPS and Polars, pulsed emission from the infrared to x-rays is produced as the funneled flows merge onto the

  19. An accurate geometric distance to the compact binary SS Cygni vindicates accretion disc theory

    Miller-Jones, J C A; Knigge, C; Körding, E G; Templeton, M; Waagen, E O

    2013-01-01

    Dwarf novae are white dwarfs accreting matter from a nearby red dwarf companion. Their regular outbursts are explained by a thermal-viscous instability in the accretion disc, described by the disc instability model that has since been successfully extended to other accreting systems. However, the prototypical dwarf nova, SS Cygni, presents a major challenge to our understanding of accretion disc theory. At the distance of 159 +/- 12 pc measured by the Hubble Space Telescope, it is too luminous to be undergoing the observed regular outbursts. Using very long baseline interferometric radio observations, we report an accurate, model-independent distance to SS Cygni that places the source significantly closer at 114 +/- 2 pc. This reconciles the source behavior with our understanding of accretion disc theory in accreting compact objects.

  20. Gravitational radiation and gamma-ray bursts from accreting neutron stars

    Mosquera Cuesta, H.J.; Araujo, J.C.N. de; Aguiar, O.D. [Instituto Nacional de Pesquisas Espaciais (INPE), Sao Jose dos Campos, SP (Brazil). Div. de Astrofisica]. E-mail: herman@das.inpe.br; jcarlos@das.inpe.br; odylio@das.inpe.br; Horvath, J.E. [Sao Paulo Univ., SP (Brazil). Inst. Astronomico e Geofisico]. E-mail: foton@orion.iagusp.usp.br

    2000-07-01

    It is well known that hydrodynamic instabilities can be induced in rapidly rotating low magnetic field neutron stars, which accrete mass from a companion in both high and low mass X-ray binaries. (author)

  1. The microphysics of collisionless shock waves

    Marcowith, A; Bykov, A; Dieckman, M E; Drury, L O C; Lembege, B; Lemoine, M; Morlino, G; Murphy, G; Pelletier, G; Plotnikov, I; Reville, B; Riquelme, M; Sironi, L; Novo, A Stockem

    2016-01-01

    Collisionless shocks, that is shocks mediated by electromagnetic processes, are customary in space physics and in astrophysics. They are to be found in a great variety of objects and environments: magnetospheric and heliospheric shocks, supernova remnants, pulsar winds and their nebul\\ae, active galactic nuclei, gamma-ray bursts and clusters of galaxies shock waves. Collisionless shock microphysics enters at different stages of shock formation, shock dynamics and particle energization and/or acceleration. It turns out that the shock phenomenon is a multi-scale non-linear problem in time and space. It is complexified by the impact due to high-energy cosmic rays in astrophysical environments. This review adresses the physics of shock formation, shock dynamics and particle acceleration based on a close examination of available multi-wavelength or in-situ observations, analytical and numerical developments. A particular emphasize is made on the different instabilities triggered during the shock formation and in a...

  2. Multi‐instrument observations from Svalbard of a traveling convection vortex, electromagnetic ion cyclotron wave burst, and proton precipitation associated with a bow shock instability

    Engebretson, M. J.; Yeoman, T. K.; Oksavik, K.;

    2013-01-01

    An isolated burst of 0.35 Hz electromagnetic ion cyclotron (EMIC) waves was observed at four sites on Svalbard from 0947 to 0954 UT 2 January 2011, roughly 1 h after local noon. This burst was associated with one of a series of ~50 nT magnetic impulses observed at the northernmost stations of the...... IMAGE magnetometer array. Hankasalmi SuperDARN radar data showed a west-to-east (antisunward) propagating vortical ionospheric flow in a region of high spectral width ~ 1–2° north of Svalbard, confirming that this magnetic impulse was the signature of a traveling convection vortex. Ground...... of Earth observed a steady solar wind and predominantly radial interplanetary magnetic field orientation before and during this event, data from Geotail (near the morning bow shock) showed large reorientations of the interplanetary magnetic field and substantial decreases in ion density several...

  3. Constraints on active galactic nucleus accretion disc viscosity derived from continuum variability

    R.L.C. Starling; A. Siemiginowska; P. Uttley; R. Soria

    2004-01-01

    We estimate a value of the viscosity parameter in active galactic nucleus (AGN) accretion discs for the Palomar-Green quasar sample. We assume that optical variability on time-scales of months to years is caused by local instabilities in the inner accretion disc. Comparing the observed variability t

  4. Chaotic cold accretion onto black holes

    Gaspari, M; Oh, S Peng

    2013-01-01

    Using 3D AMR simulations, linking the 50 kpc to the sub-pc scales over the course of 40 Myr, we systematically relax the classic Bondi assumptions in a typical galaxy hosting a SMBH. In the realistic scenario, where the hot gas is cooling, while heated and stirred on large scales, the accretion rate is boosted up to two orders of magnitude compared with the Bondi prediction. The cause is the nonlinear growth of thermal instabilities, leading to the condensation of cold clouds and filaments when t_cool/t_ff 0.2) induces the formation of thermal instabilities, even in the absence of heating, while in the transonic regime turbulent dissipation inhibits their growth (t_turb/t_cool < 1). When heating restores global thermodynamic balance, the formation of the multiphase medium is violent, and the mode of accretion is fully cold and chaotic. The recurrent collisions, shearing and tidal motions between clouds, filaments and the central torus cause a significant reduction of angular momentum, boosting accretion. ...

  5. Instabilities in astrophysical jets

    Instabilities in astrophysical jets are studied in the nonlinear regime by performing 2D numerical classical gasdynamical calculations. The instabilities which arise from unsteadiness in output from the central engine feeding the jets, and those which arise from a beam in a turbulent surrounding are studied. An extra power output an order of magnitude higher than is normally delivered by the engine over a time equal to (nozzle length)/(sound velocity at centre) causes a nonlinear Kelvin-Helmholtz instability in the jet walls. Constrictions move outwards, but the jet structure is left untouched. A beam in turbulent surroundings produces internal shocks over distances of a few beam widths. If viscosity is present the throughput of material is hampered on time scales of a few beam radius sound travel times. The implications are discussed. (Auth.)

  6. Life shocks and homelessness.

    Curtis, Marah A; Corman, Hope; Noonan, Kelly; Reichman, Nancy E

    2013-12-01

    We exploited an exogenous health shock-namely, the birth of a child with a severe health condition-to investigate the effect of a life shock on homelessness in large cities in the United States as well as the interactive effects of the shock with housing market characteristics. We considered a traditional measure of homelessness, two measures of housing instability thought to be precursors to homelessness, and a combined measure that approximates the broadened conceptualization of homelessness under the 2009 Homeless Emergency Assistance and Rapid Transition to Housing Act (2010). We found that the shock substantially increases the likelihood of family homelessness, particularly in cities with high housing costs. The findings are consistent with the economic theory of homelessness, which posits that homelessness results from a conjunction of adverse circumstances in which housing markets and individual characteristics collide. PMID:23868747

  7. Supernova Light Curves Powered by Fallback Accretion

    Dexter, Jason

    2012-01-01

    Some fraction of the material ejected in a core collapse supernova explosion may remain bound to the compact remnant, and eventually turn around and fall back. We show that the late time (> days) power associated with the accretion of this "fallback" material may significantly affect the optical light curve, in some cases producing super-luminous or otherwise peculiar supernovae. We use spherically symmetric hydrodynamical models to estimate the accretion rate at late times for a range of progenitor masses and radii and explosion energies. The accretion rate onto the proto-neutron star or black hole decreases as Mdot ~ t^-5/3 at late times, but its normalization can be significantly enhanced at low explosion energies, in very massive stars, or if a strong reverse shock wave forms at the helium/hydrogen interface in the progenitor. If the resulting super-Eddington accretion drives an outflow which thermalizes in the outgoing ejecta, the supernova debris will be re-energized at a time when photons can diffuse o...

  8. STEREO interplanetary shocks and foreshocks

    Blanco-Cano, X. [Instituto de Geofisica, UNAM, CU, Coyoacan 04510 DF (Mexico); Kajdic, P. [IRAP-University of Toulouse, CNRS, Toulouse (France); Aguilar-Rodriguez, E. [Instituto de Geofisica, UNAM, Morelia (Mexico); Russell, C. T. [ESS and IGPP, University of California, Los Angeles, 603 Charles Young Drive, Los Angeles, CA 90095 (United States); Jian, L. K. [NASA Goddard Space Flight Center, Greenbelt, MD and University of Maryland, College Park, MD (United States); Luhmann, J. G. [SSL, University of California Berkeley (United States)

    2013-06-13

    We use STEREO data to study shocks driven by stream interactions and the waves associated with them. During the years of the extended solar minimum 2007-2010, stream interaction shocks have Mach numbers between 1.1-3.8 and {theta}{sub Bn}{approx}20-86 Degree-Sign . We find a variety of waves, including whistlers and low frequency fluctuations. Upstream whistler waves may be generated at the shock and upstream ultra low frequency (ULF) waves can be driven locally by ion instabilities. The downstream wave spectra can be formed by both, locally generated perturbations, and shock transmitted waves. We find that many quasiperpendicular shocks can be accompanied by ULF wave and ion foreshocks, which is in contrast to Earth's bow shock. Fluctuations downstream of quasi-parallel shocks tend to have larger amplitudes than waves downstream of quasi-perpendicular shocks. Proton foreshocks of shocks driven by stream interactions have extensions dr {<=}0.05 AU. This is smaller than foreshock extensions for ICME driven shocks. The difference in foreshock extensions is related to the fact that ICME driven shocks are formed closer to the Sun and therefore begin to accelerate particles very early in their existence, while stream interaction shocks form at {approx}1 AU and have been producing suprathermal particles for a shorter time.

  9. Accretion Rates for T Tauri Stars Using Nearly Simultaneous Ultraviolet and Optical Spectra

    Ingleby, Laura; Herczeg, Gregory; Blaty, Alex; Walter, Frederick; Ardila, David; Alexander, Richard; Edwards, Suzan; Espaillat, Catherine; Gregory, Scott G; Hillenbrand, Lynne; Brown, Alexander

    2013-01-01

    We analyze the accretion properties of 21 low mass T Tauri stars using a dataset of contemporaneous near ultraviolet (NUV) through optical observations obtained with the Hubble Space Telescope Imaging Spectrograph (STIS) and the ground based Small and Medium Aperture Research Telescope System (SMARTS), a unique dataset because of the nearly simultaneous broad wavelength coverage. Our dataset includes accreting T Tauri stars (CTTS) in Taurus, Chamaeleon I, $\\eta$ Chamaeleon and the TW Hydra Association. For each source we calculate the accretion rate by fitting the NUV and optical excesses above the photosphere, produced in the accretion shock, introducing multiple accretion components characterized by a range in energy flux (or density) for the first time. This treatment is motivated by models of the magnetospheric geometry and accretion footprints, which predict that high density, low filling factor accretion spots co-exist with low density, high filling factor spots. By fitting the UV and optical spectra wi...

  10. Shoulder Instability

    ... Risk Factors Is shoulder instability the same as shoulder dislocation? No. The signs of dislocation and instability might ... the same to you--weakness and pain. However, dislocation occurs when your shoulder goes completely out of place. The shoulder ligaments ...