Cosmic ray ionization rate versus dust fraction: Which plays a crucial role in the early evolution of the circumstellar disc?

Abstract

ABSTRACTWe study the formation and early evolution of young stellar objects (YSOs) using three-dimensional non-ideal magnetohydrodynamic (MHD) simulations to investigate the effect of cosmic ray ionization rate and dust fraction (or amount of dust grains) on circumstellar disc formation. Our simulations show that a higher cosmic ray ionization rate and a lower dust fraction lead to (i) a smaller magnetic resistivity of ambipolar diffusion, (ii) a smaller disc size and mass, and (iii) an earlier timing of outflow formation and a greater angular momentum of the outflow. In particular, at a high cosmic ray ionization rate, the discs formed early in the simulation are dispersed by magnetic braking on a time-scale of about 104 yr. Our results suggest that the cosmic ray ionization rate has particularly a large impact on the formation and evolution of discs, while the impact of the dust fraction is not significant.