The magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse (1702.05688v1)

Abstract: The grand question of star and planet formation is the distribution of magnetic flux in the protoplanetary disks. To answer it, a detailed self-consistent chemical evolution is needed to describe the magnetic dissipation in the collapsing core accurately. We use a chemo-dynamical version of RAMSES to follow the evolution of collapsing dense cores for a range in the dust size assumptions. The number density of dust and it's mean size are affecting the efficiency of the charge capturing and thus the chemistry. The chemical abundances for the range of dust sizes are produced by RAMSES and serve as an input to calculations of Ohmic, ambipolar and Hall diffusivity terms. We find that Ohmic resistivity only plays a role at the late stage of the collapse where gas density exceeds few times $10^{13}\rm cm^{-3}$. Ambipolar diffusion is a dominant magnetic diffusivity term in cases where mean dust size is a typical ISM value or larger. We show that the assumption of a fixed 'dominant ion' mass can change the ambipolar diffusion up to one order of magnitude. 'Negative' Hall effect is dominant during the collapse in case of mean dust size of 0.02 $\mu$m and smaller, the effect which we connect to the dominance of negatively charged grains. We find that the Hall effect's sign reversal is depending on relative contribution of negatively charged dust to the hall conductivity. The dust grain mean size appears to be the parameter dividing the collapsing clouds in Hall-dominated and ambipolar-dominated clouds, and thus affecting the size of the new-born disks. We propose to link the dust properties and the occurance and size of disk structures in Class 0 YSO's. The proper accounting for dust grain growth in the radiative magneto-hydrodynamical collapse models appear to be be as important as coupling the dynamics of the collapse with the chemistry.