Toroidal angular momentum transport with non-axisymmetric magnetic fields

2016 ◽  
Vol 23 (5) ◽  
pp. 054504 ◽  
Author(s):  
J. Seol ◽  
B. H. Park
Keyword(s):  
Angular Momentum ◽  
Magnetic Fields ◽  
Momentum Transport ◽  
Angular Momentum Transport

scholarly journals Be Star Discs: Inflow or Outflow?

2000 ◽  
Vol 175 ◽  
pp. 617-620
Author(s):  
John M. Porter
Keyword(s):  
Angular Momentum ◽  
Magnetic Fields ◽  
Radiation Field ◽  
Accretion Discs ◽  
Momentum Transport ◽  
Angular Momentum Transport ◽  
Stellar Radiation ◽  
Be Star ◽  
Viscous Stresses

AbstractIt is assumed that the dynamics of Be star discs is dominated by the effects of viscous stresses. By examining angular momentum transport in discs, we show that many, if not all observed Be star discs should be accretion discs unless (i) the disc is acted upon by another agent (e.g. magnetic fields or the stellar radiation field), or (ii) the disc cools significantly as it flows outwards.

scholarly journals Galactic Dynamics and Magnetic Field Amplification

1993 ◽  
Vol 157 ◽  
pp. 395-401 ◽  
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.

scholarly journals Angular Momentum Transport and Mixing by Magnetic Fields

2004 ◽  
Vol 215 ◽  
pp. 356-365 ◽  
Author(s):  
H. C. Spruit
Keyword(s):  
Angular Momentum ◽  
Magnetic Fields ◽  
Internal Gravity Waves ◽  
Momentum Transport ◽  
Dynamo Model ◽  
Angular Momentum Transport ◽  
Field Amplification ◽  
Amplification Loop ◽  
Transport And Mixing ◽  
Rotating Star

Magnetic fields can be created in stably stratified (non-convective) layers in a differentially rotating star. A magnetic instability in the toroidal field (wound up by differential rotation) replaces the role of convection in closing the field amplification loop. A dynamo model is developed from these ingredients, and applied to the problem of angular momentum transport in stellar interiors. It produces a predominantly horizontal field. The process is found to be more effective in transporting angular momentum than the known hydrodynamic mechanisms, with the possible exception of transport by internal gravity waves.

scholarly journals Close binary evolution

2017 ◽  
Vol 609 ◽  
pp. A3 ◽  
Author(s):  
H. F. Song ◽  
G. Meynet ◽  
A. Maeder ◽  
S. Ekström ◽  
P. Eggenberger ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Binary System ◽  
Magnetic Fields ◽  
Close Binary System ◽  
Momentum Transport ◽  
Close Binary ◽  
Surface Magnetic Field ◽  
Angular Momentum Transport ◽  
Magnetic Braking

Context. Massive stars with solar metallicity lose important amounts of rotational angular momentum through their winds. When a magnetic field is present at the surface of a star, efficient angular momentum losses can still be achieved even when the mass-loss rate is very modest, at lower metallicities, or for lower-initial-mass stars. In a close binary system, the effect of wind magnetic braking also interacts with the influence of tides, resulting in a complex evolution of rotation. Aims. We study the interactions between the process of wind magnetic braking and tides in close binary systems. Methods. We discuss the evolution of a 10 M⊙ star in a close binary system with a 7 M⊙ companion using the Geneva stellar evolution code. The initial orbital period is 1.2 days. The 10 M⊙ star has a surface magnetic field of 1 kG. Various initial rotations are considered. We use two different approaches for the internal angular momentum transport. In one of them, angular momentum is transported by shear and meridional currents. In the other, a strong internal magnetic field imposes nearly perfect solid-body rotation. The evolution of the primary is computed until the first mass-transfer episode occurs. The cases of different values for the magnetic fields and for various orbital periods and mass ratios are briefly discussed. Results. We show that, independently of the initial rotation rate of the primary and the efficiency of the internal angular momentum transport, the surface rotation of the primary will converge, in a time that is short with respect to the main-sequence lifetime, towards a slowly evolving velocity that is different from the synchronization velocity. This “equilibrium angular velocity” is always inferior to the angular orbital velocity. In a given close binary system at this equilibrium stage, the difference between the spin and the orbital angular velocities becomes larger when the mass losses and/or the surface magnetic field increase. The treatment of the internal angular momentum transport has a strong impact on the evolutionary tracks in the Hertzsprung-Russell Diagram as well as on the changes of the surface abundances resulting from rotational mixing. Our modelling suggests that the presence of an undetected close companion might explain rapidly rotating stars with strong surface magnetic fields, having ages well above the magnetic braking timescale. Our models predict that the rotation of most stars of this type increases as a function of time, except for a first initial phase in spin-down systems. The measure of their surface abundances, together, when possible, with their mass-luminosity ratio, provide interesting constraints on the transport efficiencies of angular momentum and chemical species. Conclusions. Close binaries, when studied at phases predating any mass transfer, are key objects to probe the physics of rotation and magnetic fields in stars.

scholarly journals The angular momentum transport by unstable toroidal magnetic fields

2014 ◽  
Vol 573 ◽  
pp. A80 ◽  
Author(s):  
G. Rüdiger ◽  
M. Gellert ◽  
F. Spada ◽  
I. Tereshin
Keyword(s):  
Angular Momentum ◽  
Magnetic Fields ◽  
Momentum Transport ◽  
Angular Momentum Transport

scholarly journals Electric heating and angular momentum transport in laminar models of protoplanetary discs

2020 ◽  
Vol 494 (4) ◽  
pp. 6103-6119 ◽  
Author(s):  
William Béthune ◽  
Henrik Latter
Keyword(s):  
Angular Momentum ◽  
Magnetic Fields ◽  
Electric Heating ◽  
Central Star ◽  
Momentum Transport ◽  
Temperature Structure ◽  
Internal Source ◽  
Vertical Temperature ◽  
Order Unity ◽  
Angular Momentum Transport

ABSTRACT The vertical temperature structure of a protoplanetary disc bears on several processes relevant to planet formation, such as gas and dust grain chemistry, ice lines, and convection. The temperature profile is controlled by irradiation from the central star and by any internal source of heat such as might arise from gas accretion. We investigate the heat and angular momentum transport generated by the resistive dissipation of magnetic fields in laminar discs. We use local 1D simulations to obtain vertical temperature profiles for typical conditions in the inner disc (0.5–4 au). Using simple assumptions for the gas ionization and opacity, the heating and cooling rates are computed self-consistently in the framework of radiative non-ideal magnetohydrodynamics. We characterize steady solutions that are symmetric about the mid-plane and which may be associated with saturated Hall-shear unstable modes. We also examine the dissipation of electric currents driven by global accretion-ejection structures. In both cases we obtain significant heating for a sufficiently high opacity. Strong magnetic fields can induce an order-unity temperature increase in the disc mid-plane, a convectively unstable entropy profile, and a surface emissivity equivalent to a viscous heating of α ∼ 10−2. These results show how magnetic fields may drive efficient accretion and heating in weakly ionized discs where turbulence might be inefficient, at least for a range of radii and ages of the disc.

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