Simulation of Mass Transport in Disk Galaxies

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 and Dynamo Effect in Kepler Disks

Symposium - International Astronomical Union ◽

10.1017/s0074180900225473 ◽

2001 ◽

Vol 200 ◽

pp. 410-414

Author(s):

Günther Rüdiger ◽

Udo Ziegler

Keyword(s):

Angular Momentum ◽

Accretion Disks ◽

Large Scale ◽

Spherical Model ◽

Momentum Transport ◽

Rotational Instability ◽

Jet Formation ◽

Angular Momentum Transport ◽

Dynamo Effect ◽

Background Shear

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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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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Galactic Dynamics and Magnetic Field Amplification

Symposium - International Astronomical Union ◽

10.1017/s0074180900174492 ◽

1993 ◽

Vol 157 ◽

pp. 395-401 ◽

Cited By ~ 1

Author(s):

Harald Lesch

Keyword(s):

Magnetic Field ◽

Angular Momentum ◽

Magnetic Fields ◽

Time Scale ◽

Numerical Models ◽

Momentum Transport ◽

Angular Momentum Transport ◽

Field Amplification ◽

Gravitational Instabilities ◽

Magnetic Diffusivity

Stimulated by recent high frequency radio polarization measurements of M83 and M51, we consider the influence of non-axisymmetric features (bars, spiral arms, etc…) on galactic magnetic fields. The time scale for the field amplification due to the non-axisymmetric velocity field is related to the time scale of angular momentum transport in the disk by the non-axisymmetric features. Due to its dissipational character (cooling and angular momentum transport) the gas plays a major role for the excitation of non-axisymmetric instabilities. Since it is the gaseous component of the interstellar gas in which magnetic field amplification takes place we consider the interplay of gasdynamical processes triggered by gravitational instabilities and magnetic fields. A comparison with the time scale for dynamo action in a disk from numerical models for disk dynamos gives the result that field amplification by non-axisymmetric features is faster in galaxies like M83 (strong bar) and M51 (compagnion and very distinct spiral structure), than amplification by an axisymmetric dynamo. Furthermore, we propose that axisymmetric gravitational instabilities may provide the turbulent magnetic diffusivity ηT. Based on standard galaxy models we obtain a radially dependent diffusivity whose numerical value rises from 1025cm2s−1 to 1027cm2s−1, declining for large radii.

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Dynamical effects of cometary bombardment of Saturn’s rings and moons

International Astronomical Union Colloquium ◽

10.1017/s0252921100083901 ◽

1985 ◽

Vol 83 ◽

pp. 181-182

Author(s):

Jack J. Lissauer

Keyword(s):

Angular Momentum ◽

Mass Transport ◽

Momentum Transport ◽

Rear Window ◽

Small Bodies ◽

Angular Momentum Transport ◽

Saturn’S Rings ◽

Ring Particle ◽

Physical Changes ◽

Saturn's Rings

Extended AbstractSaturn’s ring particles and airless moons are exposed to a large flux of interplanetary debris, principally comets and comet dust. Collisions with this debris are responsible for both physical and dynamical changes in objects orbiting about Saturn. Physical changes include cratering of large bodies and catastrophic disruption of small bodies. Dynamical changes, which are analyzed in this paper, include orbital alteration (principally of ring particles) and changes in spin state (which are only important for moons, as ring particle spins are continually altered by mutual collisions).Saturn’s rings are rapidly being eroded by impacts of hypervelocity meteoroids in cometary orbits. Ejecta from these impacts will, in most cases, remain in orbit about Saturn and eventually be reaccreted by the rings, possibly at a different radial location. The resulting mass transport has been suggested as the cause of some of the features observed in Saturn’s rings (see Durisen 1984 for a review). Previous attempts to model this transport have used numerical simulations which have not included effects of angular momentum transport coincident with this mass transport. I have developed an analytic model for ballistic mass transport in Saturn’s rings. The model includes the effects of angular momentum advection and shows that the net material movement due to the combined effects of mass and angular momentum transport is roughly half that calculated when angular momentum advection is ignored. See Lissauer (1984) for further details.All of Saturn’s mid-sized moons are rotating synchronously with their orbital period; thus, the same hemisphere of these moons always faces the planet, and the same point is always at the center of the satellite’s leading hemisphere (the apex). The satellites orbit Saturn with velocities ranging from 14 km/sec for Mimas to 8.5 km/sec for Rhea and 3.3 km/sec for Iapetus. These speeds are a significant fraction of the encounter velocities between comets and the Saturn system (~ 10−25 km/sec); thus, due to a type of “windshield effect” (more raindrops hit the windshield of a moving car than hit the rear window), more comets will collide with the moons’ leading hemispheres than with their trailing hemispheres; also, higher relative velocities between comets and the moons will lead to larger craters for impacts by comets of a given mass on the leading hemispheres. The combination of these effects suggests that regions of the satellites’ surfaces near the apex should be much more heavily cratered than regions near the antapex (Shoemaker and Wolfe 1982). Such a major cratering asymmetry has not been observed (Plescia and Boyce 1983). A similar situation exists for Jupiter’s moons Ganymede and Callisto (Passey and Shoemaker 1982).McKinnon (1981) suggested that stochastic reorientation of the moons by impact-induced “spinup” during the establishment of the cratering record tended to equilibrate the crater densities between hemispheres. I have re-examined the dynamics of this problem, and I conclude that most impacts large enough to have caused “spinup” would have catastrophically disrupted the moons in question (Lissauer 1985).

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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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