Formation and structure of magnetized protostellar jets
Protostellar jets most probably originate in the closest environment of a fully convective young stellar object which presumably carries a magnetosphere built up by a strong stellar dynamo and is surrounded by an accretion disk. Interaction between the magnetic field and the accretion disk leads to the formation of a gap between the stellar surface and the disk. The observed jet opening angles may be less than 5°.
We present numerical solutions treeting the relativistic 2D force-balance of the magnetic field, described by the Grad-Schlüter-Shafranov (GSS) equation, and the steady motion of a cold plasma flow along the calculated magnetic flux surfaces in the collimation domain of the jet. Our model takes into account the topology of the star-disk-jet scenario mentioned above. The resulting flows have a finite asymptotic jet radius. From the observed rotational periods of T Tauri stars it follows that the derived light cylinder of a stellar magnetosphere is of the order of the observed jet radii. This fact requires a relativistic treatment although the jet velocities are clearly non-relativistic.
The resulting magnetic field structure allows simultaneously for wind outflow towards an asymptotically cylindrical jet and for mass accretion towards the central star along dipolar field lines. The outflow is initially poorly collimated near the source with an opening angle of 65° and then rapidly collimates within a distance of 0.3 jet radii along the jet axis.
The 2D velocity structure of the flow along the flux surfaces strongly depends on the magnetization σ of the plasma flow. For the asymptotic poloidal jet velocity u ∞ we find a power law u ∞.=Acσ 1/3 Since the factor A≥1, this implies that the acceleration along the collimated flux surfaces is more efficient than in a purely conical magnetic structure. The asymptotic fast-magnetosonic Mach-number of the flow turned out to be independent of the magnetization and generally is of the order of 2.5. This has far-reaching consequences for the interpretation of the knot-spacing in protostellar jets.
KeywordsAccretion Disk Flux Surface Stellar Surface Magnetic Field Structure Stellar Magnetic Field
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- Camenzind, M. (1990), Magnetized disk-winds and the origin of bipolar outflows, in: Klare, G. (ed) Rev. Mod. Astron., 3, Springer, Heidelberg, p. 234Google Scholar
- Camenzind, M., Fendt, C., Paatz, G. (1994), in preparationGoogle Scholar
- Fendt, C. (1994), PhD thesis, University of HeidelbergGoogle Scholar
- Fendt, C., Camenzind, M. (1995), in preparationGoogle Scholar
- Michel, F.C. (1991), Theory of neutron star magnetospheres, The University of Chicago Press, ChicagoGoogle Scholar
- Montmerle, T., Feigelson, E.D., Bouvier, J., André, P. (1993), Magnetic fields, activity and circumstellar material around young stellar objects, in: Levy,E.H., Lunine, J.I. (eds) Protostars and Planets III, The University of Arizona Press, p. 689Google Scholar
- Paatz, G., Camenzind, M. (1994), submitted to A&AGoogle Scholar