Mechanism of Magnetic Flux Loss in Molecular Clouds

We investigate the detailed processes at work in the drift of magnetic fields in molecular clouds. To the frictional force, whereby the magnetic force is transmitted to neutral molecules, ions contribute more than half only at cloud densities n_H ≲ 10⁴ cm^-3, and charged grains contribute more than about 90% at n_H ≳ 10⁶ cm^-3. Thus, grains play a decisive role in the process of magnetic flux loss. Approximating the flux loss time t_B by a power law t_B ∝ B^-γ, where B is the mean field strength in the cloud, we find γ ≈ 2, characteristic of ambipolar diffusion, only at n_H ≲ 10⁷ cm^-3, at which ions and the smallest grains are pretty well frozen to the magnetic fields. At n_H > 10⁷ cm^-3, γ decreases steeply with n_H, and finally at n_H ≈ n_dec ≈ a few × 10¹¹ cm^-3, at which the magnetic fields effectively decouple from the gas, γ ≪ 1 is attained, reminiscent of Ohmic dissipation, although flux loss occurs about 10 times faster than by pure Ohmic dissipation. Because even ions are not very well frozen at n_H > 10⁷ cm^-3, ions and grains drift slower than the magnetic fields. This insufficient freezing makes t_B more and more insensitive to B as n_H increases. Ohmic dissipation is dominant only at n_H ≳ 1 × 10¹² cm^-3. While ions and electrons drift in the direction of the magnetic force at all densities, grains of opposite charges drift in opposite directions at high densities, at which grains are major contributors to the frictional force. Although magnetic flux loss occurs significantly faster than by Ohmic dissipation even at very high densities, such as n_H ≈ n_dec, the process going on at high densities is quite different from ambipolar diffusion, in which particles of opposite charges are supposed to drift as one unit.