|
We present recent progress of numerical simulations of contraction of
magnetized rotating clouds. Adopting a barotropic equation of state,
we calculated the evolution of the isothermal runaway collapse, the
first core phase and the second collapse. This shows us that a simple
asymptotic relation between magnetic field strength and rotation speed
(B-omega relation) holds at the first core formation epoch. This is
also valid for the case angular momentum and magnetic field is not
necessarily parallel. As for the fragmentation it is shown the
magnetic field stabilizes the first core. That is, a first core in
strong magnetic field does not fragment (since the cloud reaches a
magnetic-dominated branch of the B-omega relation and rotation is
unimportant in this core). In this case, a single star is expected to
form. In the case of weak magnetic field, the evolution of first core
is well characterized by a spin parameter, which is defined as the
ratio of specific angular momentum to the mass. A cloud with a small
spin parameter begins a second runaway collapse after the central gas
temperature exceeds the H2 dissociation temperature. A cloud with a
large spin parameter, mass and angular momentum increase and thus a
non-axisymmetric instability appears (T/|W|>0.273) below the
dissociation density. When binary fragments are formed, each fragment
continues to contract, which lead to binary formation. Since the
non-axisymmetric pattern removes the angular momentum from the core,
the core continues further contraction.
|
