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I will first briefly address the physical conditions that must prevail within near Keplerian accretion discs in order to steadily launch cold self-collimated jets. The most important condition is the presence of a large scale magnetic field whose vertical component is smaller but close to equipartition with the disc thermal pressure. Depending upon their history, some astrophysical systems may never...
Results of three-dimensional time-dependent magneto-hydrodynamic simulations of T Tauri jets, launched magneto-centrifugally from the surface of Keplerian disk is presented. We extend the calculations to scales probed by HST (50 AU) thereby allowing a direct comparison between simulations and observations. We explore the effects of different initial magnetic field configurations on evolution and stability...
Observations of jets from accretion disks have constrained the launching radius to be confined to a zone close to the centre of the disk. The time evolution of a viscous, resistive accretion disk with a radially varying magnetisation distribution is studied. The magnetic field is advected into the centre of the disk over two accretion timescales. The magnetic field distribution evolves towards a power...
The available analytical MHD models for jets, characterized by the symmetries of radial self-similarity (ADO, Analytical Disk Outflow solutions) in general have two geometrical shortcomings, a singularity at the jet axis and the non-existence of an intrinsic scale, i.e., the jets formally extend to radial infinity. The present study focuses on imposing an outer ejecting radius of the underlying accreting...
Using axisymmetric MHD simulations we investigate how the overall jet formation is affected by a variation in the disk magnetic flux profile and/or the existence of a central stellar magnetosphere. Our simulations evolve from an initial, hydrostatic equilibrium state in a force-free magnetic field configuration. We find a unique relation between the collimation degree and the disk wind magnetization...
Axisymmetric resistive MHD simulations for radially self-similar initial conditions are performed, using the NIRVANA code. The magnetic diffusivity could occur in outflows above an accretion disk, being transferred from the underlying disk into the disk corona by MHD turbulence (anomalous turbulent diffusivity), or as a result of ambipolar diffusion in partially ionized flows. We introduce, in addition...
Based on the principle of magneto-centrifugal acceleration, the interaction of an electrically conducting accretion disk with the magnetosphere of a young star will naturally give rise to X-winds and funnel flows. A distinguishing feature of the X-wind model is the trapped flux in a small region at the inner edge of the disk, where both the X-wind and the funnel flow originate. We will review some...
We investigate outflows from the disk-magnetosphere boundary of rotating magnetized stars in cases where the magnetic field of a star is bunched into an X-type configuration using axisymmetric and full 3D MHD simulations. Such configuration appears if viscosity in the disk is larger than diffusivity, or if the accretion rate in the disk is enhanced. Conical outflows flow from the inner edge of the...
Different numerical models for the launching of jets from Young Stellar Objects (YSO) are presented. I will show numerical magnetohydrodynamic (MHD) simulations of outflows launched from the accretion disk (disk winds), from the stellar surface (stellar winds) and from the region of interaction between the stellar magnetosphere and the accretion disk (magnetospheric ejections). I will characterize...
Stellar winds seem to be very efficient at removing angular momentum from stars. By means of analytical axisymmetric solutions of the ideal MHD equations for steady outflows, we show via a specific example how collimated stellar winds can brake Weak T Tauri stars in a reasonable time. This result can be generalized to Classical T Tauri stars provided that part of the accreted angular momentum is removed...
X-ray observations have shown extensive flaring activity in young stellar associations such as the Orion nebula. Observed flares are often very long and intense, and have been associated to very long magnetic loops, which may connect the stellar surface to the circumstellar disk. As such, these loops are candidate to be also the channel of star accretion from the disk, and one then wonders whether...
We present the results of global 2D and 3D magnetohydrodynamic simulations of jet formation from a gas disk rotating around a central object. In a disk-star system, differential rotation twists magnetic loops connecting a star and its disk. As magnetic twist accumulates, the magnetic loops inflate and form current sheets inside the loops. Magnetic reconnection taking place in the current sheet can...
We report on experiments in which magnetically driven radiatively cooled plasma jets were produced by a 1 MA, 250 ns current pulse on the MAGPIE pulsed power facility. The jets were driven by the pressure of a toroidal magnetic field in a “magnetic tower” jet configuration. This scenario is characterized by the formation of a magnetically collimated plasma jet on the axis of a magnetic cavity, confined...
Outflows in the form of jets is a widespread phenomenon in astrophysics. Their main driving mechanism is likely related to magnetic fields. These fields are able to tap the rotational energy of the central object and its surrounding disk, and accelerate and collimate matter ejecta. To zeroth order these outflows can be described within the theory of steady, axisymmetric, ideal magnetohydrodynamics...
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