Fast Reconnection and Reconnection Diffusion: Implications for Star   Formation
  Fast reconnection of magnetic field in turbulent fluids allows magnetic field to change its topology and connections. As a result, the traditional concept of magnetic fields being frozen into the plasma is no longer applicable. The diffusion of plasmas and magnetic field is enabled by reconnection and therefore is termed "reconnection diffusion". We explore the consequences of reconnection diffusion for star formation. In the paper we explain the physics of reconnection diffusion both from macroscopic and microscopic points of view. We quantify the reconnection diffusion rate both for weak and strong MHD turbulence and address the problem of reconnection diffusion acting together with ambipolar diffusion. In addition, we provide a criterion for correctly representing the magnetic diffusivity in simulations of star formation. We show that the role of the plasma effects is limited to "breaking up lines" on small scales and does not affect the rate of reconnection diffusion. We address the existing observational results and demonstrate how reconnection diffusion can explain the puzzles presented by observations, in particular, the observed higher magnetization of cloud cores in comparison with the magnetization of envelopes. We also outline a possible set of observational tests of the reconnection diffusion concept and discuss how the application of the new concept changes our understanding of star formation and its numerical modeling. Finally, we outline the differences of the process of reconnection diffusion and the process of accumulation of matter along magnetic field lines that is frequently invoked to explain the results of numerical simulations 
A. Lazarian
astro-ph.GA astro-ph.CO astro-ph.SR