Outflow Of Angular Momentum Research Articles

Winds and Disk Turbulence Exert Equal Torques on Thick Magnetically Arrested Disks

The conventional accretion disk lore is that magnetized turbulence is the principal angular momentum transport process that drives accretion. However, when dynamically important large-scale magnetic fields thread an accretion disk, they can produce mass and angular momentum outflows, known as winds, that also drive accretion. Yet, the relative importance of turbulent and wind-driven angular momentum transport is still poorly understood. To probe this question, we analyze a long-duration (1.2 × 105 r g /c) simulation of a rapidly rotating (a = 0.9) black hole feeding from a thick (H/r ∼ 0.3), adiabatic, magnetically arrested disk (MAD), whose dynamically important magnetic field regulates mass inflow and drives both uncollimated and collimated outflows (i.e., winds and jets, respectively). By carefully disentangling the various angular momentum transport processes within the system, we demonstrate the novel result that disk winds and disk turbulence both extract roughly equal amounts of angular momentum from the disk. We find cumulative angular momentum and mass accretion outflow rates of L̇∝r0.9 and Ṁ∝r0.4 , respectively. This result suggests that understanding both turbulent and laminar stresses is key to understanding the evolution of systems where geometrically thick MADs can occur, such as the hard state of X-ray binaries, low-luminosity active galactic nuclei, some tidal disruption events, and possibly gamma-ray bursts.

Boundary Layers of Circumplanetary Disks around Spinning Planets. I. Effects of Rossby Waves

Gas giant planets are believed to accrete from their circumplanetary disks (CPDs). The CPDs usually involve accretion through the boundary layer (BL) in the vicinity of planets. Prior studies have concentrated on the BL of nonspinning planets. We investigate the influence of planetary spin on the wave behaviors within the BL. The rotation profile in such BLs would show a sharp transition from the rigid rotation to the Keplerian rotation. We examine the angular momentum transport in these BLs in terms of linear perturbation analysis. We find that the global inertia-acoustic mode associated with spinning planets would give rise to the inflow of angular momentum and the accretion of gas. In this work, we identify a new kind of global mode, namely the Rossby mode. The Rossby mode can lead to the outflow of angular momentum and the decretion of gas from a spinning planet. The Rossby mode provides a negative feedback that regulates the planetary spin and mass. We compare the growth rate of the two modes as a function of the width of BL, the Mach number, and the spin rate of planets. Our results reveal the underlying hydrodynamic mechanism of terminal spins and asymptotic mass of the giant planets.

The effect of external pressure & magnetic field in star formation: The critical mass model

Theoretical work addressing the role of external pressure with magnetic fields in collapsing molecular clouds is important in building a comprehensive theory of star formation(SF). In many SF studies turbulence, magnetic fields, and self-gravity are described as the key dynamical processes involved in SF. However, the importance of external pressure in collapsing strongly magnetized clouds has not yet been particularly explained. Magnetic fields transport excess angular momentum from the central core while external pressure compresses the cloud. Thus the outflow of angular momentum, and on the other hand, the compression made by external pressure are the cause for matter falling onto the central core from the envelope. Therefore, external pressure facilitates the collapse of gas and the inflow of matter. In this work, we show theoretically how the strong magnetic field is dragged inward by the external pressure during the core collapse and formulate the critical mass of the core in the presence of external pressure.

Moving away from the near-horizon attractor of the extreme Kerr force-free magnetosphere

We consider force-free magnetospheres around the extreme Kerr black hole. In this case there is no known exact analytic solution to force free electrodynamics which is stationary, axisymmetric and magnetically-dominated. However, any stationary, axisymmetric and regular force-free magnetosphere in extreme Kerr black hole approaches the same attractor solution in the near-horizon extreme Kerr (NHEK) limit with null electromagnetic field. We show that by moving away from the attractor solution in the NHEK region, one finds magnetically-dominated solutions in the extreme Kerr black hole with finite angular momentum outflow. This result is achieved using a perturbative analysis up to the second order.

Comparisons of MHD propeller model with observations of cataclysmic variable AE Aqr

ABSTRACT We have developed a numerical magnetohydrodynamic (MHD) model of the propeller candidate star AE Aqr using axisymmetric MHD simulations. We suggest that AE Aqr is an intermediate polar-type star, where the magnetic field is relatively weak and an accretion disc may form around the white dwarf. The star is in the propeller regime, and many of its observational properties are determined by the disc–magnetosphere interaction. Comparisons of the characteristics of the observed versus modelled AE Aqr star show that the model can explain many observational properties of AE Aqr. In a representative model, the magnetic field of the star is B ≈ 3.3 × 105 G and the time-averaged accretion rate in the disc is 5.5 × 1016 g s−1. Most of this matter is ejected into conically shaped winds. The numerical model explains the rapid spin-down of AE Aqr through the outflow of angular momentum from the surface of the star to the wind, corona, and disc. The energy budget in the outflows, 9 × 1033 erg s−1, is sufficient for explaining the observed flaring radiation in different wavebands. The time-scale of ejections into the wind matches the short time-scale variability in the light curves of AE Aqr.

Mass ratio in SS433 revisited

We revisit the determination of binary mass ratio in the Galactic microquasar SS433 based on recent GRAVITY VLTI measurements of mass and angular momentum outflow through a circumbinary disc. The new observations combined with the constancy of the binary orbital period over $\sim 30$ yrs confirm that the mass ratio in SS433 is $q=M_\mathrm{x}/M_\mathrm{v}\gtrsim 0.6$. For the assumed optical star mass $M_\mathrm{v}$ ranging from $\sim 8$ to 15 $M_\odot$ such a mass ratio suggests a low limit of the compact object mass of $M_\mathrm{x}\sim 5-9 M_\odot$, placing the compact object in SS433 as a stellar-mass black hole.

Role of magnetic field in star formation

AbstractMagnetic fields are a key component in star formation theories. Nevertheless, their exact role in the formation of stars is still a matter of debate. The process of angular momentum transportation by the disturbance caused during magnetic field reconnection still needs theoretical formulation in terms of the collapsing cloud’s parameters. The purposes of this study are: to model the critical mass of a magnetized, gravitating and turbulent star forming molecular cloud (MC) and to formulate the momentum carried out by a magnetic field through magnetic field reconnection in terms of the MC’s parameters. By applying theoretical modeling, we show how angular momentum transported via an Alfvén wave can be described in terms of mass, radius and dispersion velocity of a collapsing cloud core and a model equation of the critical mass for a gravitating, turbulent, and magnetized molecular cloud core. The outflow of angular momentum by magnetic fields facilitates the inflow of mass. On the other side, magnetic pressure prevents collapse. Therefore, magnetic fields have a dual purpose in the process of star formation. This momentum outflow triggers the inflow of mass to conserve angular momentum. The results show that Alfvén waves are like a machine that extracts angular momentum from a magnetized collapsing cloud core. Thus the total angular momentum transported by magnetic field at a distance R from the core’s center depends on the size, mass and turbulent velocity dispersion of the collapsing cloud core.

Advection of matter and B-fields in alpha-discs

We have carried out and analyzed a set of axisymmetric MHD simulations of the evolution of a turbulent/diffusive accretion disc around an initially unmagnetized star. The disc is initially threaded by a weak magnetic field where the magnetic pressure is significantly less than the kinetic pressure in the disc. The viscosity and magnetic diffusivity are modelled by two "alpha" parameters, while the coronal region above the disc is treated using ideal MHD. The initial magnetic field is taken to consist of three poloidal field loops threading the disc. The motivation for this study is to understand the advection of disc matter and magnetic field by the turbulent/diffusive disc. At early times the innermost field loop twists and its field lines become open. The twisting of the opened field lines leads to the formation of both an inner collimated, magnetically-dominated jet, and at larger distances from the axis a matter dominated uncollimated wind. For later times, the strength of the magnetic field decreases owing to field reconnection and annihilation in the disc. For the early times, we have derived from the simulations both the matter accretion speed in the disc and the accretion speed of the magnetic field. We show that the derived matter accretion speed agrees approximately with the predictions of a model where the accretion speed is the sum of two terms, one due to the disc's viscosity (which gives a radial outflow of angular momentum in the disc), and a second due to the twisted magnetic field at the disc's surface (which gives a vertical outflow of angular momentum). For early times we find that the magnetic contribution is roughly twice the viscous contribution for the case where the alpha parameters are both equal to 0.1. At later times the magnetic contribution to the matter speed becomes small compared to the viscous contribution.

T Tauri Jet Physics Resolved Near the Launching Region with theHubble Space Telescope

We present an analysis of the gas physics at the base of jets from five T Tauri stars based on high angular resolution optical spectra, using the Hubble Space Telescope Imaging Spectrograph (HST STIS). The spectra refer to a region within 100 AU of the star, i.e., where the collimation of the jet has just taken place. We form position-velocity (PV) images of the line ratios to get a global picture of the flow excitation. We then apply a specialized diagnostic technique to find the electron density, ionization fraction, electron temperature, and total density. Our results are in the form of PV maps of the obtained quantities, in which the gas behavior is resolved as a function of both radial velocity and distance from the jet axis. They highlight a number of interesting physical features of the jet collimation region, including regions of extremely high density, asymmetries with respect to the axis, and possible shock signatures. Finally, we estimate the jet mass and angular momentum outflow rates, both of which are fundamental parameters in constraining models of accretion-ejection structures, particularly if the parameters can be determined close to the jet footpoint. Comparing mass flow rates for cases where the mass accretion rate is available in the literature (i.e., for DG Tau, RW Aur, and CW Tau) reveals a mass ejection-to-accretion ratio of 0.01-0.07. Finally, where possible (i.e., for DG Tau and CW Tau), both mass and angular momentum outflow rates are resolved into higher and lower velocity jet material. For the clearer case of DG Tau, this reveals that the more collimated higher velocity component plays a dominant role in mass and angular momentum transport.

Numerical simulations of the interaction of accretion disks with young star magnetospheres

Aims. We present the results of the numerical simulations of the interaction between a magnetized star and an imperfectly conducting accretion disk. The star is rotating with constant angular velocity. The differentially rotating Keplerian disk is treated as a boundary condition. We are interested in the magnetic field topology dependence on the electrical conductivity of the disk. Methods. To analyze the “star–disk” interaction we numerically investigate the MHD equations using Godunov-type high resolution numerical methods. Results. It was found that in our model the “star–disk” interaction occurs with a quasi-periodic reconnection of the magnetic field coronal loops and plasmoid ejections. In the case of the perfect disk conductivity, the evolution of the coronal magnetic field leads to the periodic outflow of angular momentum from the disk. In the case of an imperfectly conducting disk, the configuration of the magnetic field is formed such that the disk angular momentum carried by the magnetic field is balanced by angular momentum carried by matter. It should be noted that we used the ideal MHD equation to obtain the solutions. The reconnection process in the disk corona depends on the numerical diffusivity that exists in our numerical code. Our simulations treat reconnection as occurring in current sheets. The thickness of the current sheet is broadened by numerical resistivity. Nevertheless, we suppose that the reconnection and plasmoid ejection takes place as well for real magnetic diffusivity. To verify the method and results we also used several more detailed grids to estimate the numerical diffusivity of the scheme. It is turned out that the setup model presented in the paper quite reasonable satisfies the goal of this paper, i.e., to investigate the regime of interaction between the magnetized star and the disk.

Prediction of Flow Characteristics and Energy Balance for a Variety of Downflow Impellers

A computational fluid dynamics (CFD) model has been developed for prediction of the flow characteristics based on the forces generated on the impeller blades. A good agreement between the CFD predictions and the experimental measurements at several locations in the vessel has been observed. The model has been validated for a wide variety of axial downflow impellers. An eddy identification scheme has been proposed to estimate the values of the turbulent length scales, eddy diffusivity, and turbulent energy dissipation rate. The impeller power number has been calculated by three methods: (i) the integration of local values of ε predicted from CFD, (ii) a method based on the forces acting on the impeller blades, and (iii) the outflow of angular momentum from the impeller zone. A good agreement has been observed between the predicted values of the power number by the previous three methods and reported values for a wide variety of axial flow impellers.

Simultaneous Implosive Accretion and Jet Formation in Quasars: Correlation of Optical Outbursts by VLBI Jets

A model and simulation code have been developed for time-dependent axisymmetric disk accretion onto a compact object including for the first time the influence of an ordered magnetic field and magnetically driven outflow of energy and angular momentum in (±z) directions (see also Lovelace et al., 1993). It was shown that the system behaviour crucially depends on the amplitude of the poloidal magnetic field fluctuation Bp, compared to the critical value Bcr ∼ (α2T1/2σ/R3/2)1/2, where T(r, t) is the temperature, σ(r, t) the surface density of the disk, R the radial distance, α the alpha coefficient of Shakura-Sunyaev disk model. If the fluctuation is small, Bp < Bcr, then it diffuses outwards with decreasing the amplitude and eventually disappears. In the opposite case Bp > Bcr, a soliton-like structure forms in the disk density, temperature, and magnetic field and propagates implosively inward. In this case the radial accretion speed u(r, t) is shown to be the sum of the usual viscous contribution and magneitic contribution ∼ R3/2B2p/σ. The essential part of angular momentum and energy is going to the jet from the region of fluctuation. Compression of matter in the propagating wave leads to enhancement of magnetic field and more effective angular momentum outflow. This leads in turn to accelerated accretion and subsequent enhancement of magnetic field. It gives the implosive nature of the process, which can be observed as: simultaneous burst in the radiation and outflow. The model is pertinent to the formation of discrete components observed in VLBI jets which appear to originate at times of optical outbursts at some quasars.

The angular momentum behavior in the dissipationless collapse of triaxial systems

We present N-body simulations of the collapse of six triaxial systems having the angular momentum aligned with one of their principal axes. Two different initial density distributions are chosen. The final relaxed states of the simulations show (1) a net outflow of angular momentum from the inner regions to the outer halo, consistent with the loss expected during the formation of elliptical galaxies; (2) a general misalignment of the average spin of the internal mass fractions with respect to that of the total system. Finally, some considerations are given concerning the occurrence of the necessary condition for the formation of a black hole at the center of these systems

Intrinsically asymmetric astrophysical jets

Previous work on the origin of self-collimated electromagnetic jets is extended to the general case where there is no reflection symmetry of the magnetic field about the equatorial plane of the disk. The axisymmetric field structure inside the disk is obtained by solving for the magnetic flux function, and the toroidal magnetic field, from the generalized thin-disk induction equation, which is derived. The asymptotic (large-z) magnetic field structure outside the disk is obtained by solving the force-free Grad-Shafranov equation semianalytically. Jet solutions are found in which the power flow is carried mainly by the Poynting flux of the electromagnetic field and the angular momentum outflow from the disk is carried by the magnetic field. The ratio of jet luminosities (top/bottom) depends directly on the degree of asymmetry of the field and can easily be much greater than unity. It is argued that the degree of field asymmetry in the disk is determined by the asymmetry of the weak galactic field fed into the disk at large distances over long periods of time.

Flow of Momentum and Mass across the High-Pressure Belt of the Earth's Atmosphere

THE atmospheric angular momentum generated at the earth's surface between latitudes ± ϕ must, over a long period, be balanced by an equivalent outflow of angular momentum across these parallels. It was argued by Jeffreys1 that the main agents of this efflux are rotations of cyclonic dimensions lying side by side, at least in the latitudes of the prevailing surface westerlies. Extended to all latitudes, this view encounters the difficulty that the maximal flux must occur across the belts of minimal disturbance around lat. 30°–35° N. and S.; nevertheless, it is widely held that this is effected by a marked horizontal asymmetry in the anticyclones comprising the high-pressure belt.