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) less than or similar to 10(4) cm(-3), and charged grains contribute more than about 90% at n(H) greater than or similar to 10(6) 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) proportional to B-gamma where B is the mean field strength in the cloud, we find gamma approximate to 2, characteristic of ambipolar diffusion, only at n(H) less than or similar to 10(7) cm(-3), at which ions and the smallest grains are pretty well frozen to the magnetic fields. At n(H) > 10(7) cm(-3), gamma decreases steeply with n(H), and finally at n(H) approximate to n(dec) a few x 10(11) cm(-3), at which the magnetic fields effectively decouple from the gas, gamma much less than 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) > 10(7) 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) greater than or similar to 1 x 10(12) 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) approximate to 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.