N ov 2 00 8 Magnetohydrodynamic jets from different magnetic field configurations

Using axisymmetric MHD simulations we investigate how the o v rall jet formation is affected by a variation in the disk magnetic flux profile and/or the existence of a central stellar magnetosphere. Our simulati ons evolve from an initial, hydrostatic equilibrium state in a force-free magnetic fiel d configuration. We find a unique relation between the collimation degree and the dis k wind magnetization power law exponent. The collimation degree decreases for st eeper disk magnetic field profiles. Highly collimated outflows resulting from a fla t profile tend to be unsteady. We further consider a magnetic field superposed of a stellar dipole and a disk field in parallel or anti-parallel alignment. Both ste llar and disk wind may evolve in a pair of outflows, however, a reasonably strong dis k wind component is essential for jet collimation. Strong flares may lead to a sud den change in mass flux by a factor two. We hypothesize that such flares may eventuall y trigger jet knots. 1 Jets as collimated MHD flows Astrophysical jets are launched by magnetohydrodynamic (M HD) processes in the close vicinity of the central object – an accretion disk surr o nding a protostar or a compact object [1,2, 9, 20, 21, 24]. Numerical simulations o f MHD jet formation are essential for our understanding of the physical process es involved. In general, simulations may be distinguished in those taking into accou nt the evolution of the disk structure and others considering the disk surface as a fi xed-in-time boundary condition for the jet. The first approach allows to directly i nvestigate the mechanism lifting matter from the disk into the outflow [3, 10, 11, 1 4, 17, 18, 22, 24] This approach is computationally expensive and still somewhat l imi ed by spatial and time resolution. In order to study the acceleration and coll imation of a disk/stellar Christian Fendt Max Planck Institute for Astronomy, Königstuhl 17, D-6911 7 Heidelberg, e-mail: fendt@mpia.de