MHD simulations of jet acceleration from Keplerian accretion disks

The effects of disk resistivity

¹ Laboratoire d'Astrophysique de Grenoble, 414 rue de la Piscine, BP 53, 38041 Grenoble, France e-mail: Claudio.Zanni@obs.ujf-grenoble.fr
² Dipartimento di Fisica Generale dell'Università, via Pietro Giuria 1, 10125 Torino, Italy e-mail: ferrari@ph.unito.it
³ Department of Astronomy and Astrophysics, University of Chicago, 5640 S. Ellis Av., Chicago, IL 60637, USA e-mail: r-rosner@uchicago.edu
⁴ Osservatorio Astronomico di Torino, Viale Osservatorio 20, 10025 Pino Torinese, Italy e-mail: bodo@to.astro.it

Received: 14 September 2006
Accepted: 24 February 2007

Abstract

Context.Accretion disks and astrophysical jets are used to model many active astrophysical objects, such as young stars, relativistic stars, and active galactic nuclei. However, existing proposals for how these structures may transfer angular momentum and energy from disks to jets through viscous or magnetic torques do not yet provide a full understanding of the physical mechanisms involved. Thus, global stationary solutions have not explained the stability of these structures; and global numerical simulations that include both the disk and jet physics have so far been limited to relatively short time scales and narrow (and possibly astrophysically unlikely) ranges of viscosity and resistivity parameters that may be crucial to defining the coupling of the inflow-outflow dynamics.

Aims.We present self-consistent, time-dependent simulations of supersonic jets launched from magnetized accretion disks, using high-resolution numerical techniques. In particular we study the effects of the disk's magnetic resistivity, parametrized through an α-prescription, in determining the properties of the inflow-outflow system. Moreover we analyze under which conditions steady state solutions of the type proposed in the self-similar models of Blandford & Payne can be reached and maintained in a self-consistent nonlinear stage.

Methods.We used the resistive MHD FLASH code with adaptive mesh refinement (AMR), allowing us to follow the evolution of the structure on a long enough time scale to reach steady state. A detailed analysis of the initial configuration state is given.

Results.We obtain the expected solutions within the axisymmetric (2.5 D) limit. Assuming a magnetic field around equipartition with the thermal pressure of the disk, we show how the characteristics of the disk-jet system, such as the ejection efficiency and the energetics, are affected by the anomalous resistivity acting inside the disk.