Magnetohydrodynamics Of Accretion To Rotating Magnetized Stars

Abstract

Many astrophysical systems consist of a central object, an accretion disc and a large scale magnetic field. Understanding their interaction, in the case where the central object is a young star, is important for undertanding stellar evolution and planet formation. The key process in this system is accretion - matter in the disc falls into the potential well of the central object, providing energy to drive the system. The key problem is understanding how angular momentum is transported in the disc, allowing matter to accrete onto the star. We investigate several astrophysical scenarios to better understand angular momentum transport in accretion discs. One scenario consists of a large scale "looped" magnetic field threading the disc - this magnetic field advects matter towards the star by driving angular momentum outwards along the field lines in the form of matter and Poynting jets. This enhances accretion rates at the surface of the star by a factor of a few. Another scenario consists of feeding matter that is counter rotating with respect to the main part of the disc, as might happen if it were sourced by a nearby counter-rotating binary or molecular cloud. A shear layer forms at the interface of this rotating/counter-rotating matter, which depends sensitively on the viscous properties of the fluid. Accretion rates can be enhanced by factors of 10 [-] 104 . Observations suggest that jets and winds are asymmetric about the disc plane. We investigate possible magnetic field configurations which exhibit these asymmetric outflows by considering combinations of stellar dipole fields and an aligned or misaligned disc field. We find that the inclusion of a dipole field or a strong enough disc field produces asymmetric outflows at late times despite initial conditions being symmetric. We explore one possible mechanism for generating the large scale magnetic field required by these systems to produce outflows, the [alpha]-dynamo. We find that an [alpha]-dynamo operating in the disc produces primarily dipole magnetic fields around the star. Seed fields initially too weak to launch outflows can grow sufficiently to launch outflows with densities consistent with observations.