What does it take to launch self-confined jets from astrophysical accretion discs?
Résumé
Well collimated jets are observed from young stellar objects (YSO), active galactic nuclei (AGN) and some galactic compacts objects (X-ray Binaries). These jets are powerful in the sense that they seem to carry a sizable fraction of the power released in the underlying accretion disc. The most complete and reliable jet models are actually disc winds, launched from the innermost disc regions: accretion and ejection are naturally interdependent, through the action of large scale magnetic fields anchored in the disc.In the last decade, disc winds have been quite successfully compared to observed YSO jets. Indeed, these jets are well monitored and provide several valuable diagnostics (plasma temperature, density and velocity stratification) that put stringent constraints on all available jet models. However, even if disc winds provide a satisfactory "first order" explanation for YSO jets, they cannot explain alone the whole phenomenology of these jets and some interaction with the central object must be taken into account.Disc wind models rely on the presence of a large scale vertical field close to equipartition. It will be shown that fulfilling such a condition is not an easy task. A complex interplay between advection and turbulent diffusion of the field is probably the cause of long term variability, as observed in X-ray binaries for instance. We also show, for the first time, that the outer parts in accretion discs could be highly magnetized. This may be the reason of ubiquitous massive, low velocity winds observed in YSOs, AGN and XrBs.