Effect of angular momentum alignment and strong magnetic fields on the formation of protostellar discs

Abstract

Star forming molecular clouds are observed to be both highly magnetized and turbulent. Consequently the formation of protostellar disks is largely dependent on the complex interaction between gravity, magnetic fields, and turbulence. Studies of non-turbulent protostellar disk formation with realistic magnetic fields have shown that these fields are efficient in removing angular momentum from the forming disks, preventing their formation. However, once turbulence is included, disks can form in even highly magnetized clouds, although the precise mechanism remains uncertain. Here we present several high resolution simulations of turbulent, realistically magnetized, high-mass molecular clouds with both aligned and random turbulence to study the role that turbulence, misalignment, and magnetic fields have on the formation of protostellar disks. We find that when the turbulence is artificially aligned so that the angular momentum is parallel to the initial uniform field, no rotationally supported disks are formed, regardless of the initial turbulent energy. We conclude that turbulence and the associated misalignment between the angular momentum and the magnetic field are crucial in the formation of protostellar disks in the presence of realistic magnetic fields.

Disc formation in magnetized dense cores with turbulence and ambipolar diffusion

“…The exact reason for this difference is unclear. We also note that the lack of a persistent rotationally supported disk is broadly consistent with the work of Gray et al (2018), who found that such a disk does not form unless there is a large misalignment between the turbulence-induced angular momentum (which is set to zero for our simulation as a whole) and the magnetic field. We note that Kuffmeier et al (2017) found multiple spirals in the circumstellar regions in some of their ideal MHD disk formation simulations (see the left column of their Fig.…”

“…This is a general concern for disk formation simulations that focus on the protostellar mass accretion phase where a sink particle treatment is needed, particularly for the relatively low-resolution simulations presented in this manuscript, because the angular momentum of the material accreted onto the central star (sink particle) from the sink region is lost as far as the disk formation is concerned (e.g. Machida et al 2014;Gray et al 2018). We have carried out a crude resolution study, by running the uniform-grid simulations well into the protostellar accretion phase without zoom-in (256 cells in 5000 au).…”

“…How large (∼100 au scale), persistent disks form and evolve during the later, main protostellar mass accretion phase of star formation is far less certain, as stressed by Tsukamoto (2016) and, more recently, Gray et al (2018). One potential difficulty in this phase is that, as more and more magnetized core material collapses onto the central protostellar system, more and more magnetic flux should be dragged by the collapsing material to the circumstellar region, making the magnetic field there increasingly stronger and magnetic braking increasingly more efficient, unless the magnetic flux can be effectively redistributed outward relative to the infalling matter.…”

“…Sink (or equivalent) treatment has been employed in magnetized protostellar disk formation simulations using both SPH (e.g., Wurster et al 2016;Lewis & Bate 2018) and grid-based MHD codes. The latter include both ideal MHD simulations with turbulence (e.g., Seifried et al 2013;González-Casanova et al 2016;Kuffmeier et al 2017;Gray et al 2018;Kuruwita & Federrath 2019), and non-ideal MHD simulations with Ohmic dissipation (e.g., Machida et al 2011Machida et al , 2014Tomida et al 2017;Kölligan & Kuiper 2018), Ohmic dissipation and turbulence (e.g., Matsumoto et al 2017), ambipolar diffusion (e.g., Masson et al 2016;Hennebelle et al 2016;Zhao et al 2016Zhao et al , 2018, and all three non-ideal MHD effects but no turbulence (e.g., Li et al 2011). Ideally, one would want to include both turbulence and all three non-ideal MHD effects and follow the disk formation and evolution to the end of the protostellar mass accretion phase.…”

Formation and Evolution of Disks Around Young Stellar Objects

“…This disordered magnetic field, leads to inefficient magnetic braking. Note that this conclusion is different from the one reached by Gray et al (2018) who concluded that the dominant effect is the misalignment between rotation and magnetic field axis.…”