Collimation of Astrophysical Jets: The Role of the Accretion Disk Magnetic Field Distribution

We have applied axisymmetric MHD simulations to investigate the impact of theaccretion disk magnetic flux profile on the jet collimation. Using the ZEUS-3Dcode modified for magnetic diffusivity, our simulations evolve from an initialhydrostatic equilibrium state in a force-free magnetic field. Considering apower law for the disk poloidal magnetic field profile Bp ~ r^{-mu} and for thedisk wind density profile rho ~ r^{-mu_rho} we performed a systematic studyover a wide parameter range mu and mu_rho. We find a degree of collimation(ratio of mass flow rates in axial and lateral direction) decreasing forsteeper disk magnetic field profiles (increasing mu). Varying the totalmagnetic flux doesn't change the degree of jet collimation substantially, itonly affects the time scale of outflow evolution and the terminal jet speed. Asour major result we find a general relation between the collimation degree withthe disk wind magnetization power law exponent. Outflows with high collimationdegree resulting from a flat disk magnetic field profile tend to be unsteady,producing axially propagating knots as discussed earlier. Depending slightly onthe inflow density profile this unsteady behavior sets in for mu < 0.4. We alsoperformed simulations of jet formation with artificially enhanced decay of thetoroidal magnetic field in order to investigate the idea of a purely "poloidalcollimation" discussed in the literature. These outflows remain weaklycollimated and propagate with lower velocity. Thanks to our large numericalgrid size (7x14 AU for protostars), we may apply our results to recentlyobserved hints of jet rotation (DG Tau) indicating a relatively flat diskmagnetic field profile, mu ~ 0.5. In general, our results are applicable toboth stellar and extragalactic sources of MHD jets.Comment: accepted by ApJ, high resolution version under www.mpia-hd.mpg.de/homes/fendt