M. Küker

Funnel Flows from Protoplanetary Disks

Klassische T Tauri-Sterne sind junge Sterne, die von Akkretionsscheiben umgeben sind. Besitzt ein solcher Stern ein hinreichend starkes Magnetfeld, so bestimmt dieses die Gasbewegung in der unmittelbaren Nachbarschaft des Sterns. Es kann dann zur Ausbildung sogenannter Funnel Flows kommen, in denen das Gas die Scheibenebene verlässt und entlang den Magnetfeldlinien auf die Polkappen des Sterns fliesst. Anhand von Simulationsrechnungen werden die Bedingungen für die Ausbildung der Funnel Flows sowie ihr Einfluss auf die zeitliche Entwicklung der Sternrotation untersucht.

Classical T Tauri Stars are young stellar objects surrounded by accretion discs. Some of these stars have been observed to be magnetically active. A large-scale stellar magnetic field can cause the disruption of the disc close to the star and the launching of outflows from the system. In case of a disrupted disc the accretion flow is lifted out of the disc plane and directed towards the polar caps of the star. It then hits the stellar surface at high latitudes, causing a bright ring.

To study the interaction between the disc and the star we have carried out numerical simulations. The setup is axisymmetric and contains the star, the disc, the halo above the disc where the density is low but finite, and the stellar magnetic field. Initially, the stellar magnetic field is a pure dipole that threads the disc, which is truncated at the corotation radius. As the system evolves, the magnetic field is wound up by the rotational shear between the star and the disc. The inner edge of the disc moves inwards while farther away from the star gas is driven away from the star and the field lines break up, leaving the outer parts of the disc disconnected from the star.

After the initial phase a state is reached where the system switches between two configurations. In one state the disc extends down to the stellar equator. The torque on the star is dominated by the accretion torque and the star is spun up. In the other state the accretion flow is lifted out of the disc plane and forms a funnel flow towards the polar caps of the star. The poloidal field is compressed and forms a magnetosphere of closed loops that are not loaded with gas from the disc. The funnel flow is essentially torqueless, with both the magnetic and the accretion torques much smaller than in the disc accretion state. In the funnel flow state the location of the inner edge of the disc is determined by the equilibrium between the magnetic pressure of the poloidal field and the gas pressure in the disc. As the funnel flow is an intermittent state and the star is spun up in the phases of (undisrupted) disc accretion the net effect on the stellar rotation is an acceleration of the latter.


Density distribution and the poloidal magnetic field in the immediate vicinity of the star in the funnel flow state. The colour contour plots show the density distribution, the white lines the poloidal magnetic field.