Scientific interests

Magnetohydrodynamic (MHD) structure of astrophysical jets Astrophysical jets are defined as highly collimated beams of matter moving with high velocity . It is now generally accepted that magnetic fields are responsible for both acceleration and collimation of the flow. As a general property observed in astrophysical jet sources, there is the additional signature for the existence of an accretion disk. This holds for jets sources of all different scales of energy or spatial dimension: Active galactic nuclei, compact stars, and protostellar objects. Applying numerical simulations we calculate the time-dependent structure of the jet magnetosphere and the dynamics of the plasma dynamics in the collimation region of magnetised jets. Below you see example results of time-dependent MHD jet formation simulations from a Keplerian disk using the ZEUS-3D code and the PLUTO code. These simulations follow the general framework of Blandford & Payne (1982) and Ouyed & Pudritz (1997). For details we refer to our published papers. Model scenario of a magnetised young star-disk system (Fendt 1994, Fendt & Camenzind 1995). MHD jet formation simulations Using the ZEUS-3D code in the axisymmetry option we calculated the jet formation from an accretion disk into a hydrostatic disk corona. The model of Ouyed and Pudritz (1997) is applied and used as reference for further simulations, e.g. of dipolar-type magnetic fields interacting with an accretion disk (below). The jet runs from left (accretion disk) to the right (hydrostatic corona). Shown are density (colors) and poloidal field lines (black). The jet ACCELERATES and COLLIMATES. Movie of the jet simulation (~1MB) MHD simulations: dipolar magnetosphere Using the ZEUS 3D code in the axisymmetry option the evolution of a stellar dipolar-type magnetic field interacting with an accretion disk is calculated. The boundary condition is an inflow from a Keplerian disk (as in Ouyed and Pudritz 1997). In the movies the accretion disk is at the lower boundary. The initial magnetic field is locked in a rigidly rotating stellar surface and the disk. The size of the domain is 20x20 inner disk radii. Shown are density (colors) and poloidal field lines (black). In the first simulation, the star is at rest. 100 Keplerian periods of the inner disk are calculated. A bubble forms disrupting the dipole. A disk wind accelerates and slowly collimates, indicating a possible final stationary state (Fendt & Elstner, 1999, A&A 349, L61, pdf-file ). Dipolar field, star at rest (~2MB) In the second example the star rotates with a corotation radius at the inner disk radius. The initial field structure is a force-free dipole quenched along the equatorial plane. A two-component outflow (disk wind and stellar wind) is formed which is uncollimated. More than 2500 Keplerian periods of the inner disk are calculated in order to obtain a quasi-stationary final state (run S2). A larger stellar wind mass flow rate stabilizes the flow along the axis (run L5) (see Fendt & Elstner, 2000, A&A 363, 208 pdf-file). The following gif-animations show the long-term evolution of the flow (use xanim). Small frames, long run (~11MB) Small frames, final time steps (~3MB) Large frames, final time steps (~9MB) Medium size frames, long run (~25MB) Large frames, long run (~35MB) MHD simulations: magnetic diffusivity and jet collimation In another paper we investigated the relation between the magnitude of jet magnetic diffusivity and the degree of jet collimation. (see Fendt & Cemeljic, 2002, A&A 395, 1045, pdf-file). Diffusive jets are generally less collimated and there seems to be a critical value for the diffusivity (eta) above which the "jet" remains uncollimated. The jet velocities become faster with increasing diffusivity but the bow shock propagates slower (see density contours below with normalized eta = 0, 0.01, 0.1) MHD simulations: disk magnetization profile and jet collimation This paper investigates the interrelation between the profile of the accretion disk magnetic field profile (and the disk wind density profile) and the degree of jet collimation. (see Fendt, 2006, ApJ 651, 272, pdf-file). Disk density profile and magnetic flux profile are prescribed as power law profiles. Both variables can be unifoed in the disk magnetisation profile. We find a unique relation between the magnetization profile power law index and the jet degeree of collimation (measured by the ratio of axial to lateral mass fluxes). Flat magnetic field / magnetization profiles collimate the outflow to a higher degree. Steep profiles such as a dipolar-like field distribution or a so-called X-wind do not produce highly collimated jets. MHD simulations: disk- jet magnetospheres and jet collimation This paper investigates time-dependent evolution of a superposed disk magnetic field with a stellar dipolar magnetosphere. (see Fendt, 2009, 692, 346 pdf-file). Field distributions with aligned and anti-aligned magnetic axis of disk and stellar field are investigated. We use our version of ZEUS-3D extended for physical magnetic diffusivity, thus being able to follow reconnection events. The X-point of where anit-aligned disk field and stellar field meet along the disk surface gives rise to strong reconnection events with strong flares emerging across the jet magnetosphere. The flares evolve similar to coronal mass ejections changing the mass flux and velocity profile across the jet and also changing the mass flux number value by a factor of two to four with in short time. Our hypothesis is is that these flares initiate the observed jet knots. For our magnetic diffusivity model the time scales somehow fit. The following gif-animations show the long-term evolution of the flow (use xanim). Large frames, initial time steps (~3MB) Medium size frames, long run (~15MB) Fewer frames, long run (~34MB) Large frames, long run (~35MB)

©2010 Christian Fendt - last modified July 2010