Observations of protostellar disks indicate the presence of the magnetic field of thermal (or superthermal) strength. In such a strong magnetic field, many MHD instabilities responsible for turbulent transport of the angular momentum are suppressed. We consider the shear-driven instability that can occur in protostellar disks even if the field is superthermal. This instability is caused by the combined influence of shear and compressibility in a magnetized gas and can be an efficient mechanism to generate turbulence in disks. The typical growth time is of the order of several rotation periods.
Aspects of turbulence in protostellar accretion discs are being reviewed. The emergence of dead zones due to poor ionization and alternatives to the magneto-rotational instability are discussed. The coupling between dust and gas in protostellar accretion discs is explained and turbulent drag is compared with laminar drag in the Stokes and Epstein regimes. Finally, the significance of magnetic field generation in turbulent discs is emphasized in connection with driving outflows and with star-disc coupling.
We study a self-similar circulation model for protostellar bipolar outflows. The model is axisymmetric and stationary, and now includes Poynting flux. Compared to an earlier version of the model, this addition produces faster and more collimated outflows. Moreover the luminosity needed for the radiative heating is smaller. The model has been applied to both low and high mass protostars. Monte Carlo methods have been used to explore systematically the parameter space. An inflow/outflow pattern including collimation of high speed material and an infalling toroidal disc arises naturally. The disc…
The link between turbulence in star formatting environments and protostellar jets remains controversial. To explore issues of turbulence and fossil cavities driven by young stellar outflows we present a series of numerical simulations tracking the evolution of transient protostellar jets driven into a turbulent medium. Our simulations show both the effect of turbulence on outflow structures and, conversely, the effect of outflows on the ambient turbulence. We demonstrate how turbulence will lead to strong modifications in jet morphology. More importantly, we demonstrate that individual transie…
We calculate the proton distribution function due to heating by subsonic (Alfvenic) turbulence in a weakly magnetized collisionless plasma. The distribution function is nonthermal. For non-relativistic energies, it is an exponential of the magnitude of the proton velocity. For ultra-relativistic energies, it can be characterized as a power law with a momentum-dependent slope.
We investigate the conditions for the presence of a magnetically inactive dead zone in protostellar disks, using 3-D shearing-box MHD calculations including vertical stratification, Ohmic resistivity and time-dependent ionization chemistry. Activity driven by the magnetorotational instability fills the whole thickness of the disk at 5 AU, provided cosmic ray ionization is present, small grains are absent and the gas-phase metal abundance is sufficiently high. At 1 AU the larger column density of 1700 g/cm^2 means the midplane is shielded from ionizing particles and remains magnetorotationally …
The prevailing theory for the origin of cosmic magnetic fields is that they have been amplified to their present values by the turbulent dynamo inductive action in the protogalactic and galactic medium. Up to now, in calculation of the turbulent dynamo, it has been customary to assume that there is no back reaction of the magnetic field on the turbulence, as long as the magnetic energy is less than the turbulent kinetic energy. This assumption leads to the kinematic dynamo theory. However, the applicability of this theory to protogalaxies is rather limited. The reason is that in protogalaxies …
Using numerical methods, we systematically study in the framework of ideal MHD the effect of magnetic fields on heat transfer within a turbulent gas. We measure the rates of passive scalar diffusion within magnetized fluids and make the comparisons a) between MHD and hydro simulations, b) between different MHD runs with different values of the external magnetic field (up to the energy equipartition value), c) between thermal conductivities parallel and perpendicular to magnetic field. We do not find apparent suppression of diffusion rates by the presence of magnetic fields, which implies that …
The prevailing theory for the origin of cosmic magnetic fields is that they have been amplified to their present values by the turbulent dynamo inductive action in the protogalactic and galactic medium. Up to now, in calculation of the turbulent dynamo, it has been customary to assume that there is no the back reaction of the magnetic field on the turbulence, as long as the magnetic energy is less than the turbulent kinetic energy. This assumption leads to the kinematic dynamo theory. However, the applicability of the kinematic dynamo theory to protogalaxies is rather limited. The reason is th…
The processes of planet formation and migration depend intimately on the interaction between planetesimals and the gaseous disks in which they form. The formation of gaps in the disk can severely limit the mass of the planet and its migration toward the protostar. We investigate the process of gap formation through magnetohydrodynamic simulations in which internal stress arises self-consistently from turbulence generated by the magnetorotational instability. The simulations investigate three different planetary masses and two disk temperatures to bracket the tidal (thermal) and viscous gap ope…
Protostellar outflows crisscross the regions of star cluster formation, stirring turbulence and altering the evolution of the forming cluster. We model the stirring of turbulent motions by protostellar outflows, building on an observation that the scaling law of supersonic turbulence implies a momentum cascade analogous to the energy cascade in Kolmogorov turbulence. We then generalize this model to account for a diversity of outflow strengths, and for outflow collimation, both of which enhance turbulence. For a single value of its coupling coefficient the model is consistent with turbulence s…
We use ZEUS-MP to perform high resolution, three-dimensional, super-Alfvenic turbulent simulations in order to investigate the role of magnetic fields in self-gravitating core formation within turbulent molecular clouds. Statistical properties of our super-Alfvenic model without gravity agree with previous similar studies. Including self-gravity, our models give the following results. They are consistent with the turbulent fragmentation prediction of the core mass distribution of Padoan & Nordlund. They also confirm that local gravitational collapse is not prevented by magnetohydrodynamic wave…
A new theory is presented for the development of circular polarization as radio waves propagate through the turbulent, birefringent interstellar medium. The fourth order moments of the wavefield are calculated and it is shown that unpolarized incident radiation develops a nonzero variance in circular polarization. A magnetized turbulent medium causes the Stokes parameters to scintillate in a non-identical manner. A specific model for this effect is developed for the case of density fluctuations in a uniform magnetic field.
Turbulent accretion disks are the most probable origin of jets from young stellar objects. Using an extended version of ZEUS-3D code in the axisymmetry option we investigate the evolution of a disk wind into a collimated jet. Resistive jets propagate slower with increasing magnetic diffusivity, and the degree of collimation decreases. The resistive disk included in the simulations ejects persistent, well collimated jet.
A new mechanism of generation of large-scale magnetic fields in colliding protogalactic clouds and merging protostellar clouds is discussed. Interaction of the colliding clouds produces large-scale shear motions which are superimposed on small-scale turbulence. Generation of the large-scale magnetic field is due to a ''shear-current" effect (or "vorticity-current" effect), and the mean vorticity is caused by the large-scale shear motions of colliding clouds. This effect causes the generation of the mean magnetic field even in a nonrotating and nonhelical homogeneous turbulence. There is no que…
Magnetic fields are usually considered dynamically important in star formation when the dimensionless mass-to-flux ratio is close to, or less than, unity (lambda<~1). We show that, in disk formation, the requirement is far less stringent. This conclusion is drawn from a set of 2D (axisymmetric) simulations of the collapse of rotating, magnetized, singular isothermal cores. We find that a weak field corresponding to 1ambda~100 can begin to disrupt the rotationally supported disk through magnetic braking, by creating regions of rapid, supersonic collapse in the disk. These regions are separated …
(Abridged) We present the results of three dimensional calculations for the MHD evolution of an adiabatic supernova remnant in both a uniform and turbulent interstellar medium using the RIEMANN framework of Balsara. In the uniform case, which contains an initially uniform magnetic field, the density structure of the shell remains largely spherical, while the magnetic pressure and synchrotron emissivity are enhanced along the plane perpendicular to the field direction. This produces a bilateral or barrel-type morphology in synchrotron emission for certain viewing angles. We then consider a case…
The origin of brown dwarfs (BDs) is an important component of the theory of star formation, because BDs are approximately as numerous as solar mass stars. It has been suggested that BDs originate from the gravitational fragmentation of protostellar disks, a very different mechanism from the formation of hydrogen burning stars. We propose that BDs are instead formed by the process of turbulent fragmentation, like more massive stars. In numerical simulations of turbulence and star formation we find that gravitationally unstable density peaks of BD mass are commonly formed by the turbulent flow. …
Star formation is usually accompanied by outflow phenomena. There is strong evidence that these outflows and jets are launched from the protostellar disk by magneto-rotational processes. Here, we report on our three dimensional, adaptive mesh, magneto-hydrodynamic simulations of collapsing, rotating, magnetized Bonnor-Ebert-Spheres whose properties are taken directly from observations. In contrast to the pure hydro case where no outflows are seen, our present simulations show an outflow from the protodisk surface at ~ AU and a jet at ~ 0.07 AU after a strong toroidal magnetic field build up. T…
We develop a model of thin turbulent accretion discs supported by magnetic pressure of turbulent magnetic fields. This applies when the turbulent kinetic and magnetic energy densities are greater than the thermal energy density in the disc. Whether such discs survive in nature or not remains to be determined, but here we simply demonstrate that self-consistent solutions exist when the alpha-prescription for the viscous stress, similar to that of the original Shakura-Sunyaev model, is used. We show that α\sim 1 for the strongly magnetized case and we calculate the radial structure and emission …
The molecular component of the Galaxy is comprised of turbulent, magnetized clouds, many of which are self-gravitating and form stars. To understand how these clouds' evolution may depend on their level of turbulence, mean magnetization, and degree of self-gravity, we perform a survey of direct numerical MHD simulations in 2.5 dimensions. Two of our independent survey parameters allow us to model clouds which either meet or fail conditions for magneto-Jeans stability and magnetic criticality. Our third survey parameter allows us to initiate turbulence of either sub- or super-Alfvénic amplitude…
We investigate the effects of heating generated by damping of Alfven waves on protostellar accretion disks. Two mechanisms of damping are investigated, nonlinear and turbulent, which were previously studied in stellar winds (Jatenco-Pereira & Opher 1989a, b). For the nominal values studied, f=delta v/v_{A}=0.002 and F=varpi/Omega_{i}=0.1, where delta v, v_{A} and varpi are the amplitude, velocity and average frequency of the Alfven wave, respectively, and Omega_{i} is the ion cyclotron frequency, we find that viscous heating is more important than Alfven heating for small radii. When the radiu…
Magnetically mediated disk outflows are a leading paradigm to explain winds and jets in a variety of astrophysical sources, but where do the fields come from? Since accretion of mean magnetic flux may be disfavored in a thin turbulent disk, and only fields generated with sufficiently large scale can escape before being shredded by turbulence, in situ field production is desirable. Nonlinear helical inverse dynamo theory can provide the desired fields for coronae and outflows. We discuss the implications for contemporary protostellar disks, where the MRI (magneto-rotational instability) can dri…
The set of equations for magnetohydrodynamic (MHD) waves in a shear flow is consecutively derived. The proposed scenario involves the presence of a self-sustained turbulence and magnetic field. In the framework of Langevin--Burgers approach the influence of the turbulence is described by an additional external random force in the MHD system. Kinetic equation for the spectral density of the slow magnetosonic (Alfvenic) mode is derived in the short wavelength (WKB) approximation. The results show a pressing need for conduction of numerical Monte Carlo (MC) simulations with a random driver to tak…
Angular momentum in protostellar discs can be transported either radially, through turbulence induced by the magnetorotational instability (MRI), or vertically, through the torque exerted by a large-scale magnetic field. We present a model of steady-state discs where these two mechanisms operate at the same radius and derive approximate criteria for their occurrence in an ambipolar diffusion dominated disc. We obtain "weak field'' solutions - which we associate with the MRI channel modes in a stratified disc - and transform them into accretion solutions with predominantly radial angular-moment…
Accretion rates of order 10^-8 M_\odot/yr are observed in young protostars of approximately a solar mass with evidence of circumstellar disks. The accretion rate is significantly lower for protostars of smaller mass, approximately proportional to the second power of the stellar mass, \dot{M}_accr\propto M^2. The traditional view is that the observed accretion is the consequence of the angular momentum transport in isolated protostellar disks, controlled by disk turbulence or self--gravity. However, these processes are not well understood and the observed protostellar accretion, a fundamental a…
Magnetic fields are believed to play a crucial role in the process of star formation. We compare high-angular resolution observations of the submillimeter polarized emission of NGC 1333 IRAS 4A, tracing the magnetic field around a low-mass protostar, with models of the collapse of magnetized molecular cloud cores. Assuming a uniform dust alignment efficiency, we computed the Stokes parameters and synthetic polarization maps from the model density and magnetic field distribution by integrations along the line-of-sight and convolution with the interferometric response. The synthetic maps are in …
We investigate the behavior of the magnetic pressure, $b^2$, in fully turbulent MHD flows in ``1+2/3'' dimensions by means of its effect on the probability density function (PDF) of the density field. We start by reviewing our previous results for general polytropic flows, according to which the value of the polytropic exponent $γ$ determines the functional shape of the PDF. A lognormal density PDF appears in the isothermal ($γ=1$) case, but a power-law tail at either large or small densities appears for large Mach numbers when $γ>1$ and $γ< 1$, respectively. In the isothermal magnetic case, t…
We present an analytic description of turbulent, MHD disk accretion around black holes that specifically addresses the relationship between radial and vertical, transport, thereby complementing and extending numerical simulations. The azimuthal-vertical component of the magnetic stress is fundamental to an understanding of disk--corona--outflow coupling: when it is important for driving the angular momentum transport and mass accretion in the disk, it also has an important influence on the disk--corona--outflow energy budget. The Poynting flux derived from the product of this term with the Kep…
This paper studies the turbulent cascade of magnetic energy in weakly collisional magnetized plasmas. A cascade model is presented, based on the assumptions of local nonlinear energy transfer in wavenumber space, critical balance between linear propagation and nonlinear interaction times, and the applicability of linear dissipation rates for the nonlinearly turbulent plasma. The model follows the nonlinear cascade of energy from the driving scale in the MHD regime, through the transition at the ion Larmor radius into the kinetic Alfven wave regime, in which the turbulence is dissipated by kine…
We use global magnetohydrodynamic simulations, in a pseudo-Newtonian potential, to investigate the temporal variability of accretion discs around Schwarzschild black holes. We use the vertically-averaged magnetic stress in the simulated disc as a proxy for the rest-frame dissipation, and compute the observed emission by folding this through the transfer function describing the relativistic beaming, light bending and time delays near a non-rotating black hole. The temporal power spectrum of the predicted emission from individual annuli in the disc is described by a broken power law, with indice…
We discuss the transport of angular momentum induced by tidal effects in a disk surrounding a star in a pre-main sequence binary system. We consider the effect of both density and bending waves. Although tidal effects are important for truncating protostellar disks and for determining their size, it is unlikely that tidally-induced angular momentum transport plays a dominant role in the evolution of protostellar disks. Where the disk is magnetized, transport of angular momentum is probably governed by MHD turbulence. In a non self-gravitating laminar disk, the amount of transport provided by t…
We review recent observational and theoretical results on the relationship between circumstellar accretion disks and jets in young stellar objects. We then present a theoretical framework that interprets jets as accretion-powered, centrifugally driven winds from magnetized accretion disks. Recent progress in the numerical simulation of such outflows is described. We also discuss the structure of the underlying magnetized protostellar disks, emphasizing the role that large-scale, open magnetic fields can play in angular momentum transport.
We describe the results of a sequence of simulations of gravitational collapse in a turbulent magnetized region. The parameters are chosen to be representative of molecular cloud material. We find that several protostellar cores and filamentary structures of higher than average density form. The filaments inter-connect the high density cores. Furthermore, the magnetic field strengths are found to correlate positively with the density, in agreement with recent observations. We make synthetic channel maps of the simulations and show that material accreting onto the cores is channelled along the …
We present steady-state calculations of self-similar magnetized accretion discs driving cold, adiabatic, non-relativistic jets. For the first time, both the magnetic torque due to the jets and a turbulent "viscous" torque are taken into account. This latter torque allows a dissipation of the accretion power as radiation at the disc surfaces, while the former predominantly provides jets with power. The parameter space of these structures has been explored. It is characterized by four free parameters, namely the disc aspect ratio and three MHD turbulence parameters, related to the anomalous magn…
We review theoretical models of the early stages of star formation, in which gravitational collapse is strongly regulated by magnetic fields and the associated process of ambipolar diffusion. We discuss results of numerical simulations and analytical studies of core formation and collapse, which can be directly tested against observation. We also focus on recent data which are relevant to this theory of star formation, such as: observations of extended infall in protostellar cores, estimates of evolutionary timescales ~ 1 Myr for cores, measured mass-to-flux ratios of cores, and the relative a…
We present a physics-based statistical theory of a force-free magnetic field in the corona above a turbulent accretion disk. The field is represented by a statistical ensemble of loops tied to the disk. Each loop evolves under several physical processes: Keplerian shear, turbulent random walk of the disk footpoints, and reconnection with other loops. To build a statistical description, we introduce the distribution function of loops over their sizes and construct a kinetic equation that governs its evolution. This loop kinetic equation is formally analogous to Boltzmann's kinetic equation, wit…
