"Star Formation and the Hall Effect"

The breakdown of magnetic flux-freezing in molecular clouds and protostellar discs is usually approximated by ambipolar diffusion at low densities or by resistive diffusion at high densities. However, there is an intermediate regime in which the Hall term in the conductivity tensor is significant, and the vector evolution of the magnetic field -- and therefore the evolution of the system under consideration -- is dramatically altered. I begin by outlining how MHD differs in the Hall regime, and then present calculations of charged particle abundances in dense molecular gas in different environments that demonstrate that Hall diffusion is important over a surprisingly broad range of conditions. The implications for the dynamics of shock waves, dense cores, and protostellar discs are discussed.