Numerical simulations of MHD jets from Keplerian accretion disks: I. Recollimation shocks

Context. The most successful scenario for the origin of astrophysical jets requires a large-scale magnetic field anchored in a rotating object (black hole or star) and/or its surrounding accretion disk. Platform jet simulations, where the mass load onto the magnetic field is not computed by solving the vertical equilibrium of the disk but is imposed as a boundary condition, are very useful for probing the jet acceleration and collimation mechanisms. The drawback of such simulations is the very large parameter space: despite many previous attempts, it is very difficult to determine the generic results that can be derived from them. Aims. We wish to establish a firm link between jet simulations and analytical studies of magnetically driven steady-state jets from Keplerian accretion disks. In particular, the latter have predicted the existence of recollimation shocks – due to the dominant hoop stress –, which have so far never been observed in platform simulations. Methods. We performed a set of axisymmetric magnetohydrodynamics (MHD) simulations of nonrelativistic jets using the PLUTO code. The simulations are designed to reproduce the boundary conditions generally expected in analytical studies. We vary two parameters: the magnetic flux radial exponent α and the jet mass load κ . In order to reach the huge unprecedented spatial scales implied by the analytical solutions, we used a new method allowing us to boost the temporal evolution. Results. We confirm the existence of standing recollimation shocks at large distances. As in self-similar studies, their altitude evolves with the mass load κ . The shocks are weak and correspond to oblique shocks in a moderately high, fast magnetosonic flow. The jet emitted from the disk is focused toward the inner axial spine, which is the outflow connected to the central object. The presence of this spine is shown to have a strong influence on jet asymptotics. We also argue that steady-state solutions with α ≥ 1 are numerically out of range. Conclusions. Internal recollimation shocks may produce observable features such as standing knots of enhanced emission and a decrease in the flow rotation rate. However, more realistic simulations (e.g. fully three-dimensional) must be carried out in order to investigate nonaxisymmetric instabilities and with ejection only from a finite zone in the disk, so as to to verify whether these MHD recollimation shocks and their properties are maintained.