A self-sustaining nonlinear dynamo process in Keplerian shear flows

A three-dimensional nonlinear dynamo process is identified in rotating plane Couette flow in the Keplerian regime. It is analogous to the hydrodynamic self-sustaining process in non-rotating shear flows and relies on the magneto-rotational instability of a toroidal magnetic field. Steady nonlinear solutions are computed numerically for a wide range of magnetic Reynolds numbers but are restricted to low Reynolds numbers. This process may be important to explain the sustenance of coherent fields and turbulent motions in Keplerian accretion disks, where all its basic ingredients are present.Comment: 4 pages, 7 figures, accepted for publication in Physical Review Letter

Development of a scanning electron mirror microscope

Scanning electron mirrors microscope design and developmen

Viscoresistive MHD Configurations of Plasma in Accretion Disks

We present a discussion of two-dimensional magneto-hydrodynamics (MHD) configurations, concerning the equilibria of accretion disks of a strongly magnetized astrophysical object. We set up a viscoresistive scenario which generalizes previous two-dimensional analyses by reconciling the ideal MHD coupling of the vertical and the radial equilibria within the disk with the standard mechanism of the angular momentum transport, relying on dissipative properties of the plasma configuration. The linear features of the considered model are analytically developed and the non-linear configuration problem is addressed, by fixing the entire disk profile at the same order of approximation. Indeed, the azimuthal and electron force balance equations are no longer automatically satisfied when poloidal currents and matter fluxes are included in the problem. These additional components of the equilibrium configuration induce a different morphology of the magnetic flux surface, with respect to the ideal and simply rotating disk.Comment: 19 pages, 4 figures. To appear on the Proceedings of the Second Italian-Pakistani Workshop on Relativistic Astrophysic

Robustly Unstable Eigenmodes of the Magnetoshearing Instability in Accretion Disk

The stability of nonaxisymmetric perturbations in differentially rotating astrophysical accretion disks is analyzed by fully incorporating the properties of shear flows. We verify the presence of discrete unstable eigenmodes with complex and pure imaginary eigenvalues, without any artificial disk edge boundaries, unlike Ogilvie & Pringle(1996)'s claim. By developing the mathematical theory of a non-self-adjoint system, we investigate the nonlocal behavior of eigenmodes in the vicinity of Alfven singularities at omega_D=omega_A, where omega_D is the Doppler-shifted wave frequency and omega_A=k_// v_A is the Alfven frequency. The structure of the spectrum of discrete eigenmodes is discussed and the magnetic field and wavenumber dependence of the growth rate are obtained. Exponentially growing modes are present even in a region where the local dispersion relation theory claims to have stable eigenvalues. The velocity field created by an eigenmode is obtained, which explains the anomalous angular momentum transport in the nonlinear stage of this stability.Comment: 11pages, 11figures, to be published in ApJ. For associated eps files, see http://dino.ph.utexas.edu/~knoguchi

A Manager’s Actions? An Exercise for Exploring Sexual Harassment

This exercise is designed to enhance students’ understanding of sexual harassment issues in the workplace, as well as to extend these issues to customer interactions in retail settings. Through research and class discussion, participants explore their perceptions and assumptions about a manager’s potentially harassing behaviors. Furthermore, discussions of the manager’s actions provide additional opportunities to explore issues dealing with legal foundations of sexual harassment as well as gender differences in response to harassment. Multiple variations are presented making this scenario/exercise suitable to various courses and all types of participants in order to facilitate a healthy discussion about the issues associated with sexual harassment

Magnetohydrodynamic turbulence in warped accretion discs

Warped, precessing accretion discs appear in a range of astrophysical systems, for instance the X-ray binary Her X-1 and in the active nucleus of NGC4258. In a warped accretion disc there are horizontal pressure gradients that drive an epicyclic motion. We have studied the interaction of this epicyclic motion with the magnetohydrodynamic turbulence in numerical simulations. We find that the turbulent stress acting on the epicyclic motion is comparable in size to the stress that drives the accretion, however an important ingredient in the damping of the epicyclic motion is its parametric decay into inertial waves.Comment: to appear in the proceedings of the 20th Texas Symposium on Relativistic Astrophysics, J. C. Wheeler & H. Martel (eds.

The response of a turbulent accretion disc to an imposed epicyclic shearing motion

We excite an epicyclic motion, whose amplitude depends on the vertical position, $z$, in a simulation of a turbulent accretion disc. An epicyclic motion of this kind may be caused by a warping of the disc. By studying how the epicyclic motion decays we can obtain information about the interaction between the warp and the disc turbulence. A high amplitude epicyclic motion decays first by exciting inertial waves through a parametric instability, but its subsequent exponential damping may be reproduced by a turbulent viscosity. We estimate the effective viscosity parameter, $\alpha_{\rm v}$, pertaining to such a vertical shear. We also gain new information on the properties of the disc turbulence in general, and measure the usual viscosity parameter, $\alpha_{\rm h}$, pertaining to a horizontal (Keplerian) shear. We find that, as is often assumed in theoretical studies, $\alpha_{\rm v}$ is approximately equal to $\alpha_{\rm h}$ and both are much less than unity, for the field strengths achieved in our local box calculations of turbulence. In view of the smallness ($\sim 0.01$) of $\alpha_{\rm v}$ and $\alpha_{\rm h}$ we conclude that for $\beta = p_{\rm gas}/p_{\rm mag} \sim 10$ the timescale for diffusion or damping of a warp is much shorter than the usual viscous timescale. Finally, we review the astrophysical implications.Comment: 12 pages, 18 figures, MNRAS accepte

Aligning spinning black holes and accretion discs

We consider the alignment torque between a spinning black hole and an accretion disc whose angular momenta are misaligned. This situation must hold initially in almost all gas accretion events on to supermassive black holes, and may occur in binaries where the black hole receives a natal supernova kick. We show that the torque always acts to align the hole's spin with the total angular momentum without changing its magnitude. The torque acts dissipatively on the disc, reducing its angular momentum, and aligning it with the hole if and only if the angle theta between the angular momenta J_d of the disc and J_h of the hole satisfies the inequality cos theta > -J_d / 2 J_h. If this condition fails, which requires both theta > pi/2 and J_d < 2 J_h, the disc counteraligns.Comment: MNRAS, in pres

The Excitation, Propagation and Dissipation of Waves in Accretion Discs: The Non-linear Axisymmetric Case

We analyse the non-linear propagation and dissipation of axisymmetric waves in accretion discs using the ZEUS-2D hydrodynamics code. The waves are numerically resolved in the vertical and radial directions. Both vertically isothermal and thermally stratified accretion discs are considered. The waves are generated by means of resonant forcing and several forms of forcing are considered. Compressional motions are taken to be locally adiabatic ($\gamma = 5/3$). Prior to non-linear dissipation, the numerical results are in excellent agreement with the linear theory of wave channelling in predicting the types of modes that are excited, the energy flux by carried by each mode, and the vertical wave energy distribution as a function of radius. In all cases, waves are excited that propagate on both sides of the resonance (inwards and outwards). For vertically isothermal discs, non-linear dissipation occurs primarily through shocks that result from the classical steepening of acoustic waves. For discs that are substantially thermally stratified, wave channelling is the primary mechanism for shock generation. Wave channelling boosts the Mach number of the wave by vertically confining the wave to a small cool region at the base of the disc atmosphere. In general, outwardly propagating waves with Mach numbers near resonance {\cal M}_{\rm r} \ga 0.01 undergo shocks within a distance of order the resonance radius.Comment: 28 pages, 21 figures - 8 as GIF, 13 embedded postscript, Accepted for publication in MNRAS. Full postscript version available from http://www.astro.ex.ac.uk/people/mbat

Viscous Effects on the Interaction between the Coplanar Decretion Disc and the Neutron Star in Be/X-Ray Binaries

We study the viscous effects on the interaction between the coplanar Be-star disc and the neutron star in Be/X-ray binaries, using a three-dimensional, smoothed particle hydrodynamics code. For simplicity, we assume the Be disc to be isothermal at the temperature of half the stellar effective temperature. In order to mimic the gas ejection process from the Be star, we inject particles with the Keplerian rotation velocity at a radius just outside the star. Both Be star and neutron star are treated as point masses. We find that the Be-star disc is effectively truncated if the Shakura-Sunyaev viscosity parameter alpha_SS >> 1, which confirms the previous semi-analytical result. In the truncated disc, the material decreted from the Be star accumulates, so that the disc becomes denser more rapidly than if around an isolated Be star. The resonant truncation of the Be disc results in a significant reduction of the amount of gas captured by the neutron star and a strong dependence of the mass capture rate on the orbital phase. We also find that an eccentric mode is excited in the Be disc through direct driving due to a one-armed bar potential of the binary. The strength of the mode becomes greater in the case of a smaller viscosity. In a high-resolution simulation with alpha_SS=0.1, the eccentric mode is found to precess in a prograde sense. The mass capture rate by the neutron star modulates as the mode precesses.Comment: 15 pages, including 20 figures and 1 table, accepted for publication in MNRA