The Role of Magnetic Fields in the Formation of Protostellar Discs

Abstract

The formation of a protostellar disc is a natural outcome during the star formation process. As gas in a molecular cloud core collapses under self-gravity, the angular momentum of the gas will slow its collapse on small scales and promote the formation of a protostellar disc. Although the angular momenta of dense star-forming cores remain to be fully characterized observationally, existing data indicates that typical cores have enough angular momenta to form relatively large, 100 au-scale, rotationally supported discs, as illustrated by hydrodynamic simulations. However, the molecular clouds are observed to be permeated by magnetic fields, which can in principle strongly affect the evolution of angular momentum during the core collapse through magnetic braking. Indeed, in the ideal magnetohydrodynamic (MHD) limit, magnetic braking has been shown to be so efficient as to remove essentially all of the angular momentum of the material close to the forming star such that disc formation is suppressed. This failure to produce discs in idealized cores is known as the magnetic braking catastrophe. The catastrophe must be averted in order for the all-important rotationally supported discs to appear, but when and how this happens remains debated. We review the resolutions proposed to date, with emphasis on misalignment, turbulence and especially non-ideal effects. Non-ideal MHD accounts for charged and neutral species, making it a natural extension to the ideal MHD approximation, since molecular clouds are only weakly ionized. The dissipative non-ideal effects diffuse the magnetic field to weaken it, and the dispersive term redirects the magnetic field to promote or hinder disc formation, dependent upon the magnetic geometry. When self-consistently applying non-ideal processes, rotationally supported discs of at least tens of au form, thus preventing the magnetic braking catastrophe. The non-ideal processes are sensitive to the magnetic field strength, cosmic ray ionization rate, and gas and dust grain properties, thus a complete understanding of the host molecular cloud is required. Therefore, the properties of the host molecular cloud -and especially its magnetic field -cannot be ignored when numerically modelling the formation and evolution of protostellar discs.

What determines the formation and characteristics of protoplanetary discs?

“…One of the major outcome of the first calculations of magnetized collapse (e.g. Allen et al 2003;Galli et al 2006;Price & Bate 2007;Hennebelle & Fromang 2008;Li et al 2014;Wurster & Li 2018;Hennebelle & Inutsuka 2019), is that magnetic braking could be so efficient that disc formation may be entirely prevented, a process named as catastrophic braking. Further studies have shown that the aligned configuration assumed in these calculations was however a significant oversimplification and that discs should form in magnetized clouds, although the discs are smaller in size and fragment less than in the absence of magnetic field.…”

Numerical Methods for Simulating Star Formation

“…This ambitious strategy has already been discussed at length in the previous sections, so we will not describe it here again. We also refer the reader to the review by Wurster and Li (2018) on the formation of protostellar disks.…”

“…In the previous section, we presented the work done in the ideal MHD framework, which do not appear to be well suited to the ionization state in collapsing cores, in particular at the onset of disk formation. Recent works have emphasized the imperfect coupling of the dust and gas mixture with the magnetic fields at the transition between the envelop and the disk in collapsing cores (see the review by Wurster and Li, 2018 for the work done in the context of protostellar disk formation) and a lot of effort has been devoted over the past 10 years to include the so called non-ideal effects: the ambipolar diffusion, the Ohmic diffusion, and the Hall effect. The ambipolar diffusion is the common name to describe the interaction between neutrals and charged particles.…”

“…This domain remains the subject of intense research works that we do not detail in this review. We refer readers to section 4.0.1 in the review by Wurster and Li (2018) for a scan of the works that tackle the resistivity calculations. Ambipolar diffusion and Ohmic diffusion were the first to be considered in star formation applications.…”

“…The physical conditions in starforming regions are so wide that there is room for regions where all three effects dominate. In addition, every effect needs to be tested and quantified before any conclusions on the relative importance of each of these effects (see the review by Wurster and Li, 2018).…”