Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

Table 15.1: List of the permanent SHERPA group members in 2005.
Name Grade Specialty
Jonathan Ferreira MdC accretion-ejection, MHD
Didier Fraix-Burnet CR1 astrocladistics
Gilles Henri Prof high energy phenomena
Pierre-Yves Longaretti CR1 MHD instabilities and transport
Guy Pelletier Prof (group leader) relativistic plasma physics, MHD
Pierre-Olivier Petrucci CR2 high energy phenomena
Peggy Varnière CR2 (starting Fall of 2005) MHD simulations
15.3 Accretion-Ejection
15.3.1 The self-similar model
Accretion-ejection phenomena are common-place in astrophysics. They are present in the cores of active galaxies
(AGNs) and quasars but also around compact objects such as X-ray binaries or even some cataclysmic variables
within our galaxy. It is the major physical process governing the growth rate of young stellar objects (YSOs).
Accretion-ejection is therefore a process of major importance. It has long been recognized that the high degree
of collimation exhibited by jets from AGNs or YSOs requires a self-confinement process which can only be
provided by large scale magnetic fields carried in along the jet. This gave rise to MHD models of jet formation
and collimation. On the other hand, all these objects display a correlation between accretion and ejection
observational signatures (Hartigan, Edwards & Ghandour 1995, ApJ, 452, 736). These correlations gave birth
to the idea that accretion and ejection were actually interdependent processes.
Our team was the first to identify the concept of a Magnetized Accretion-Ejection Structure (MAES, Ferreira
& Pelletier 1993, A&A, 276, 625). In such a structure a large scale magnetic field of bipolar topology is
threading the disk. The field exerts a torque which takes away the disk angular momentum thereby allowing it
to accrete towards the central object. A turbulence is needed in order to allow mass to steadily diffuse through
the magnetic field. This angular momentum and energy is then transferred back to a small fraction of the
disk material which gives rise to a self-confined MHD jet. In contrast with other teams, the MAES has been
computed by solving the exact equations of both the disk and the jets: usually, the disk is either ignored (taken
as a mere boundary condition, e.g. Blandford & Payne 1982, MNRAS, 199, 883, Shu et al. 1994, ApJ, 429, 781)
or its vertical structure is crudely approximated (e.g. Wardle & Königl 1993, 410, 218, Li 1995, ApJ, 444, 848).
By taking into account all terms (which was possible thanks to a self-similar ansatz), we were able to provide
the full parameter space of MAES with strong consequences on the level of the required MHD turbulence (Casse
& Ferreira 2000a, A&A, 353, 1115, Casse & Ferreira 2000b, A&A, 361, 1178). This work has been recently
extended by producing the only solutions of jets from accretion disks that cross the three MHD critical points
(Ferreira & Casse 2004).
We are now endowed with the only available model in the literature of disk-driven jets which provides all
fields (density, velocity and magnetic fields) as functions of the disk physical conditions in a consistent way.
15.3.2 Application to different astrophysical contexts
Since jets from YSOs are cooling by optically thin emission lines it is possible to derive strong constraints on
their dynamics and discriminate models. Thus, most past work has been devoted to YSOs. The application of
the MAES model to compact objects is only beginning, with a focus on ”microquasars”.
Young Stellar Objects.
Using the self-similar MAES model, we were able to compute synthetic observations and compare them
to real observations. A previous work done in collaboration with observers (Garcia et al. 2001a, A&A, 377,
589, Garcia et al. 2001b, 377, 609) showed that most of the T-Tauri optical jet properties (line profiles, flux,
jet velocities, evolution of the jet diameter along the distance) could be easily explained by disk-driven jets.
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