Efficiency Of Angular Momentum Transport Essay

1273 words - 6 pages

Observations indicate that at present time stars born in magnetized rotating cores of molecular clouds~\citep{shu87, dudorov91, dudorov95, mckee07}. Centrifugal and electromagnetic forces lead to the formation of a disk-like structure during the gravitational collapse of the molecular cloud. Evolution of accretion disks depends on efficiency of angular momentum escape. Turbulence \citep{ss73}, magnetic braking and outflows are the most important mechanisms of angular momentum transport in accretion disks of young stars. Turbulence in accretion disks comes probably from magnetorotational instability \citep[MRI, ][]{balbus91}. Magnetic braking mechanism is based on the process of transfer of angular momentum by torsional Alfven waves \citep{mestel56, mouschovias79}. Centrifugally driven outflows arise when ordered magnetic field lines are inclined more than 30 degrees from vertical \citep{blandford82}. Efficiency of angular momentum transport depends on strength and geometry of the magnetic field in the accretion disks, which are still remain uncertain.

Numerical simulations indicate that initial magnetic flux of molecular clouds cores is partially conserved during the process of star formation \citep[e.g.,][]{dud87}.
\cite{dudorov08} shown that an initially uniform magnetic field acquires hour-glass geometry during protostellar cloud collapse. Collapsing protostellar cloud with moderate magnetic field evolves into a flat disk according to numerical simulations of \cite{dudorov03}. They showed that the collapse of the clouds with strong magnetic field switches to a magnetostatic contraction. Based on numerical calculations of magnetized protostellar cores collapse, we believe that accretion disks of young stars should have fossil magnetic field (i.e. it is the relic of magnetic field of parent protostellar clouds).

There are some observational confirmations of predictions of theory of fossil magnetic field. For example, \cite{girart06} detected hour-glass shaped geometry of the magnetic field in the low-mass protostellar system NGC 1333 IRAS 4A using the Submillimeter Array polarimetry observations at 877 $\mu$m dust continuum emission. \cite{chapman13} detected a correlation between core magnetic field direction and protostellar disk symmetry axis in a few low-mass protostellar cores using the SHARP polarimeter at Caltech Submillimeter Observatory at 350 $\mu$m dust continuum emission. \cite{donati05} probably detected magnetic field with azimuthal component of order of 1 kGs in the FU Orionis accretion disk. In absence of sufficient number of observational data, theoretical calculations of magnetic field of accretion disks of young stars become an important task.

Dragging of an external magnetic field in the accretion disks has been investigated in several works. \cite{lubow94} and \cite{agapitou96} considered bending of an initially vertical weak magnetic field in the infinitesimally thin viscous disk. They pointed out that inclination of...

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