Angular Momentum Transport and Dynamo Effect in Kepler Disks

Properties have been demonstrated of the magneto-rotational instability for two different applications, i.e. for a global spherical model and a box simulation with Keplerian background shear flow. In both nonlinear cases a dynamo operates with a negative (positive) α-effect in the northern (southern) disk hemisphere and in both cases the angular momentum transport is outwards. Keplerian accretion disks should therefore exhibit large-scale magnetic fields with a dipolar geometry of the poloidal components favoring jet formation.

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Turbulence, Vorticity Generation and Angular Momentum Transport via the Baroclinic Instability in Accretion Disks

Symposium - International Astronomical Union ◽

10.1017/s0074180900218226 ◽

2004 ◽

Vol 202 ◽

pp. 350-352

Author(s):

Hubert Klahr ◽

Peter Bodenheimer

Keyword(s):

Angular Momentum ◽

Accretion Disks ◽

Baroclinic Instability ◽

Three Dimensional ◽

Gravitational Energy ◽

Momentum Transport ◽

Angular Momentum Transport ◽

Accretion Rates ◽

Background Shear ◽

2D And 3D

We propose the global baroclinic instability as a source for vigorous turbulence leading to angular momentum transport in Keplerian accretion disks. We know from analytical considerations and three-dimensional radiation hydro simulations that, in particular, protoplanetary disks have a negative radial entropy gradient, which makes them baroclinic. Two-dimensional numerical simulations show that this baroclinic flow is unstable and produces turbulence. These findings were tested for numerical effects by performing barotropic simulations which show that imposed turbulence rapidly decays. The turbulence in baroclinic disks draws energy from the background shear, transports angular momentum outward and creates a radially inward bound accretion of matter, thus forming a self consistent process. Gravitational energy is transformed into turbulent kinetic energy, which is then dissipated, as in the classical accretion paradigm. We measure accretion rates in 2D and 3D simulations of Ṁ = −;10−9 to −10−7 M⊙ yr−1 and viscosity parameters of α = 10−4–10−2, which fit perfectly together and agree reasonably with observations. The turbulence creates pressure waves, Rossby waves, and vortices in the (R – ø) plane of the disk. We demonstrate in a global simulation that these vortices tend to form out of little background noise and to be long-lasting features, which have already been suggested to lead to the formation of planets.

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Turbulent and wind-driven accretion in dwarf novae threaded by a large-scale magnetic field

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833921 ◽

2018 ◽

Vol 620 ◽

pp. A49 ◽

Cited By ~ 8

Author(s):

N. Scepi ◽

G. Lesur ◽

G. Dubus ◽

M. Flock

Keyword(s):

Magnetic Field ◽

Angular Momentum ◽

Large Scale ◽

Turbulent Transport ◽

Momentum Transport ◽

Rotational Instability ◽

Quiescent State ◽

Dwarf Novae ◽

Angular Momentum Transport ◽

Viscous Instability

Dwarf novae (DNe) are accreting white dwarfs that show eruptions caused by a thermal-viscous instability in the accretion disk. The outburst timescales constrain α, the ratio of the viscous stress to the thermal pressure, which phenomenologically connects to the mechanism of angular momentum transport. The eruptive state has α ≈ 0.1 while the quiescent state has α ≈ 0.03. Turbulent transport that is due to the magneto-rotational instability (MRI) is generally considered to be the source of angular momentum transport in DNe. The presence of a large-scale poloidal field threading the disk is known to enhance MRI-driven transport. Here, we perform 3D local magnetohydrodynamic (MHD) shearing-box simulations including vertical stratification, radiative transfer, and a net constant vertical magnetic flux to investigate how transport changes between the outburst and quiescent states of DNe. We find that a net vertical constant magnetic field, as could be provided by the white dwarf or by its stellar companion, provides a higher α in quiescence than in outburst, in opposition to what is expected. Including resistivity quenches MRI turbulence in quiescence, suppressing transport, unless the magnetic field is high enough, which again leads to α ≈ 0.1. A major difference between simulations with a net poloidal flux and simulations without a net flux is that angular momentum transport in the former is shared between turbulent radial transport and wind-driven vertical transport. We find that wind-driven transport dominates in quiescence even for moderately low magnetic fields ∼1 G. This can have a great impact on observational signatures since wind-driven transport does not heat the disk. Furthermore, wind transport cannot be reduced to an α prescription. We provide fits to the dependence of α with β, the ratio of thermal to magnetic pressure, and Teff, the effective temperature of the disk, as well as a prescription for the wind torque as a function of β that is in agreement with both local and global simulations. We conclude that the evolution of the thermal-viscous instability, and its consequences on the outburst cycles of CVs, needs to be thoroughly revised to take into account that most of the accretion energy may be carried away by a wind instead of being locally dissipated.

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Magnetic field transport in compact binaries

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037903 ◽

2020 ◽

Vol 641 ◽

pp. A133

Author(s):

N. Scepi ◽

G. Lesur ◽

G. Dubus ◽

J. Jacquemin-Ide

Keyword(s):

Magnetic Field ◽

Angular Momentum ◽

Magnetic Flux ◽

Large Scale ◽

Compact Object ◽

Light Curves ◽

Momentum Transport ◽

Local Magnetization ◽

Angular Momentum Transport ◽

The Impact

Context. Dwarf novæ (DNe) and low mass X-ray binaries (LMXBs) show eruptions that are thought to be due to a thermal-viscous instability in their accretion disk. These eruptions provide constraints on angular momentum transport mechanisms. Aims. We explore the idea that angular momentum transport could be controlled by the dynamical evolution of the large-scale magnetic field. We study the impact of different prescriptions for the magnetic field evolution on the dynamics of the disk. This is a first step in confronting the theory of magnetic field transport with observations. Methods. We developed a version of the disk instability model that evolves the density, the temperature, and the large-scale vertical magnetic flux simultaneously. We took into account the accretion driven by turbulence or by a magnetized outflow with prescriptions taken, respectively, from shearing box simulations or self-similar solutions of magnetized outflows. To evolve the magnetic flux, we used a toy model with physically motivated prescriptions that depend mainly on the local magnetization β, where β is the ratio of thermal pressure to magnetic pressure. Results. We find that allowing magnetic flux to be advected inwards provides the best agreement with DNe light curves. This leads to a hybrid configuration with an inner magnetized disk, driven by angular momentum losses to an MHD outflow, sharply transiting to an outer weakly-magnetized turbulent disk where the eruptions are triggered. The dynamical impact is equivalent to truncating a viscous disk so that it does not extend down to the compact object, with the truncation radius dependent on the magnetic flux and evolving as Ṁ−2/3. Conclusions. Models of DNe and LMXB light curves typically require the outer, viscous disk to be truncated in order to match the observations. There is no generic explanation for this truncation. We propose that it is a natural outcome of the presence of large-scale magnetic fields in both DNe and LMXBs, with the magnetic flux accumulating towards the center to produce a magnetized disk with a fast accretion timescale.

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Angular Momentum Transport in Accretion Disks

Cataclysmic Variables - Astrophysics and Space Science Library ◽

10.1007/978-94-011-0335-0_98 ◽

1995 ◽

pp. 349-358

Author(s):

Mario Livio

Keyword(s):

Angular Momentum ◽

Accretion Disks ◽

Momentum Transport ◽

Angular Momentum Transport

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Simulation of Mass Transport in Disk Galaxies

Symposium - International Astronomical Union ◽

10.1017/s007418090023324x ◽

1996 ◽

Vol 171 ◽

pp. 405-405 ◽

Cited By ~ 1

Author(s):

S. von Linden ◽

J. Heidt ◽

H.P. Reuter ◽

R. Wielebinski

Keyword(s):

Angular Momentum ◽

Mass Transport ◽

Large Scale ◽

Momentum Transport ◽

Disk Galaxies ◽

Angular Momentum Transport ◽

Gravitational Instabilities ◽

Gas Component

The large-scale dynamics and evolution of disk galaxies is controlled by the angular-momentum transport provided by non-axisymmetric perturbances through their gravity torques. To continuously maintain such gravitational instabilities, the presence of the gas component and its dissipative character are essential.

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Angular momentum transport in accretion disks: a hydrodynamical perspective

EAS Publications Series ◽

10.1051/eas/1982035 ◽

2019 ◽

Vol 82 ◽

pp. 391-413 ◽

Cited By ~ 5

Author(s):

S. Fromang ◽

G. Lesur

Keyword(s):

Angular Momentum ◽

Accretion Disk ◽

Accretion Disks ◽

Dynamical Evolution ◽

Theoretical Work ◽

Momentum Transport ◽

Angular Momentum Transport ◽

Recent Developments ◽

The Many ◽

The Universe

The radial transport of angular momentum in accretion disk is a fundamental process in the universe. It governs the dynamical evolution of accretion disks and has implications for various issues ranging from the formation of planets to the growth of supermassive black holes. While the importance of magnetic fields for this problem has long been demonstrated, the existence of a source of transport solely hydrodynamical in nature has proven more difficult to establish and to quantify. In recent years, a combination of results coming from experiments, theoretical work and numerical simulations has dramatically improved our understanding of hydrodynamically mediated angular momentum transport in accretion disk. Here, based on these recent developments, we review the hydrodynamical processes that might contribute to transporting angular momentum radially in accretion disks and highlight the many questions that are still to be answered.

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