Magnetocentrifugal launching of jets from accretion disks. I. Cold axisymmetric flows

We present time-dependent, numerical simulations of the magnetocentrifugal model for jet formation, in an axisymmetric geometry, using a modification of the ZEUS3D code adapted to parallel computers. The gas is supposed cold with negligible thermal pressure throughout. The number of boundary conditi ons imposed on the disk surface is that necessary and sufficient to take in to account information propagating upstream from the fast and Alfven critic al surfaces, avoiding overdetermination of the flow and unphysical effects, such as numerical "boundary layers" that otherwise isolate the disk from t he flow and produce impulsive accelerations. It is known that open magnetic field lines can either trap or propel the ga s, depending upon the inclination angle, theta, of the poloidal field to th e disk normal. This inclination is free to adjust, changing from trapping t o propelling when theta is larger than theta(c) similar to 30 degrees, howe ver, the ejected mass flux is imposed in these simulations as a function of the radius alone. As there is a region, near the origin, where the inclina tion of field lines to the axis is too small to drive a centrifugal wind, w e inject a thin, axial jet, expected to form electromagnetically near black holes in active galactic nuclei and Galactic superluminal sources. Rapid acceleration and collimation of the flow is generally observed when t he disk field configuration is propelling. We parameterize our runs using a magnetic flux Psi proportional to R-e Psi and mass flux j = rho nu(z) prop ortional to R-ej. We show in detail the steady state of a reference run wit h parameters e(Psi) = -1/2, e(j) = 3/2, finding that the wind leaves the co mputational volume in the axial direction with an Alfven number M-A similar to 4, poloidal speed nu(p) similar to 1.6 nu(K0), collimated inside an ang le theta similar to 11 degrees. We show also the thrust T, energy L, torque G, and mass discharge (M) over dot of the outgoing wind, and we illustrate the dependence of these quantities with the exponents e(Psi) and e(j).