Magnetocentrifugal Launching of Jets from Accretion Disks. I. Cold Axisymmetric Flows

Abstract

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 conditions imposed on the disk surface is that necessary and sufficient to take into account information propagating upstream from the fast and Alfvén critical surfaces, avoiding overdetermination of the flow and unphysical effects, such as numerical "boundary layers" that otherwise isolate the disk from the flow and produce impulsive accelerations. It is known that open magnetic field lines can either trap or propel the gas, depending upon the inclination angle, θ, of the poloidal field to the disk normal. This inclination is free to adjust, changing from trapping to propelling when θ is larger than θ_c ~ 30°; however, 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 inclination of field lines to the axis is too small to drive a centrifugal wind, we 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 the disk field configuration is propelling. We parameterize our runs using a magnetic flux Ψ ∝ R^(-eΨ) and mass flux j = ρv_z ∝ R^(-ej). We show in detail the steady state of a reference run with parameters e_Ψ = -1/2, e_j = 3/2, finding that the wind leaves the computational volume in the axial direction with an Alfvén number M_A ~ 4, poloidal speed v_p ~ 1.6v_(K0), collimated inside an angle θ ~ 11°. We show also the thrust T, energy L, torque G, and mass discharge Ṁ of the outgoing wind, and we illustrate the dependence of these quantities with the exponents e_Ψ and e_j.