Angular momentum transport in astrophysics and in the lab (original) (raw)

Enhanced Angular Momentum Transport in Accretion Disks

Annual Review of Astronomy and Astrophysics, 2003

▪ The status of our current understanding of angular momentum transport in accretion disks is reviewed. The last decade has seen a dramatic increase both in the recognition of key physical processes and in our ability to carry through direct numerical simulations of turbulent flow. Magnetic fields have at once powerful and subtle influences on the behavior of (sufficiently) ionized gas, rendering them directly unstable to free energy gradients. Outwardly descreasing angular velocity profiles are unstable. The breakdown of Keplerian rotation into MHD turbulence may be studied in some numerical detail, and key transport coefficients may be evaluated. Chandra observations of the Galactic Center support the existence of low luminosity accretion, which may ultimately prove amenable to global three-dimensional numerical simulation.

Angular Momentum Transport in Astrophysical Disks

The Astrophysical Journal, 2008

The evolution of astrophysical disks is dominated by instabilities of gravity perturbations (e.g., those produced by a spontaneous disturbance). We develop a hydrodynamic theory of nonresonant Jeans instability in a dynamically cold subsystem (identified as the gaseous component) of a disk. We show analytically that gravitationally unstable systems, such as disks of rotationally supported galaxies, protoplanetary disks, and, finally, the solar nebula are efficient at transporting mass and angular momentum: already on a timescale of on the order of 2-3 rotational periods an unstable disk sees a large portion of its angular momentum transferred outward, and mass transferred both inward and outward.

Investigating the transport of angular momentum from young stellar objects

Astronomy and Astrophysics, 2008

Aims. In this pilot study, we examine molecular jets from the embedded Class I sources, HH 26 and HH 72, to search, for the first time, for kinematic signatures of jet rotation from young embedded sources. Methods. High-resolution long-slit spectroscopy of the H 2 1-0 S(1) transition was obtained using VLT/ISAAC. The slit was placed perpendicular to the flow direction about 2 ′′ from the sources. Position-velocity (PV) diagrams are constructed and intensityweighted radial velocities transverse to the jet flow are measured. Results. Mean intensity-weighted velocities vary between v LSR ∼ -90 and -65 km s −1 for HH 26, and -60 and -10 km s −1 for HH 72; maxima occur close to the intensity peak and decrease toward the jet borders. Velocity dispersions are ∼ 45 and ∼ 80 km s −1 for HH 26 and HH 72, respectively, with gas motions as fast as -100 km s −1 present. Asymmetric PV diagrams are seen for both objects, which a simple empirical model of a cylindrical jet section shows could in principle be reproduced by jet rotation alone. Assuming magneto-centrifugal launching, the observed HH 26 flow may originate at a disk radius of 2-4 AU from the star with the toroidal component of the magnetic field dominant at the observed location, in agreement with magnetic collimation models. We estimate that the kinetic angular momentum transported by the HH 26 jet is ∼ 2 × 10 −5 M ⊙ yr −1 AU km s −1 . This value (a lower limit to the total angular momentum transported by the flow) already amounts to 70% of the angular momentum that has to be extracted from the disk for the accretion to proceed at the observed rate.

Angular Momentum Transport in Low Mass Accretion Disks

1993

We review mechanisms for the transport of angular momentum in accretion disks that are low mass, in the sense that the gravitational forces produced by the material in these disks has a negligible effect on disk dynamics. There is no established consensus on how this transport takes place. We note that for phenomenological reasons the traditional alpha\alphaalpha model is probably not a good description of real disks. We discuss models in which angular momentum transport is driven by shocks, and by magnetic field instabilities. The latter is more promising, but requires a dynamo. We note that the direction of angular momentum transport due to convection in a conducting disk is not known as competing mechanisms are at work. We briefly discuss a number of possible dynamo mechanisms, and their problems. We then give a detailed exposition of the internal wave driven dynamo model, in which internal waves excited at large radii drive an alpha−Omega\alpha-\Omegaalpha−Omega dynamo.

Angular Momentum Transport by MHD Turbulence in Accretion Disks

Progress of Theoretical Physics Supplement, 2004

The saturation level of the magnetorotational instability (MRI) is investigated using three-dimensional MHD simulations. The shearing box approximation is adopted and the vertical component of gravity is ignored, so that the evolution of the MRI is followed in a small local part of the disk. We focus on the dependence of the saturation level of the stress on the gas pressure, which is a key assumption in the standard α disk model. From our numerical experiments it is found that there is a weak power-law relation between the saturation level of the Maxwell stress and the gas pressure in the nonlinear regime; the higher the gas pressure, the larger the stress. Although the powerlaw index depends slightly on the initial field geometry, the relationship between stress and gas pressure is independent of the initial field strength, and is unaffected by Ohmic dissipation if the magnetic Reynolds number is at least 10. The relationship is the same in adiabatic calculations, where pressure increases over time, and nearly-isothermal calculations, where pressure varies little with time. Over the entire region of parameter space explored, turbulence driven by the MRI has many characteristic ratios such as that of the Maxwell stress to the magnetic pressure. We also find the amplitudes of the spatial fluctuations in density and the time variability in the stress are characterized by the ratio of magnetic pressure to gas pressure in the nonlinear regime. Our numerical results are qualitatively consistent with an idea that the saturation level of the MRI is determined by a balance between the growth of the MRI and the dissipation of the field through reconnection. The quantitative interpretation of the pressure-stress relation, however, may require advances in the theoretical understanding of non-steady magnetic reconnection.

Stability of quasi-Keplerian shear flow in a laboratory experiment

Astronomy & Astrophysics, 2012

Context. Subcritical transition to turbulence has been proposed as a source of turbulent viscosity required for the associated angular momentum transport for fast accretion in Keplerian disks. Previously cited laboratory experiments in supporting this hypothesis were performed either in a different type of flow than Keplerian or without quantitative measurements of angular momentum transport and mean flow profile, and all of them appear to suffer from Ekman effects, secondary flows induced by nonoptimal axial boundary conditions. Such Ekman effects are expected to be absent from astronomical disks, which probably have stress-free vertical boundaries unless strongly magnetized. Aims. To quantify angular momentum transport due to subcritical hydrodynamic turbulence, if exists, in a quasi-Keplerian flow with minimized Ekman effects. Methods. We perform a local measurement of the azimuthal-radial component of the Reynolds stress tensor in a novel laboratory apparatus where Ekman effects are minimized by flexible control of axial boundary conditions. Results. We find significant Ekman effects on angular momentum transport due to nonoptimal axial boundary conditions in quasi-Keplerian flows. With the optimal control of Ekman effects, no statistically meaningful angular momentum transport is detected in such flows at Reynolds number up to two millions. Conclusions. Either a subcritical transition does not occur, or, if a subcritical transition does occur, the associated radial transport of angular momentum in optimized quasi-Keplerian laboratory flows is too small to directly support the hypothesis that subcritical hydrodynamic turbulence is responsible for accretion in astrophysical disks. Possible limitations in applying laboratory results to astrophysical disks due to experimental geometry are discussed.

Instability, turbulence, and enhanced transport in accretion disks

Reviews of Modern Physics, 1998

Recent years have witnessed dramatic progress in our understanding of how turbulence arises and transports angular momentum in astrophysical accretion disks. The key conceptual point has its origins in work dating from the 1950s, but its implications have been fully understood only in the last several years: the combination of a subthermal magnetic field (any nonpathological configuration will do) and outwardly decreasing differential rotation rapidly generates magnetohydrodynamic (MHD) turbulence via a remarkably simple linear instability. The result is a greatly enhanced effective viscosity, the origin of which had been a long-standing problem. The MHD nature of disk turbulence has linked two broad domains of magnetized fluid research: accretion theory and dynamos. The understanding that weak magnetic fields are not merely passively acted upon by turbulence, but actively generate it, means that the assumptions of classical dynamo theory break down in disks. Paralleling the new conceptual understanding has been the development of powerful numerical MHD codes. These have taught us that disks truly are turbulent, transporting angular momentum at greatly enhanced rates. We have also learned, however, that not all forms of disk turbulence do this. Purely hydrodynamic turbulence, when it is imposed, simply causes fluctuations without a significant increase in transport. The interplay between numerical simulation and analytic arguments has been particularly fruitful in accretion disk theory and is a major focus of this article. The authors conclude with a summary of what is now known of disk turbulence and mention some knotty outstanding questions (e.g., what is the physics behind nonlinear field saturation?) for which we may soon begin to develop answers. [S0034-6861(98)00501-7] CONTENTS 42 5. The evolution of an initially random field 42 6. Shear vs vorticity 43 7. Density stratification 44 C. MHD simulations: a summary 45 VI. Accretion Disk Dynamos 45 A. The dynamo-electric machine 45 B. A brief review of mean-field dynamo theory 46 C. Mean-field theory and nonlinear evolution of the magnetorotational instability 47 D. Saturation 49 VII. Summary 50 Acknowledgments 50 References 51

Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets

Plasma and Fusion Research, 2009

Astrophysical accretion is arguably the most prevalent physical process in the Universe; it occurs during the birth and death of individual stars and plays a pivotal role in the evolution of entire galaxies. Accretion onto a black hole, in particular, is also the most efficient mechanism known in nature, converting up to 40% of accreting rest mass energy into spectacular forms such as high-energy (X-ray and gamma-ray) emission and relativistic jets. Whilst magnetic fields are thought to be ultimately responsible for these phenomena, our understanding of the microphysics of MHD turbulence in accretion flows as well as large-scale MHD outflows remains far from complete. We present a new theoretical model for astrophysical disk accretion which considers enhanced vertical transport of momentum and energy by MHD winds and jets, as well as transport resulting from MHD turbulence. We also describe new global, 3D simulations that we are currently developing to investigate the extent to which non-ideal MHD effects may explain how small-scale, turbulent fields (generated by the magnetorotational instability-MRI) might evolve into large-scale, ordered fields that produce a magnetized corona and/or jets where the highest energy phenomena necessarily originate.

Non-linear energy transfers in accretion discs MRI turbulence

Astronomy & Astrophysics, 2011

The magnetorotational instability (MRI) is believed to be responsible for most of the angular momentum transport in accretion discs. However, molecular dissipation processes may drastically change the efficiency of MRI turbulence in realistic astrophysical situations. The physical origin of this dependency is still poorly understood as linear and quasi linear theories fail to explain it. In this paper, we look for the link between molecular dissipation processes and MRI transport of angular momentum in unstratified shearing box simulations, including a mean vertical field. We show that magnetic helicity is unimportant in the model we consider. We perform a spectral analysis on the simulations tracking energy exchanges in spectral space when turbulence is fully developed. We find that the energy exchanges are essentially direct (from large to small scale) whereas some non-linear interactions appear to be non-local in spectral space. We speculate that these non-local interactions are responsible for the correlation between turbulent transport and molecular dissipation. We argue that this correlation should then disappear when a significant scale separation is achieved, and we discuss several methods by which one can test this hypothesis.

Seismic diagnostics for transport of angular momentum in stars

Astronomy & Astrophysics, 2013

Context. Rotational splittings are currently measured for several main sequence stars and a large number of red giants with the space mission Kepler. This will provide stringent constraints on rotation profiles. Aims. Our aim is to obtain seismic constraints on the internal transport and surface loss of the angular momentum of oscillating solar-like stars. To this end, we study the evolution of rotational splittings from the pre-main sequence to the red-giant branch for stochastically excited oscillation modes. Methods. We modified the evolutionary code CESAM2K to take rotationally induced transport in radiative zones into account. Linear rotational splittings were computed for a sequence of 1.3 M models. Rotation profiles were derived from our evolutionary models and eigenfunctions from linear adiabatic oscillation calculations. Results. We find that transport by meridional circulation and shear turbulence yields far too high a core rotation rate for red-giant models compared with recent seismic observations. We discuss several uncertainties in the physical description of stars that could have an impact on the rotation profiles. For instance, we find that the Goldreich-Schubert-Fricke instability does not extract enough angular momentum from the core to account for the discrepancy. In contrast, an increase of the horizontal turbulent viscosity by 2 orders of magnitude is able to significantly decrease the central rotation rate on the red-giant branch. Conclusions. Our results indicate that it is possible that the prescription for the horizontal turbulent viscosity largely underestimates its actual value or else a mechanism not included in current stellar models of low mass stars is needed to slow down the rotation in the radiative core of red-giant stars.