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

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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Be Star Discs: Inflow or Outflow?

International Astronomical Union Colloquium ◽

10.1017/s0252921100056645 ◽

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.

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Testing viscous disc theory using the balance between stellar accretion and external photoevaporation of protoplanetary discs

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slaa110 ◽

2020 ◽

Vol 497 (1) ◽

pp. L40-L45

Author(s):

Andrew J Winter ◽

Megan Ansdell ◽

Thomas J Haworth ◽

J M Diederik Kruijssen

Keyword(s):

Angular Momentum ◽

Mass Loss ◽

Central Star ◽

Outer Edge ◽

Momentum Transport ◽

Angular Momentum Transport ◽

Accretion Rates ◽

Novel Method ◽

Viscous Models ◽

Protoplanetary Discs

ABSTRACT The nature and rate of (viscous) angular momentum transport in protoplanetary discs (PPDs) have important consequences for the formation process of planetary systems. While accretion rates on to the central star yield constraints on such transport in the inner regions of a PPD, empirical constraints on viscous spreading in the outer regions remain challenging to obtain. Here, we demonstrate a novel method to probe the angular momentum transport at the outer edge of the disc. This method applies to PPDs that have lost a significant fraction of their mass due to thermal winds driven by UV irradiation from a neighbouring OB star. We demonstrate that this external photoevaporation can explain the observed depletion of discs in the 3–5 Myr old σ Orionis region, and use our model to make predictions motivating future empirical investigations of disc winds. For populations of intermediate-age PPDs, in viscous models we show that the mass flux outwards due to angular momentum redistribution is balanced by the mass-loss in the photoevaporative wind. A comparison between wind mass-loss and stellar accretion rates therefore offers an independent constraint on viscous models in the outer regions of PPDs.

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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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Angular Momentum Transport and Mixing by Magnetic Fields

Symposium - International Astronomical Union ◽

10.1017/s0074180900195889 ◽

2004 ◽

Vol 215 ◽

pp. 356-365 ◽

Cited By ~ 1

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.

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Angular Momentum Transport and Magnetic Fields in the Solar Interior

Inside the Sun - Astrophysics and Space Science Library ◽

10.1007/978-94-009-0541-2_34 ◽

1990 ◽

pp. 415-423 ◽

Cited By ~ 6

Author(s):

H. C. Spruit

Keyword(s):

Angular Momentum ◽

Magnetic Fields ◽

Solar Interior ◽

Momentum Transport ◽

Angular Momentum Transport

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Close binary evolution

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731073 ◽

2017 ◽

Vol 609 ◽

pp. A3 ◽

Cited By ~ 9

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.

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Superrotation Maintained by Meridional Circulation and Waves in a Venus-Like AGCM

Journal of the Atmospheric Sciences ◽

10.1175/jas3859.1 ◽

2006 ◽

Vol 63 (12) ◽

pp. 3296-3314 ◽

Cited By ~ 48

Author(s):

Masaru Yamamoto ◽

Masaaki Takahashi

Keyword(s):

Angular Momentum ◽

General Circulation ◽

Meridional Circulation ◽

Circulation Model ◽

Shear Instability ◽

Momentum Transport ◽

Lower Atmosphere ◽

Cloud Base ◽

Temperature Structure ◽

Angular Momentum Transport

Fully developed superrotation—60 times faster than the planetary rotation (243 days)—is simulated using a Venus-like atmospheric general circulation model (AGCM). The angular momentum of the superrotation is pumped up by the meridional circulation with the help of waves, which accelerate the equatorial zonal flow. The waves generated by solar heating and shear instability play a crucial role in the atmospheric dynamics of the Venusian superrotation. Vertical and horizontal momentum transports of thermal tides maintain the equatorial superrotation in the middle atmosphere, while equatorward eddy momentum flux due to shear instability raises the efficiency of upward angular momentum transport by the meridional circulation in the lower atmosphere. In addition to the superrotation, some waves simulated in the cloud layer are consistent with the observations. The planetary-scale Kelvin wave identified as the near-infrared (NIR) oscillation with periods of 5–6 days is generated by the shear instability near the cloud base, and the temperature structure of the diurnal tide is similar to the infrared (IR) observation near the cloud top. Sensitivities to the bottom boundary conditions are also examined in this paper, since the surface physical processes are still unknown. The decrease of the equator–pole temperature difference and the increase of the surface frictional time constant result in the weaknesses of the meridional circulation and superrotation. In the cases of the weak superrotation, the vertical angular momentum transport due to the meridional circulation is inefficient and the equatorward eddy angular momentum transport is absent near 60-km altitude.

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