Magnetic Field of a Contracting Protostar

Extract

In a recent paper, Mestel and Spitzer discussed the problem of star formation from cool matter in the presence of a magnetic field of energy density comparable with the thermal energy density. If the field were frozen into the gas, the magnetic pressure would put a lower limit of order 10³M_⊙ to the mass that could be gravitationally bound, and this limit is unaltered by contraction of the cloud. However, in a lightly ionized gas, the field moves not with the gas as a whole, but with the plasma, and the motion of the plasma through the cloud is determined by a balance between mgnetic force and friction between neutral gas and plasma. A cloud containing sufficient dust can extinguish the galactic ionizing radiation; the plasma density decays quickly enough for the plasma and neutral gas to become uncoupled during the time of gravitational contraction. In this way high densities are built up without correspondingly high magnetic energy, and the cloud can break up into stars.