Although the powering mechanism for quasars is now widely recognized to be the accretion of matter in a geometrically thin disk, the transport of matter to the inner region of the disk where luminosity is emitted remains an unsolved question. Miralda-Escudé & Kollmeier (2005) proposed a model whereby quasars are fuelled when stars are captured by the accretion disk as they plunge through the gas. Such plunging stars can then be destroyed and deliver their mass to the accretion disk.

Here we present the first detailed calculations for the capture of stars originating far from the accretion disk near the zone of influence of the central black hole. In particular we examine the effect of adding a perturbing mass to a fixed stellar cusp potential on bringing stars into the accretion disk where they can be captured. The work presented here will be discussed in detail in an upcoming publication Kennedy et al. (2010).

The source of the cosmological magnetic field is still unknown because the widely invoked dynamo processes are only able to regenerate and amplify some initial magnetic field seeds. In the hot and highly ionized intergalactic matter such magnetic field seeds can easily be produced by the (electro-)magnetic instabilities of Weibel type. Here we discuss suplementary mechanisms that can make these Weibel created fields to evolve at large scales presently observed in galaxies and clusters and can also enhance these magnetic field seeds after the dissipation.

I present axisymmetric numerical simulations of the solar interior, with differential rotation imposed in the convection zone and tachocline and a dipolar poloidal field confined to the radiative interior. In these simulations toroidal field reversals which are equator-ward propagating are driven in the absence of a dynamo. These reversals are driven in the tachocline and are seen at the top of the convection zone. While not solar-like in many ways, these reversals do show some solar-like properties not previously seen in full MHD simulations.

Main-sequence massive stars possess convective cores that likely harbor strong dynamo action. To assess the role of core convection in building magnetic fields within these stars, we employ the 3-D anelastic spherical harmonic (ASH) code to model turbulent dynamics within a 10 M⊙ main-sequence (MS) B-type star rotating at 4 Ω⊙. We find that strong (900 kG) magnetic fields arise within the turbulence of the core and penetrate into the stably stratified radiative zone. These fields exhibit complex, time-dependent behavior including reversals in magnetic polarity and shifts between which hemisphere dominates the total magnetic energy.

Recently, Silvers et al. (2009b), using numerical simulations, confirmed the existence of a double diffusive magnetic buoyancy instability of a layer of horizontal magnetic field produced by the interaction of a shear velocity field with a weak vertical field. Here, we demonstrate the longer term nonlinear evolution of such an instability in the simulations. We find that a quasi two-dimensional interchange instability rides (or “surfs”) on the growing shear-induced background downstream field gradients. The region of activity expands since three-dimensional perturbations remain unstable in the wake of this upward-moving activity front, and so the three-dimensional nature becomes more noticeable with time.

We review some of the recent results obtained in MHD turbulence, as encountered in many astrophysical objects. We focus attention on the lack of universality in such flows, including in the simplest case (no externally imposed magnetic field, no forcing, unit magnetic Prandtl number). Several parameters can foster such a breakdown of classical Kolmogorov scaling, such as the presence of velocity-magnetic field correlations, or of magnetic helicity and the role of the interplay between nonlinear eddies and Alfvén waves. A link with avalanche processes is also discussed. These findings have led to the conjecture of the emergence of a new paradigm for MHD turbulence, as a possibly unsettled competition between several dynamical phenomena.

The particle dynamics and in the stellar magnetosphere during gravitational collapse is investigated. The formations of relativistic jets and the generation of the radiation bursts in the stellar magnetosphere by gravitational collapse are considered. As follows from results, the stars on the stage of gravitational collapse must be powerful sources of the relativistic jets and the non-thermal radiation. These jets will formed in the polar caps of collapsing stars magnetospheres, when the stellar magnetic field increases during collapse and the charged particles will be accelerate. These jets will generate the non-thermal radiation. The radiation flux grows with decreasing stellar radius and can be observed in the form of radiation burst in wide band wave- from radio to gamma-ray. These bursts radiation can be observed as gamma- and X- rays bursts.

The formation scenario of ring galaxies is addressed in this paper. We focus on the P-type ring galaxies presented in Madore, Nelson & Petrillo (2009), particularly on the axis-symmetric ones. Our simulations show that a ring can form through the collision of disc and dwarf galaxies, and the locations, widths, and density contrasts of the ring are well determined. We find that a ring galaxy such as AM 2302-322 can be produced by this collision scenario.

3D high resolution simulations for the convective zone of a 4Myr old 0.7 M⊙ pre-main sequence star in gravitational contraction are carried out with different radial density contrast using the pseudo spectral ASH code (Brun et al. 2004). We extract giant cells signal from the complex surface convective patterns by using a wavelet analysis. We then characterize them by estimating their lifetime and rotation rate according to the density contrast.

Internal gravity waves are excited at the interface of convection and radiation zones of a solar-type star, by the tidal forcing of a short-period planet. The fate of these waves as they approach the centre of the star depends on their amplitude. We discuss the results of numerical simulations of these waves approaching the centre of a star, and the resulting evolution of the spin of the central regions of the star and the orbit of the planet. If the waves break, we find efficient tidal dissipation, which is not present if the waves perfectly reflect from the centre. This highlights an important amplitude dependence of the (stellar) tidal quality factor Q′, which has implications for the survival of planets on short-period orbits around solar-type stars, with radiative cores.

We review the observational knowledge that has built up over the past 25 years on the interstellar magnetic field within ~ 150 pc of the Galactic center. We also provide a critical discussion of the main observational findings and comment on their possible theoretical interpretations. To conclude, we propose a coherent view of the interstellar magnetic field near the Galactic center, which accounts at best for the vast body of observations.

We present the results of 3D simulations, performed with the ASH code, of the nonlinear, magnetic coupling between the convective and radiative zones in the Sun, through the tachocline. Contrary to the predictions of Gough & McIntyre (1998), a fossil magnetic field, deeply buried initially in the solar interior, will penetrate into the convection zone. According to Ferraro's law of iso-rotation, the differential rotation of the convective zone will thus expand into the radiation zone, along the field lines of the poloidal field.

Three-dimensional (3D) hydrodynamic simulations of shell oxygen burning by Meakin & Arnett (2007b) exhibit bursty, recurrent fluctuations in turbulent kinetic energy. These are shown to be due to a global instability in the convective region, which has been suppressed in simulations of stellar evolution which use mixing-length theory (MLT). Quantitatively similar behavior occurs in the model of a convective roll (cell) of Lorenz (1963), which is known to have a strange attractor that gives rise to random fluctuations in time. An extension of the Lorenz model, which includes Kolmogorov damping and nuclear burning, is shown to exhibit bursty, recurrent fluctuations like those seen in the 3D simulations. A simple model of a convective layer (composed of multiple Lorenz cells) gives luminosity fluctuations which are suggestive of irregular variables (red giants and supergiants, see Schwarzschild (1975). Details and additional discussion may be found in Arnett & Meakin (2011).

Apparent inconsistencies between Arnett, Meakin, & Young (2009) and Nordlund, Stein, & Asplund (2009) on the nature of convective driving have been resolved, and are discussed.

As compressible convection has inherent up/down asymmetry, overshooting above and below a convection zone behave differently. In downward overshooting, the narrow down-flow columns dynamically play an important role. It is customary, and reasonable, to use the downward flux of kinetic energy as a proxy for overshooting. In the upward situation, the flux of kinetic energy can take on different signs near the upper boundary of the convection zone, and its magnitude is generally small. It cannot make a good proxy for overshooting. This paper discusses the results of a set of numerical experiments that investigate the problem of overshooting above a convection zone. Particle tracing and color advection are used to follow the mixing process. The overshoot region above a convection zone is found to contain multiple counter cell layers.

The large-scale dynamics of the solar convection zone have been inferred using both global and local helioseismology applied to data from the Global Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI) on board SOHO. The global analysis has revealed temporal variations of the “torsional oscillation” zonal flow as a function of depth, which may be related to the properties of the solar cycle. The horizontal flow field as a function of heliographic position and depth can be derived from ring diagrams, and shows near-surface meridional flows that change over the activity cycle. Time-distance techniques can be used to infer the deep meridional flow, which is important for flux-transport dynamo models. Temporal variations of the vorticity can be used to investigate the production of flare activity. This paper summarizes the state of our knowledge in these areas.

We report on the extension of the ASH code to include an atmospheric stable layer (i.e not convective). This layer is meant to model the sun's chromosphere within the anelastic approximation limits while coping with the wide range of densities, time and spatial scales between r = 0.7 R⊙ and r = 1.03 R⊙. Convective overshoot into the stable atmospheric layer is observed in a region ~ 0.01 R⊙ thick, exciting waves which propagate upwards into the atmosphere.

We revisit a phenomenological description of turbulent thermal convection along the lines proposed originally by Gough (1965) in which eddies grow solely by extracting energy from the unstably stratified mean state and are subsequently destroyed by internal shear instability. This work is part of an ongoing investigation for finding a procedure to calculate the turbulent fluxes of heat and momentum in the presence of a shearing background flow in stars.

Records of the solar magnetic field extend back for millennia, and its surface properties have been observed for centuries, while helioseismology has recently revealed the Sun's internal rotation and the presence of a tachocline. Dynamo theory has developed to explain these observations, first with idealized models based on mean-field electrodynamics and, more recently, by direct numerical simulation, notably with the ASH code at Boulder. These results, which suggest that cyclic activity relies on the presence of the tachocline, and that its modulation is chaotic (rather than stochastic), will be critically reviewed. Similar theoretical approaches have been followed in order to explain the magnetic properties of other main-sequence stars, whose fields can be mapped by Zeeman-Doppler imaging. Of particular interest is the behaviour of fully convective, low-mass stars, which lack any tachocline but are nevertheless extremely active.

We have made 3-D models of the collision of binary star winds and followed their interaction over multiple orbits. This allows us to explore how the wind-wind interaction shapes the circumstellar environment. Specifically, we can model the highly radiative shock that occurs where the winds collide. We find that the shell that is created at the collision front between the two winds can be highly unstable, depending on the characteristics of the stellar winds.

A dynamo is a process by which fluid motions sustain magnetic fields against dissipative effects. Dynamos occur naturally in many astrophysical systems. Theoretically, we have a much more robust understanding of the generation and maintenance of magnetic fields at the scale of the fluid motions or smaller, than that of magnetic fields at scales much larger than the local velocity. Here, via numerical simulations, we examine one example of an “essentially nonlinear” dynamo mechanism that successfully maintains magnetic field at the largest available scale (the system scale) without cascade to the resistive scale. In particular, we examine whether this new type of dynamo at the system scale is still effective in the presence of other smaller-scale dynamics (turbulence).