The conductivity of dense molecular gas

We evaluate the conductivity tensor for molecular gas at densities ranging from 10⁴ to 10¹⁵ cm^-3 for a variety of grain models. The Hall contribution to the conductivity has generally been neglected in treatments of the dynamics of molecular gas. We find that it is not important if only 0.1-μm grains are considered, but for a Mathis-Rumpl-Nordsieck grainsize distribution (with or without PAHs) it becomes important for densities between 10⁷ and 10¹¹ cm^-3. If PAHs are included, this range is reduced to 10⁹-10¹⁰ cm^-3. The consequences for the magnetic field evolution and dynamics of dense molecular gas are profound. To illustrate this, we consider the propagation of Alfvén waves under these conditions. A linear analysis yields a dispersion relation valid for frequencies below the neutral collision frequencies of the charged species. The dispersion relation shows that there is a pair of circularly polarized modes with distinct propagation speeds and damping rates. We note that the gravitational collapse of dense cloud cores may be substantially modified by the Hall term.