Magnetic fields in protoplanetary disks

Wardle, Mark

doi:10.1007/s10509-007-9575-8

Cited by 245 publications

(346 citation statements)

References 42 publications

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“…We calculate the Ohmic, Hall and ambipolar diffusivities (η O , η H , and η A , respectively) from the number densities of charged species obtained with our chemistry module (Wardle 2007) …”

Section: Magnetic Diffusivitiesmentioning

confidence: 99%

Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

Dzyurkevich

Commerçon

Lesaffre

et al. 2017

A&A

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Context. Both theory and observations of star-forming clouds require the simulations which combine the co-evolving chemistry, magneto-hydrodynamics and radiative transfer in protostellar collapse simulation. A detailed knowledge of self-consistent chemical evolution for the main charge carriers (both gas species and dust grains) allows to correctly estimate the rate and nature of magnetic dissipation in the collapsing core. Last is of crucial importance for answering the grand question of star and planet formation: the magnitude and spatial distribution of magnetic flux as the initial condition to protoplanetary disk evolution. Aims. We use a chemo-dynamical version of RAMSES, described in a companion publication, to follow the chemo-dynamical evolution of collapsing dense cores with the various dust properties and interpret the occuring differences in the magnetic diffusivity terms. Later are of crucial importance for the circumstellar disk formation. Methods. We perform 3D chemo-dynamical simulations of 1 M⊙ isolated dense core collapse for a range in the dust size assumptions. The number density of dust and it's mean size are affecting the efficiency of charge capturing and the formation of ices. The radiative hydrodynamics and dynamical evolution of chemical abundances are used to reconstruct the magnetic diffusivity terms for clouds with various magnetisation. Results. The simulations are performed for a mean dust size ranging from 0.017µm to 1µm, and we adopt both a fixed dust size and a dust size distribution. The chemical abundances for this range of dust sizes are produced by RAMSES and serve as an input to calculations of Ohmic, ambipolar and Hall diffusivity terms. Ohmic resistivity only play a role at the late stage of the collapse, in the innermost region of the cloud where gas density exceeds few times 10 13 cm −3 . Ambipolar diffusion is a dominant magnetic diffusivity term in cases where mean dust size is a typical ISM value or larger. We demonstrate that the assumption of a fixed 'dominant ion' mass can lead to one order of magnitude mismatch in the ambipolar diffusion magnitude. 'Negative' Hall effect is dominant during the collapse in case of small dust, i.e. for the mean dust size of 0.02 µm and smaller, the effect which we connect to the dominance of negatively charged grains. We find that the Hall effect reverses its sign for mean dust size of 0.1µm and smaller. The phenomenon of the sign reversal is strongly depending on the number of negatively charged dust relative to the ions, and quality of coupling of the last to the magnetic fields. We have adopted different strength of magnetic fields, β = Pgas/Pmag = 2, 5, 25. We observe that the variation on the field strength only shifts the Hall effect reversal along the radius of the collapsing cloud, but do not prevent it. Conclusions. The dust grain mean size appears to be the parameter with the strongest impact on the magnitude of the magnetic diffusivity, dividing the collapsing clouds in Hall-dominated and ambipolar-dominated cloud...

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Section: Magnetic Diffusivitiesmentioning

confidence: 99%

Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

Dzyurkevich

Commerçon

Lesaffre

et al. 2017

A&A

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“…Turbulence attenuation in the "dead" zones favours matter accumulation, gravitational instability and planet formation (e.g., Armitage 2010). There are three important non-ideal magnetohydrodynamical (MHD) effects operating in accretion disks: OD, MAD and Hall effect (e.g., Wardle 2007). Efficiency of MAD and Hall effect depends on magnetic field strength.…”

Section: Introductionmentioning

confidence: 99%

Fossil magnetic field of accretion disks of young stars

Dudorov

Khaibrakhmanov

2014

Astrophys Space Sci

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We elaborate the model of accretion disks of young stars with the fossil large-scale magnetic field in the frame of Shakura and Sunyaev approximation. Equations of the MHD model include Shakura and Sunyaev equations, induction equation and equations of ionization balance. Magnetic field is determined taking into account ohmic diffusion, magnetic ambipolar diffusion and buoyancy. Ionization fraction is calculated considering ionization by cosmic rays and X-rays, thermal ionization, radiative recombinations and recombinations on the dust grains.Analytical solution and numerical investigations show that the magnetic field is coupled to the gas in the case of radiative recombinations. Magnetic field is quasi-azimuthal close to accretion disk inner boundary and quasi-radial in the outer regions.Magnetic field is quasi-poloidal in the dusty "dead" zones with low ionization degree, where ohmic diffusion is efficient. Magnetic ambipolar diffusion reduces vertical magnetic field in 10 times comparing to the frozenin field in this region. Magnetic field is quasi-azimuthal close to the outer boundary of accretion disks for standard ionization rates and dust grain size a d = 0.1 µm. In the case of large dust grains (a d > 0.1 µm) or enhanced ionization rates, the magnetic field is quasiradial in the outer regions.It is shown that the inner boundary of dusty "dead" zone is placed at r = (0.1 − 0.6) AU for accretion disks of stars with M = (0.5 − 2) M ⊙ . Outer boundary of "dead" zone is placed at r = (3 − 21) AU and it is determined by magnetic ambipolar diffusion. Mass of solid material in the "dead" zone is more than 3 M ⊕ for stars with M ≥ 1 M ⊙ .

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“…We also often use the ambipolar diffusivity term, given by η A = η P − η O . When the charge carriers are ions and electrons, it can be shown that the following relations apply (Wardle 2007): η H = β e η O and η A = β i β e η O . Note also that η O , η A and η P are always positive (as η O is independent of B, whereas η A depends on B 2 ), but η H scales linearly with B and can, therefore, be either positive or negative depending on the field polarity (e.g.…”

Section: Magnetic Diffusivitymentioning

confidence: 99%

“…1, which shows the regions where the ambipolar, Hall and Ohmic diffusivity terms dominate in a Log B -Log n H plane (e.g. Wardle 2007), assuming that the charged particles are ions and electrons, in a weakly-ionised plasma. As expected, ambipolar diffusion is dominant at low densities and strong fields whereas the opposite is true for the Ohmic regime.…”

Section: Magnetic Diffusivitymentioning

confidence: 99%

“…This is most relevant at early stages of the accretion process, or when turbulent motions prevent the grains from settling towards the disc midplane. In fact, recent diffusivity calculations for a minimum-mass solar nebula disc at 1 AU (Wardle 2007;WS11) have shown that the magnetic coupling may be adequate over the entire disc cross-section, provided that dust grains are settled out of the gas phase.…”

Section: Magnetic Diffusivitymentioning

confidence: 99%

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Formation of stars and planets: the role of magnetic fields

Salmerón

2011

Astrophys Space Sci

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Star formation is thought to be triggered by gravitational collapse of the dense cores of molecular clouds. Angular momentum conservation during the collapse results in the progressive increase of the centrifugal force, which eventually halts the inflow of material and leads to the development of a central mass surrounded by a disc. In the presence of an angular momentum transport mechanism, mass accretion onto the central object proceeds through this disc, and it is believed that this is how stars typically gain most of their mass. However, the mechanisms responsible for this transport of angular momentum are not well understood. Although the gravitational field of a companion star or even gravitational instabilities (particularly in massive discs) may play a role, the most general mechanisms are turbulence viscosity driven by the magnetorotational instability (MRI), and outflows accelerated centrifugally from the surfaces of the disc. Both processes are powered by the action of magnetic fields and are, in turn, likely to strongly affect the structure, dynamics, evolutionary path and planet-forming capabilities of their host discs. The weak ionisation of protostellar discs, however, may prevent the magnetic field from effectively coupling to the gas and shear and driving these processes. Here I examine the viability and properties of these magnetically-driven processes in protostellar discs. The results indicate that, despite the weak ionisation, the magnetic field is able to couple to the gas and shear for fluid conditions thought to be satisfied over a wide range of radii in these discs.

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Magnetic fields in protoplanetary disks

Cited by 245 publications

References 42 publications

Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

Fossil magnetic field of accretion disks of young stars

Formation of stars and planets: the role of magnetic fields

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