Cloud Dissipation and Disk Wind in the Late Phase of Star Formation

Abstract

We perform a long-term simulation of star and disk formation using three-dimensional nonideal magnetohydrodynamics. The simulation starts from a prestellar cloud and proceeds through the long-term evolution of the circumstellar disk until ∼1.5 × 105 yr after protostar formation. The disk has size ≲50 au and little substructure in the main accretion phase because of the action of magnetic braking and the magnetically driven outflow to remove angular momentum. The main accretion phase ends when the outflow breaks out of the cloud, causing the envelope mass to decrease rapidly. The outflow subsequently weakens as the mass accretion rate also weakens. While the envelope-to-disk accretion continues, the disk grows gradually and develops transient spiral structures, due to gravitational instability. When the envelope-to-disk accretion ends, the disk becomes stable and reaches a size ≳300 au. In addition, about 30% of the initial cloud mass has been ejected by the outflow. A significant finding of this work is that after the envelope dissipates a revitalization of the wind occurs, and there is mass ejection from the disk surface that lasts until the end of the simulation. This mass ejection (or disk wind) is generated because the magnetic pressure significantly dominates both the ram pressure and thermal pressure above and below the disk at this stage. Using the angular momentum flux and mass-loss rate estimated from the disk wind, the disk dissipation timescale is estimated to be ∼106 yr.