Relativistic Jets from Accretion Disks

The jets observed to emanate from many compact accreting objects may arise from the twisting of a magnetic field threading a differentially rotating accretion disk which acts to magnetically extract angular momentum and energy from the disk. Two main regimes have been discussed, hydromagnetic jets, which have a significant mass flux and have energy and angular momentum carried by both matter and electromagnetic field and, Poynting jets, where the mass flux is small and energy and angular momentum are carried predominantly by the electromagnetic field. Here, we describe recent theoretical work on the formation of relativistic Poynting jets from magnetized accretion disks. Further, we describe new relativistic, fully-electromagnetic, particle-in-cell simulations of the formation of jets from accretion disks. Analog Z-pinch experiments may help to understand the origin of astrophysical jets.Comment: 7 pages, 3 figures, Proc. of High Energy Density Astrophysics Conf., 200

Kelvin-Helmholtz Instability of the Magnetopause of Disc-Accreting Stars

This work investigates the short wavelength stability of the magnetopause between a rapidly-rotating, supersonic, dense accretion disc and a slowly-rotating low-density magnetosphere of a magnetized star. The magnetopause is a strong shear layer with rapid changes in the azimuthal velocity, the density, and the magnetic field over a short radial distance and thus the Kelvin-Helmholtz (KH) instability may be important. The plasma dynamics is treated using non-relativistic, compressible (isentropic) magnetohydrodynamics. It is necessary to include the displacement current in order that plasma wave velocities remain less than the speed of light. We focus mainly on the case of a star with an aligned dipole magnetic field so that the magnetic field is axial in the disc midplane and perpendicular to the disc flow velocity. However, we also give results for cases where the magnetic field is at an arbitrary angle to the flow velocity. For the aligned dipole case the magnetopause is most unstable for KH waves propagating in the azimuthal direction perpendicular to the magnetic field which tends to stabilize waves propagating parallel to it. The wave phase velocity is that of the disc matter. A quasi-linear theory of the saturation of the instability leads to a wavenumber ($k$) power spectrum $\propto k^{-1}$ of the density and temperature fluctuations of the magnetopause, and it gives the mass accretion and angular momentum inflow rates across the magnetopause. For self-consistent conditions this mass accretion rate will be equal to the disc accretion rate at large distances from the magnetopause.Comment: 8 pages, 7 figure

Spinning-Down of Moving Magnetars in the Propeller Regime

We use axisymmetric magnetohydrodynamic simulations to investigate the spinning-down of magnetars rotating in the propeller regime and moving supersonically through the interstellar medium. The simulations indicate that magnetars spin-down rapidly due to this interaction, faster than for the case of a non-moving star. From many simulation runs we have derived an approximate scaling laws for the angular momentum loss rate, \dot{L} \propto \~\eta_m^{0.3}\mu^{0.6}\rho^{0.8}{\cal M}^{-0.4} \Omega_*^{1.5}, where \rho is the density of the interstellar medium, \cal M is Mach number, \mu is the star's magnetic moment, \Omega_* is its angular velocity, and \eta_m is magnetic diffusivity. A magnetar with a surface magnetic field of 10^{13} - 10^{15} G is found to spin-down to a period P > 10^5-10^6 s in \sim 10^4 - 10^5 years. There is however uncertainty about the value of the magnetic diffusivity so that the time-scale may be longer. We discuss this model in respect of Soft Gamma Repeaters (SGRs) and the isolated neutron star candidate RXJ1856.5-3754.Comment: 10 pages, 4 figures, accepted by MNRAS. See version with better resolution figures and animation at http://astrosun2.astro.cornell.edu/us-rus/propeller.ht

Large space antennas: A systems analysis case history

The value of systems analysis and engineering is aptly demonstrated by the work on Large Space Antennas (LSA) by the NASA Langley Spacecraft Analysis Branch. This work was accomplished over the last half-decade by augmenting traditional system engineering, analysis, and design techniques with computer-aided engineering (CAE) techniques using the Langley-developed Interactive Design and Evaluation of Advanced Spacecraft (IDEAS) system. This report chronicles the research highlights and special systems analyses that focused the LSA work on deployable truss antennas. It notes developmental trends toward greater use of CAE techniques in their design and analysis. A look to the future envisions the application of improved systems analysis capabilities to advanced space systems such as an advanced space station or to lunar and Martian missions and human habitats

Boundary Between Stable and Unstable Regimes of Accretion

We investigated the boundary between stable and unstable regimes of accretion and its dependence on different parameters. Simulations were performed using a "cubed sphere" code with high grid resolution (244 grid points in the azimuthal direction), which is twice as high as that used in our earlier studies. We chose a very low viscosity value, with alpha-parameter alpha=0.02. We observed from the simulations that the boundary strongly depends on the ratio between magnetospheric radius r_m (where the magnetic stress in the magnetosphere matches the matter stress in the disk) and corotation radius r_cor (where the Keplerian velocity in the disk is equal to the angular velocity of the star). For a small misalignment angle of the dipole field, Theta=5 degrees, accretion is unstable if r_cor/r_m>1.35, and is stable otherwise. In cases of a larger misalignment angle of the dipole, Theta=20 degrees, instability occurs at slightly larger values, r_cor/r_m>1.41.Comment: 4 pages, 4 figures, conference proceedings: "Physics at the Magnetospheric Boundary", Geneva, Switzerland, 25-28 June, 201

Jets and Disk-Winds from Pulsar Magnetospheres

We discuss axisymmetric force-free pulsar magnetospheres with magnetically collimated jets and a disk-wind obtained by numerical solution of the pulsar equation. This solution represents an alternative to the quasi-spherical wind solutions where a major part of the current flow is in a current sheet which is unstable to magnetic field annihilation.Comment: 6 figures, accepted for publication in the Ap

MHD simulations of disk-star interaction

We discuss a number of topics relevant to disk-magnetosphere interaction and how numerical simulations illuminate them. The topics include: (1) disk-magnetosphere interaction and the problem of disk-locking; (2) the wind problem; (3) structure of the magnetospheric flow, hot spots at the star's surface, and the inner disk region; (4) modeling of spectra from 3D funnel streams; (5) accretion to a star with a complex magnetic field; (6) accretion through 3D instabilities; (7) magnetospheric gap and survival of protoplanets. Results of both 2D and 3D simulations are discussed.Comment: 12 pages, 10 figures, Star-Disk Interaction in Young Stars, Proceedings of the International Astronomical Union, IAU Symposium, Volume 243. See animations at http://astro.cornell.edu/~romanova/projects.htm and at http://astro.cornell.edu/us-rus