Birth of magnetized low-mass protostars and circumstellar disks

Abstract

Although protostars and disks are often studied separately owing to numerical and observational challenges, breakthroughs in recent years have highlighted the need to study both objects in concert. The role of magnetic fields in this regard must be investigated. We aim to describe the birth of the protostar and that of its disk, as well as their early joint evolution following the second collapse. We wish to study the structure of the nascent star-disk system, while focusing on the innermost sub-AU region. We carry out high resolution 3D RMHD simulations, describing the collapse of dense cloud cores to stellar densities. The calculations reach $\approx 2.3$ yr after protostellar birth. Our simulations are also compared to their hydro counterpart to better isolate the role of magnetic fields. When accounting for ambipolar diffusion, the efficiency of magnetic braking is drastically reduced and the nascent protostar reaches breakup velocity, thus forming a rotationally supported disk. The diffusion of the magnetic field also allows for the implantation of a $\sim \mathrm{kG}$ field in the protostar, which is thereafter maintained. The magnetic field is mainly toroidal in the star-disk system, although a notable vertical component threads it. We also show that the nascent disk is prone to the MRI, although our resolution is inadequate to capture the mechanism. We note a sensitivity of the disk's properties with regards to the angular momentum inherited prior to the second collapse, as well as the magnetic field strength. These calculations carry multiple implications on several issues in stellar formation theory, and offer perspectives for future modeling of the system. Should the fossil field hypothesis to explain the origins of magnetic fields in young stellar objects hold, we show that a $\sim \mathrm{kG}$ field strength may be implanted and maintained in the protostar at birth.