Magnetohydrodynamic Simulations of Disk–Magnetized Star Interactions in the Quiescent Regime: Funnel Flows and Angular Momentum Transport

Abstract

Magnetohydrodynamic (MHD) simulations have been used to study disk accretion to a rotating magnetized star with an aligned dipole moment. Quiescent initial conditions were developed in order to avoid the fast initial evolution seen in earlier studies. A set of simulations was performed for different stellar magnetic moments and rotation rates. Simulations have shown that the disk structure is significantly changed inside a radius r br where magnetic braking is significant. In this region the disk is strongly inhomogeneous. Radial accretion of matter slows as it approaches the area of strong magnetic field and a dense ring and funnel flow form at the magnetospheric radius r m where the magnetic pressure is equal to the total, kinetic plus thermal, pressure of the matter.Funnel flows (FF), where the disk matter moves away from the disk plane and flows along the stellar magnetic field, are found to be stable features during many rotations of the disk. The dominant force driving matter into the FF is the pressure gradient force, while gravitational force accelerates it as it approaches the star. The magnetic force is much smaller than the other forces. The funnel flow is found to be strongly sub-Alfvénic everywhere. The FF is subsonic close to the disk, but it becomes supersonic well above the disk. Matter reaches the star with a velocity close to that of free-fall.Angular momentum is transported to the star dominantly by the magnetic field. In the disk the transport of angular momentum is mainly by the matter, but closer to the star the matter transfers its angular momentum to the magnetic field and the magnetic field is dominant in transporting angular momentum to the surface of the star. For slowly rotating stars we observed that magnetic braking leads to the deceleration of the inner regions of the disk and the star spins up. For a rapidly rotating star, the inner regions of the disk rotate with a super-Keplerian velocity, and the star spins-down. The average torque is found to be zero when the corotation radius r cor ≈ 1.5r m .The evolution of the magnetic field in the corona of the disk depends on the ratio of magnetic to matter energies in the corona and in the disk. Most of the simulations were performed in the regime of a relatively dense corona where the matter energy density was larger than the magnetic energy density. In this case the coronal magnetic field gradually opens but the velocity and density of outflowing matter are small. In a test case where a significant part of the corona was in the field dominated regime, more dramatic opening of the magnetic field was observed with the formation of magneto-centrifugally driven outflows.Numerical applications of our simulation results are made to T Tauri stars. We conclude that our quasi-stationary simulations correspond to the classical T Tauri stage of evolution. Our results are also relevant to cataclysmic variables and magnetized neutron stars in X-ray binaries.

Magnetically Channeled Accretion in T Tauri Stars

“…Aly & Kuijpers 1990;Goodson et al 1997;Goodson & Winglee 1999). Magnetospheric inflation cycles are thus expected to develop and to be accompanied by violent episodic outflows as field lines open and reconnect as well as time dependent accretion rate onto the star (Hayashi et al 1996;Romanova et al 2002). The most recent 3D MHD simulations of disk accretion onto an inclined stellar magnetosphere are presented in Romanova et al (2003) and illustrate well the extreme complexity of the process.…”

Accretion dynamics in the classical T Tauri star V2129 Ophiuchi

“…characteristics of CTTSs (Shu et al 1994;Hartmann et al 1994;Muzerolle et al 2001;Kurosawa et al 2006;Lima et al 2010) and are currently the most accepted ways to describe these systems. In the past decade, magnetohydrodynamic (MHD) simulations have followed the rotational evolution of CTTSs over as many as hundreds of rotational periods, presenting a dynamical view of the star-disk interaction (Goodson et al 1999;Romanova et al 2002;Long et al 2008;Zanni & Ferreira 2009) that can be tested with synoptic observations. Accretion has a long-lasting role in early stellar evolution, by providing mass and helping to regulate the angular momentum transfer from the star to the disk.…”

Accretion disk coronae of intermediate polar cataclysmic variables

“…In this model α can be assumed to be a measure of the strength of the turbulent angular momentum transport and varies in the range 0.01 − 0.6 (Balbus 2003). In the simulations here presented, we assumed constant α = 0.02 (Romanova et al 2002). Thermal conduction appears in the energy Eq.…”

“…black holes). In particular for CVs, there are no remarkable differences in the properties of extended coronae observed from magnetic (such as IPCVs) and non-magnetic CVs, since the main features differentiating these two types of CVs are: the presence of circular polarization of the emission due to the interaction of accreting charged material with magnetic field; a spin period lower than orbital period P spin P orb ; He II emission lines due to ionization of the accretion columns by the EUV continuum of the radial accretion shock (Szkody et al 2003); and the properties of flickering (Fritz & Bruch 1998), whose origin may be mainly related to accretion, that in magnetized objects is thought to occur via accretion columns or curtains (Romanova et al 2002(Romanova et al , 2003.…”