Bipolar Jets Launched From Accretion Disks. Ii. The Formation of Asymmetric Jets and Counter Jets

Abstract

We investigate the jet launching process from accretion disks extending our recent study (paper I) to a truly bipolar setup. We perform axisymmetric MHD simulations of the disk-jet interaction on a computational domain covering both hemispheres, in particular addressing the question of an intrinsically asymmetric origin of jet / counter jet systems. Treating both hemispheres simultaneously, we overcome the equatorial plane symmetry boundary condition used in most previous studies which naturally fosters a symmetric evolution. For the magnetic diffusivity prescription we apply an αparametrisation, considering both, globally models of diffusivity, and local models. We first approve the quality of our numerical setup by generating perfectly symmetric jets, lasting over a 1000s of dynamical time scales. We then disturb the hemispheric symmetry in the disk, and investigate the subsequent evolution of the outflow. The evolution first leads to a substantial disk warping with electric currents intersecting the equatorial plane. We investigate two models, i) a disk with (initially) different thermal scale height in both hemispheres, and ii) a symmetric disk into which a local disturbance is injected in one hemisphere. In both cases the disk asymmetry results in asymmetric outflows with mass fluxes differing by 10-20%. For case i) the jets are launched asymmetric right from the beginning. However, due to the symmetric diffusivity profile and restoring gravitational forces, the outflow asymmetry dies out after several hundred dynamical time scales. For case ii) the onset of asymmetry in the outflows is delayed as the disturbance injected at a larger radius first has to propagate to smaller radii where the bulk of the outflow is launched. For a local prescription of magnetic diffusivity (following mainly the local sound speed) the outflow asymmetry is persistent for long time, implying that diffusive effects play a major role when re-establishing hemispheric symmetric by gravitational forces. We find up to 30% difference in mass flux between jet and counter jet for this setup, lasting over 1000s of dynamical time scales (i.e. lasting for the whole simulation). For comparison we have run simulations where jets are launched from a symmetric disk into an asymmetric coronal hemispheres of different density. As expected, the initial jet and counter jet are launched asymmetric, however, as soon as the jets have left the coronal asymmetry (within a few hundred dynamical time steps) a symmetric flow structure is established. In summary, our results suggest that the jet asymmetries in protostellar and extragalactic jets can indeed be generated intrinsically and maintained over long time by disk asymmetries and the standard jet launching mechanism.

Accretion-ejection connection in the young brown dwarf candidate ISO-ChaI 217

“…AlsoṀ out is larger in the red-shifted jet than in the blue-shifted, even when we account for the uncertainty introduced by the use of an upper limit for the flux of the [N ] λ6583 line in the red-shifted jet. This difference inṀ out was also found in Whelan et al (2009b) and this sort of of asymmetry can be reproduced by the lastest models which account for jet asymmetries (Fendt & Sheikhnezami 2013). -Using the improved method for estimatingṀ out we now placeṀ out /Ṁ acc (for the ISO-ChaI 217 jets combined) at 0.05 (+0.07)(−0.02).…”

Particle rings and astrophysical accretion discs

“…Remarkably, once the assumption of symmetry about the equatorial plane is dropped, the conical winds alternately come from one hemisphere and then the other even when the stellar magnetic field is a centered axisymmetric dipole [89]. This symmetry breaking has also been found by Fendt & Sheikhnezami [91]. The time-scale for the 'flipping' is the accretion time-scale of the inner part of the disc which is expected to be much less than the evolution time of the star's magnetic field.…”

The stratified two-sided jet of Cygnus A

“…This is probably an effect of the smaller beam used in the slicing, and it suggests that the counter-jet emission is less compact than the jet emission. However, the hypothesis of intrinsic asymmetry, which is indeed possible (e.g., Fendt & Sheikhnezami 2013), cannot be excluded. Based on these results, the jets in Cygnus A can still be defined as relatively broad on the examined scale of ∼2 mas.…”