EDP Sciences
Free access
Issue
A&A
Volume 508, Number 3, December IV 2009
Page(s) 1117 - 1133
Section Astrophysical processes
DOI http://dx.doi.org/10.1051/0004-6361/200912879
Published online 04 November 2009
A&A 508, 1117-1133 (2009)
DOI: 10.1051/0004-6361/200912879

MHD simulations of accretion onto a dipolar magnetosphere

I. Accretion curtains and the disk-locking paradigm
C. Zanni1, 2 and J. Ferreira2

1  INAF – Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025, Pino Torinese, Italy
    e-mail: zanni@oato.inaf.it
2  Laboratoire d'Astrophysique de Grenoble, 414 rue de la Piscine, BP 53, 38041 Grenoble, France

Received 13 July 2009 / Accepted 30 September 2009

Abstract
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 ${\sim}10\%$ 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 ( ${\sim}20\%$): despite a huge lever arm ( $R_{\rm A} \approx 19~R_\star$), the mass-loss rate ( $\dot{M}_{\rm wind} \approx 1\%\,\dot{M}_{\rm acc}$) is too low to provide an efficient torque.
Conclusions. We argue that, at least in the case of typical classical T Tauri stars ( $\dot{M}_{\rm acc} \approx 10^{-8}~M_\odot~{\rm yr}^{-1}$, $B_{\star, {\rm dipole}} \la 1~{\rm kG}$) 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 ( $\dot{M}_{\rm wind} \approx 20\%\, \dot{M}_{\rm acc}$) could in principle succeed, but its mass and energy fluxes are quite demanding, both from a theoretical and an observational point of view.


Key words: stars: rotation -- stars: magnetic fields -- accretion, accretion disks -- ISM: jets and outflows -- methods: numerical -- magnetohydrodynamics (MHD)



© ESO 2009