TY  - JOUR
A1  - Fendt, Christian
A1  - Cemeljic, Miljenko
T1  - Formation of protostellar jets : effects of magnetic diffusion
N2  - Protostellar jets most probably originate in turbulent accretion disks surrounding young stellar objects. We investigate the evolution of a disk wind into a collimated jet under the influence of magnetic diffusivity, assuming that the turbulent pattern in the disk will also enter the disk corona and the jet. Using the ZEUS-3D code in the axisymmetry option we solve the time-dependent resistive MHD equations for a model setup of a central star surrounded by an accretion disk. The disk is taken as a time-independent boundary condition for the mass flow rate and the magnetic flux distribution. We derive analytical estimates for the magnitude of magnetic diffusion in a protostellar jet connecting our results to earlier work in the limit of ideal MHD. We find that the diffusive jets propagate slower into the ambient medium, most probably due to the lower mass flow rate in the axial direction. Close to the star we find that a quasi stationary state evolves after several hundred (weak diffusion) or thousand (strong diffusion) disk rotations. Magnetic diffusivity affects the protostellar jet structure as follows. The jet poloidal magnetic field becomes de- collimated. The jet velocity increases with increasing diffusivity, while the degree of collimation for the hydrodynamic flow remains more or less the same. We suggest that the mass flux is a proper tracer for the degree of jet collimation and find indications of a critical value for the magnetic diffusivity above which the jet collimation is only weak. We finally develop a self-consistent picture in which all these effects can be explained in the framework of the Lorentz force.
Y1  - 2003
UR  - http://www.edpsciences.org/articles/aa/pdf/2002/45/aa2729.pdf
ER  - 
TY  - JOUR
A1  - Cemeljic, Miljenko
A1  - Fendt, Christian
T1  - Protostellar jets and magnetic diffusion
N2  - We investigate the evolution of a disk wind into a collimated jet under the influence of magnetic diffusivity. Using the ZEUS-3D code in the axisymmetry option we solve the time-dependent resistive MHD equations for a model setup of a central star surrounded by an accretion disk. The disk is taken as a time-independent boundary condition for the mass flow rate and the magnetic flux distribution. We find that the diffusive jets propagate slower into the ambient medium, most probably due to the lower mass flow rate in axial direction. Close to the star we find that a quasi stationary state evolves after several hundreds (weak diffusion) or thousands (strong diffusion) of disk rotations. Magnetic diffusivity affects the protostellar jet structure de-collimating it. We explain these effects in the framework of the Lorentz force.
Y1  - 2003
UR  - http://arxiv.org/pdf/astro-ph/0211513
SN  - 1-4020-1617-4
ER  - 
TY  - THES
A1  - Cemeljic, Miljenko
T1  - Resistive magnetohydrodynamic jets from protostellar accretion disks
N2  - Astrophysikalische Jets sind ausgedehnte, kollimierte Massenausflüsse von verschiedenen astronomischen Objekten. Zeitabhängige magnetohydrodynamische (MHD) Simulationen der Jet-Entwicklung müssen den Akrretionsprozess in der Scheibe berücksichtigen, da der Jet aus der Scheibenmaterie gespeist wird. Allerdings ist die simultane Berechnung der Entwicklung von Scheibe und Jet schwierig, da die charakteristischen Zeitskalen unterschiedlich sind. Selbstähnliche Modelle zeigten, daß eine Beschreibung der Jetentstehung aus einer Akkretionsscheibe durch rein magnetische Prozesse möglich ist.
N2  - In this thesis the magnetohydrodynamic jet formation and the effects of magnetic diffusion on the formation of axisymmetric protostellar jets have been investigated in three different simulation sets. The time-dependent numerical simulations have been performed, using the magnetohydrodynamic ZEUS-3D code.
T2  - Resistive magnetohydrodynamic jets from protostellar accretion disks
KW  - Jets
KW  - MHD
KW  - Akkretion
KW  - kollimation
KW  - jets
KW  - MHD
KW  - accretion
KW  - collimation
Y1  - 2005
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0001845
ER  -