Accretion and Ejection of Astrophysical Plasma

The aim of this page is to present the results of two papers devoted to the numerical study of MAES (MHD Accretion-Ejection Structures). These systems account for  astrophysical systems where collimated mass ejection called "Jets" are detected. This peculiar mechanism is always occuring  while  mass is accreting into a so-called accretion disk.  The presence of a magnetic field and some observationnal correlation between jet and disk luminosities has made quite clear that a magnetohydrodynamical (MHD) description of the problem is likely to be accurate. The different systems involved in this kind of structures are:

There are also lots of more "exotic" systems that may be sorted into the MAES class: gamma-ray bursts, planetary nebula, pulsar ....
For more informations, you should go to the LAOG's High Energy team  web page presenting these objects :  SHERPAS
The next paragraph deals with the numerical MHD simulations done with the Versatile Advection Code (VAC) designed by G. Toth and R.Keppens. This code is devoted to HD and MHD simulations in any dimensionality (1D, 1.5D, 2D, 2.5D or 3D) and geometry (cartesian, cylindrical or spherical). For more detail go to the VAC page !

In a serie of two  papers, R. Keppens and I have presented 3D axisymmetric MHD computations of such systems. The aim of these simulations was to study the evolution of a resistive accretion disk (with no initial outflows) threaded by a large-scale magnetic field. On the right plot, I have displayed the initial configuration of the simulations: the different isosurfaces stand for density isocontours, the yellow lines for some magnetic field lines and the blue line is a streamline located whithin the disk. All streamlines are initially  accreting, namely they are spirals centered on the origin. At the origin, we have designed a sink region where the central object is hidden as well as  the boundary layer (the transition region between the object and the disk). The sink set-up is such that no mass is allowed to flow from the sink into the computationnal domain. We time-evolved this system by integrating the MHD set of equations. These set contains mass, momentum and energy conservations as well as the magnetic field induction equation (plus the div. B =0 constraint). The result of the previous simulation is shown on the right picture. The picture display is identical so that now it clearly appears that some streamlines are first accreting and then turn into an ejection motion. The basic idea is that the rotation of the disk is twisting the magnetic field lines anchored on the disk. This twisting provoked a magnetic retroaction on the plasma which is slowed down, allowing then the accretion. In parallel, the magnetic twisting generates a MHD Poynting energy flux in the jet accelerating the small fraction of mass extracted from the disk by the subtle balance between magnetic and thermal pressures. For a funny illustration, click on the picture to get a Quicktime movie of what one would see if one was flowing along the streamline displayed on the picture. For more informations or details, you can first retrieve the papers here or contact me directly by mail.