Long Term Simulations of Astrophysical Jets

Abstract

We have performed self-consistent 2.5-dimensional nonsteady MHD numerical simulations of jet formation, including the dynamics of accretion disks. Although the previous nonsteady MHD simulations for astrophysical jets revealed that the characteristics of nonsteady jets are very similar to those of steady jets, the calculation time of these simulations is very short compared with the time scale of observed jets. Thus we have investigated long term evolutions of the mass accretion rate (Ṁ a), mass outflow rate (Ṁw), and energy of the toroidal magnetic field (E mgt). We found that the jet is ejected intermittently with a period of around 2π which is similar to the growth time of the magnetorotational instability (MRI). §1. IntroductionAlthough the acceleration and collimation mechanisms of the astrophysical jets are still not well understood, these objects are believed to have accretion disks in their central regions. One of the most promising models for jet formation is magnetic acceleration from accretion disks (Blandford 1993). Blandford and Payne (1982) pointed out that a magneto-centrifugally driven outflow from a Keplerian disk is possible if the angle between the disk surface and the poloidal component of the magnetic field is less than 60 • . In order to have access to the observed time scale of jets and to determine whether jet formation becomes a quasi-steady state, we performed long term nonsteady 2.5-dimensional MHD simulations of jets. We also wish to determine whether the time averaged physical quantities have the same characteristics as those found in the steady state model and previous simulations. §2. Assumptions and numerical method Our simulations incorporate the following assumptions: (1) axial symmetry around the rotational axis (∂/∂ϕ = 0) including the azimuthal components of a velocity field (v ϕ ) and a magnetic field (B ϕ ); (2) ideal MHD; (3) an inviscid perfect gas with a specific heat ratio of γ = 5/3; (4) a point-mass gravitational potential. The numerical schemes we use are the cubic interpolated propagation (CIP) method (Yabe et al. 1991) and the method of characteristics-constrained transport (MOCCT) (Stone and Norman 1992). The magnetic induction equation is solved using MOCCT and the others using CIP. §3. Results and conclusionsIn the early stage of evolution, a torsional Alfven wave is generated at the disk surface. This wave propagates up into the corona. Since this wave extracts angular at State Univ NY at Stony Brook on June 30, 2015 http://ptps.oxfordjournals.org/ Downloaded from