Global axisymmetric dynamics of thin viscous accretion disks

Umurhan, O. M.; Nemirovsky, Amikam; Regev, Oded; Shaviv, G.

doi:10.1051/0004-6361:20053960

Cited by 31 publications

(44 citation statements)

References 41 publications

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“…We initially consider a viscous accretion α-disk surrounded by a rarefied corona threaded by the stellar magnetosphere. The Keplerian accretion disk is modeled after the polytropic solution presented in Kluźniak & Kita (2000, see also Regev & Gitelman 2002Umurhan et al 2006). The density, pressure, toroidal and accretion speed, and kinematic viscosity of the disk are given respectively by…”

Section: Initial and Boundary Conditionsmentioning

confidence: 99%

MHD simulations of accretion onto a dipolar magnetosphere

Zanni

Ferreira

2013

A&A

212

247

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Aims. This paper examines the outflows associated with the interaction of a stellar magnetosphere with an accretion disk. In particular, we investigate the magnetospheric ejections (MEs) due to the expansion and reconnection of the field lines connecting the star with the disk. Our aim is to study the dynamical properties of the outflows and evaluate their impact on the angular momentum evolution of young protostars. Methods. Our models are based on axisymmetric time-dependent magnetohydrodynamic simulations of the interaction of the dipolar magnetosphere of a rotating protostar with a viscous and resistive disk, using alpha prescriptions for the transport coefficients. Our simulations are designed to model the accretion process and the formation of accretion funnels, the periodic inflation/reconnection of the magnetosphere and the associated MEs, and the stellar wind.Results. Similar to a magnetic slingshot, MEs can be powered by the rotation of both the disk and the star so that they can efficiently remove angular momentum from both. Depending on the accretion rate, MEs can extract a relevant fraction of the accretion torque and, together with a weak but non-negligible stellar wind torque, can balance the spin-up due to accretion. When the disk truncation approaches the corotation radius, the system enters a "propeller" regime, where the torques exerted by the disk and the MEs can even balance the spin-up due to the stellar contraction. Conclusions. Magnetospheric ejections can play an important role in the stellar spin evolution. Their spin-down efficiency can be compared to other scenarios, such as the Ghosh & Lamb, X-wind, or stellar wind models. Nevertheless, for all scenarios, an efficient spin-down torque requires a rather strong dipolar component, which has seldom been observed in classical T Tauri stars. A better analysis of the torques acting on the protostar must consider non-axisymmetric and multipolar magnetic components consistent with observations.

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Section: Initial and Boundary Conditionsmentioning

confidence: 99%

MHD simulations of accretion onto a dipolar magnetosphere

Zanni

Ferreira

2013

A&A

212

247

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“…The accretion disk is set following the threedimensional models of α Keplerian accretion disks originally developed in Kluźniak & Kita (2000) and further discussed by Regev & Gitelman (2002) and Umurhan et al (2006). These are polytropic (P d ∝ ρ 2 ) of the equations of viscous hydrodynamics, where = C s /V K is the disk aspect ratio, given by the ratio between the isothermal sound speed C s = P d /ρ d and the Keplerian speed V K = √ GM /r evaluated on the midplane of the disk.…”

Section: Initial Conditionsmentioning

confidence: 99%

MHD simulations of accretion onto a dipolar magnetosphere

Zanni

Ferreira

2009

A&A

123

149

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Aims. We investigate the accretion process from an accretion disk onto a magnetized rotating star with a purely dipolar magnetic field. Our main aim is to study the mechanisms that regulate the stellar angular momentum. In this work, we consider two effects that can contrast with the spin-up torque normally associated with accretion: (1) the spin-down torque exerted by an extended magnetosphere connected to the disk beyond the corotation radius; (2) the spin-down torque determined by a stellar wind flowing along the opened magnetospheric field lines. Methods. Our study is based on time-dependent axisymmetric magnetohydrodynamic numerical simulations of the interaction between a viscous and resistive accretion disk with the dipolar magnetosphere of a rotating star. We present the first example of a numerical experiment able to model at the same time the formation of accretion curtains, the effects of an extended stellar magnetosphere and the launching of a stellar wind. Results. In the examples presented, the spin-down torque related to the star-disk interaction can extract only ∼10% of the accretion torque, due to the weakness of the extended connection. Not even the spin-down torque exerted by a stellar wind is strong enough (∼20%): despite a huge lever arm (R A ≈ 19 R ), the mass-loss rate (Ṁ wind ≈ 1%Ṁ acc ) is too low to provide an efficient torque. Conclusions. We argue that, at least in the case of typical classical T Tauri stars (Ṁ acc ≈ 10 −8 M yr −1 , B ,dipole < ∼ 1 kG) rotating at 10% of their break-up speed, the disk spin-down is unlikely to balance the accretion torque ("disk locked" equilibrium). A massive stellar wind (Ṁ wind ≈ 20%Ṁ acc ) could in principle succeed, but its mass and energy fluxes are quite demanding, both from a theoretical and an observational point of view.

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“…Keplerian rotating flows are (according to the Rayleigh and other criteria) linearly stable and thus astrophysical disk turbulence can not originate from a linear instability of the kind known (and well studied) in Taylor-Couette hydrodynamical flows. Several ideas on how purely hydrodynamical processes may account for anomalous angular momentum transport in accretion disks have been put forward [9,10,11,12,13,14,15,16,17,18,19,20], but they still remain quite controversial [21,22,23,24,25,26,27, 28] and we shall not deal with this problem here.…”

Section: Introductionmentioning

confidence: 99%

Nonlinear saturation of the magnetorotational instability near threshold in a thin-gap Taylor-Couette setup

2007

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We study the saturation near threshold of the axisymmetric magnetorotational instability (MRI) of a viscous, resistive, incompressible fluid in a thin-gap Taylor-Couette configuration. A vertical magnetic field, Keplerian shear and no-slip, conducting radial boundary conditions are adopted. The weakly non-linear theory leads to a real Ginzburg-Landau equation for the disturbance amplitude, like in our previous idealized analysis. For small magnetic Prandtl number (Pm ≪ 1), the saturation amplitude scales as P and Pm respectively) is attributed to the emergence of radial boundary layers. Away from those, steady-state non-linear saturation is achieved through a modest reduction in the destabilizing shear. These results will be useful to understand MRI laboratory experiments and associated numerical simulations.

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Global axisymmetric dynamics of thin viscous accretion disks

Cited by 31 publications

References 41 publications

MHD simulations of accretion onto a dipolar magnetosphere

MHD simulations of accretion onto a dipolar magnetosphere

MHD simulations of accretion onto a dipolar magnetosphere

Nonlinear saturation of the magnetorotational instability near threshold in a thin-gap Taylor-Couette setup

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