Research on the Contraction and Fragmentation of Magnetized Interstellar Clouds.

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The following results have been obtained.1. The dissipation processes of magnetic fields may be classified into two categories, the plasma drift (ambipolar diffusion) and the Ohmic dissipation. The former is more efficient than the latter when the dominant positive- and negative-charged particles are well frozen to the magnetic fields, and the latter is more efficient otherwise.2. The dissipation rate of magnetic fields depends on the abundances of various charged particles. We have investigated their abundances for some limiting cases on the depletion of heavy elements in the gas phase, and found that the dissipation time of magnetic fields exceeds 10 times the free-fall time at the density n <10^(11)> <cm^(-3)> .3. We have investigated two typical cases with regard to the cloud configuration. A disk cloud with the magnetic fields parallel to its surface can maintain hydrostatic equilibrium due to the gas pressure even after the fields have dissipated. Therefore, such a cloud can lose the magnetic flux extensively even at low densities. However, because such a cloud is gravitatioually unstable and breaks into fragments in a time scale much shorter than the flux-loss time, the phenomena in such a cloud cannot be regarded realistic. When the magnetic flux is smaller than some critical value, the disk cloud with perpendicular magnetic fields contracts in nearly the free-fall time as long as it remains nearly isothermal. Therefore, substantial loss of magnetic flux does not occur at n <10^(11)> <cm^(-3)> .4. At n <10^(12)> <cm^(-3)> the grains with positine and negative charges and the ions are the dominant charged particles. Because they are hardly frozen to the magnetic fields at such high densities, the fields dissipate by the Ohmic heating. At such a situation the magnetic fields are nearly completely decoupled from the gas, and only nearly current-free magnetic fields can exist.