Magnetic flux loss from interstellar clouds

Abstract

The densities of charged particles in ionization–recombination equilibrium are investigated with some revised reaction rate coefficients. Especially the dissociative recombination rate of |$\text H^+_3$| with an electron is decreased by a factor 102 compared with the rate generally used before. At the density by hydrogen number |$n_\text H\lesssim 10^{11}\enspace\text {cm}^{-3}$|⁠, where electrons and ions are the dominant charged particles, the ionization fraction depends considerably on the degree of depletion of heavy elements from the gas phase. At higher densities where grains are the dominant charged particles, the abundances of some dominant charged particles depend scarcely on the degree of the element depletion. Dissipation of magnetic fields due to plasma drift (ambipolar diffusion) and ohmic loss is investigated for the cloud cores with density |$n_\text H\approx10^3-10^{13}\enspace\text {cm}^{-3}$|⁠. At |$n_\text H\lesssim 10^{11}\enspace\text {cm}^{-3}$| the dissipation time of magnetic fields is considerably greater than the free-fall time, and it is difficult to decrease the magnetic flux of the cloud core much below the critical flux for magnetohydrostatic equilibrium. At |$n_\text H\gtrsim \enspace\text{several}\times10^{11}\enspace\text {cm}^{-3}$| the magnetic field decouples from the gas due to ohmic dissipation, and only a nearly uniform field can exist in such a cloud.