Outflows and accretion in a star-disc system with stellar magnetosphere and disc dynamo

Abstract

Abstract.The interaction between a protostellar magnetosphere and a surrounding dynamo-active accretion disc is investigated using an axisymmetric mean-field model. In all models investigated, the dynamo-generated magnetic field in the disc arranges itself such that in the corona, the field threading the disc is anti-aligned with the central dipole so that no X-point forms. When the magnetospheric field is strong enough (stellar surface field strength around 2 kG or larger), accretion happens in a recurrent fashion with periods of around 15 to 30 days, which is somewhat longer than the stellar rotation period of around 10 days. In the case of a stellar surface field strength of at least a few 100 G, the star is being spun up by the magnetic torque exerted on the star. The stellar accretion rates are always reduced by the presence of a magnetosphere which tends to divert a much larger fraction of the disc material into the wind. Both, a pressure-driven stellar wind and a disc wind form. In all our models with disc dynamo, the disc wind is structured and driven by magneto-centrifugal as well as pressure forces.

The role of magnetic reconnection on jet/accretion disk systems

“…We note that previous numerical studies of star-disk interactions have shown that differential rotation between the star and the inner disk regions where the stellar magnetosphere is anchored may lead to field lines opening and reconnection, which eventually restores the initial magnetospheric configuration (e.g., Goodson & Winglee 1999;Matt et al 2002;Uzdensky et al 2002;Romanova et al 2004;von Rekowski & Brandenburg 2004;Zanni 2009; see also Alencar 2007, for a review). Magnetospheric reconnection cycles are expected by most numerical models to develop in a few Keplerian periods at the inner disk, and be accompanied by time dependent accretion onto the star and by episodic outflow events as reconnection takes place.…”

Magnetic Fields of Nondegenerate Stars

“…Several theoretical models and numerical simulations have been proposed and carried out to study this complex magnetospheric interaction and attempt to reproduce observations, in particular the typical sizes of magnetospheric gaps and rotation periods of cTTSs. Initially restricted to dipolar magnetospheres (Romanova et al 2002;von Rekowski & Brandenburg 2004; the disc to the star and of accretion spots at funnel footpoints, whose locations and geometries are found to depend strongly on the inclination of the magnetosphere with respect to the rotation axis of both the disc and the protostar (e.g., Romanova et al 2003Romanova et al , 2004 as well as on the large-scale topology of the field (e.g., Gregory et al 2006;Long et al 2008). The size of the magnetospheric gap is however still difficult to reconcile with observed large-scale field strengths Bessolaz et al 2008;Gregory et al 2008) and simulations fail to confirm that the magnetic torque from the accretion disc is strong enough to spin the star down as observations suggest (e.g., Bessolaz et al 2008); a braking contribution from an accretion-powered magnetised stellar wind may help solving the problem (Matt & Pudritz 2005).…”

Wobbling and Precessing Jets From Warped Disks in Binary Systems

“…Following this -meanwhile -standard approach, further physical effects were investigated, such as the influence of the disk magnetization (Tzeferacos et al 2009), the launching from viscous disks (Murphy et al 2010), thermal effects (Tzeferacos et al 2013), or even the launching of outflows from a magnetic field, selfgenerated by a mean-field disk dynamo (von Rekowski & Brandenburg 2004;von Rekowski et al 2003;.…”