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.

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).

Observations have shown that the Sun's magnetic field has helical structures. The helicity content in magnetic field configurations is a crucial constraint on the dynamical evolution of the system. Since helicity is connected with the number of links we investigate configurations with interlocked magnetic flux rings and one with unlinked rings. It turns out that it is not the linking of the tubes which affects the magnetic field decay, but the content of magnetic helicity.

We investigate conditions in a radially self-similar outflow in the regime of large resistivity. Using the PLUTO code, we performed simulations with proper choice of boundary conditions, relaxed at the footpoints of critical surfaces in the flow. We investigate outflow propagation in a high-resistive disk corona, and compare it to the results with small or vanishing resistivity.

We study the effects of different approximations of scattering in 3D radiation-hydrodynamics simulations on the photospheric temperature stratification of metal-poor red giant stars. We find that assuming a Planckian source function and neglecting the contribution of scattering to extinction in optically thin layers provides a good approximation of the effects of coherent scattering on the photospheric temperature balance.

In the interstellar medium the turbulence is believed to be forced mostly through supernova explosions. In a first approximation these flows can be written as a gradient of a potential being thus devoid of vorticity. There are several mechanisms that could lead to vorticity generation, like viscosity and baroclinic terms, rotation, shear and magnetic fields, but it is not clear how effective they are, neither is it clear whether the vorticity is essential in determining the turbulent diffusion acting in the ISM. Here we present a study of the role of rotation, shear and baroclinicity in the generation of vorticity in the ISM.

HD75445 was recently announced by Kochukhov et al. (2009) to be a low-amplitude roAp star, based on spectroscopic measurements. We present putative pulsation frequencies of HD75445 determined from 22 hours of Johnson B photometry obtained in 2008, 2009 and 2010. We present the first photometric periodicities detected in this star. We make a marginal detection of one of Kochukhov et al. (2009)'s spectroscopic periods, along with a range of confidently detected periodicities covering the low-frequency end of the roAp instability spectrum and the high-frequency end of the Delta Scuti instability spectrum.

We present numerical results concerning the behavior of the density and the velocity power spectrum in turbulent thermally bistable flows for different Mach numbers.

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 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.

Turbulent reconnection is studied by means of two-dimensional (2D) compressible magnetohydrodynamical numerical calculations. The process of homogeneous turbulence is set up by adding two-dimensional random forcing implemented in the spectral space at small wave numbers with no correlation between velocity and forcing. We apply the initial Harris current sheet configuration together with a density profile calculated from the numerical equilibrium of magnetic and gas pressures. We assume that there is no external driving of the reconnection. The reconnection develops as a result of the initial vector potential perturbation. We use open boundary conditions. Our main goal is to find the dependencies of reconnection rate on the uniform resistivity. We present that the reconnection speed depends on the Lindquist number in 2D in the case of low as well as high resolution. When we apply more powerful turbulence the reconnection is faster, however the speed of reconnection is smaller than in the case of our three-dimensional numerical simulations.

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.

The young star clusters we observe today are the building blocks of a new generation of stars and planets in our Galaxy and beyond. Despite their fundamental role we still lack knowledge about the initial conditions under which star clusters form and the impact of these often harsh environments on the formation and evolution of their stellar and substellar members.

We present recent results showing that mass segregation in realistic models of young star clusters occurs very quickly for subvirial spherical systems without substructure. This finding is a critical step to resolve the controversial debate on mass segregation in young star clusters and provides strong constraints on their initial conditions. The rapid concentration of massive stars is usually associated with strong gravitational interactions early on during cluster evolution and the subsequent formation of multiple systems and ejection of stars.

We study the complexity of supergranular cells using the intensity patterns obtained at the Kodaikanal solar observatory during the solar maximum. Our data consists of visually identified supergranular cells, from which a fractal dimension D is obtained according to the relation P ∝ AD/2 where A is the area and P is the perimeter of the cells. We find a difference in the fractal dimension between the active and the quiet region cells which is conjectured to be due to the magnetic activity level.

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 study the connections between the sun's convection zone evolution and the dynamics of the solar wind and corona. We input the magnetic fields generated by a 2.5D axisymmetric kinematic dynamo code (STELEM) into a 2.5D axisymmetric coronal MHD code (DIP). The computations were carried out for an 11 year cycle. We show that the solar wind's velocity and mass flux vary in latitude and in time in good agreement with the well known time-latitude assymptotic wind speed diagram. Overall sun's mass loss rate, momentum flux and magnetic breaking torque are maximal near the solar minimum.

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.

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.

We perform 3D radiative hydrodynamic simulations to study convection in low-mass main-sequence stars with the aim of improving stellar models. Comparing models from a 0.90 M⊙ evolutionary track with 3D simulations reveals distinct differences between simulations and mixing length theory. The simulations show obvious structural differences throughout the superadiabatic layer where convection is inefficient at transporting energy. The discrepancy between MLT and simulation changes as the star evolves and the dynamical effects of turbulence increase. Further, the simulations reveal a T-tau relation that is sensitive to the strength of the turbulence, which is in contrast to 1D stellar models that use the same T-tau relation across the HR diagram.

Direct numerical simulations of convective stellar envelopes, are divided between two different physical regimes, that are rather difficult to reconcile — at least with the computational power of present-day computers. This paper outlines an attempt at bridging the gap between surface and interior simulations of convection.