Disc formation in magnetized dense cores with turbulence and ambipolar diffusion

Abstract

Disks are essential to the formation of both stars and planets, but how they form in magnetized molecular cloud cores remains debated. This work focuses on how the disk formation is affected by turbulence and ambipolar diffusion (AD), both separately and in combination, with an emphasis on the protostellar mass accretion phase of star formation. We find that a relatively strong, sonic turbulence on the core scale strongly warps but does not completely disrupt the well-known magnetically-induced flattened pseudodisk that dominates the inner protostellar accretion flow in the laminar case, in agreement with previous work. The turbulence enables the formation of a relatively large disk at early times with or without ambipolar diffusion, but such a disk remains strongly magnetized and does not persist to the end of our simulation unless a relatively strong ambipolar diffusion is also present. The AD-enabled disks in laminar simulations tend to fragment gravitationally. The disk fragmentation is suppressed by initial turbulence. The ambipolar diffusion facilitates the disk formation and survival by reducing the field strength in the circumstellar region through magnetic flux redistribution and by making the field lines there less pinched azimuthally, especially at late times. We conclude that turbulence and ambipolar diffusion complement each other in promoting disk formation. The disks formed in our simulations inherit a rather strong magnetic field from its parental core, with a typical plasma-β of order a few tens or smaller, which is 2-3 orders of magnitude lower than the values commonly adopted in MHD simulations of protoplanetary disks. To resolve this potential tension, longer-term simulations of disk formation and evolution with increasingly more realistic physics are needed.

The JCMT BISTRO Survey: Alignment between Outflows and Magnetic Fields in Dense Cores/Clumps

“…MHD simulations show that when the magnetic field is misaligned with the rotational axis in a collapsing dense core, the efficiency of magnetic braking decreases, and more angular momentum can be transferred to the vicinity of the central protostar, resulting in the formation of a larger rotationally supported disk (Joos et al 2012;Li et al 2013;Hirano et al 2020). Similar effects are also seen in MHD simulations with turbulence, where the local turbulence in a collapsing core can cause misalignment between the magnetic field and the rotational axis (e.g., Gray et al 2018;Lam et al 2019). Theoretically, an outflow is expected to launch along the rotational axis of a stardisk system (e.g., Blandford & Payne 1982;Pudritz & Norman 1983).…”

“…In contrast, protostellar sources with well aligned magnetic fields and outflows can also exhibit significant rotational motion on a 1000 au scale, such as L1448IRS2 (Yen et al 2015;Kwon et al 2019;Gaudel et al 2020). Thus, overall, the misalignment is unlikely a primary mechanism, and other mechanisms, such as nonideal MHD effects, could play a more important role in disk formation and growth (Tsukamoto et al 2017;Zhao et al 2016Zhao et al , 2018Masson et al 2016;Matsumoto et al 2017;Lam et al 2019;.…”

Formation and Evolution of Disks Around Young Stellar Objects

“…Basically, the ambipolar diffusion becomes efficient at high density and operates on a time scale shorter than the free-fall time. Such a magnetic diffusion barrier is conceptually similar to the AD-shock ; see also Lam et al 2019), but is less obstructive to the infalling flow and not obviously enhancing the magnetic field strength across the barrier. The discrepancy likely originates, as pointed out in Zhao et al (2016), from the grain size distribution in their chemical network , which enhances the ambipolar diffusivity by a factor of ∼10 as compared to that used in Li et al (2011) and Mellon and Li (2009).…”

Collapse of turbulent massive cores with ambipolar diffusion and hybrid radiative transfer

“…However, this picture may change when turbulence and nonideal MHD are accounted for. Because both effects individually (see, e.g., Joos et al 2013;Hennebelle et al 2016a) and together (Lam et al 2019) solve the magnetic catastrophe, we would expect the disk-magnetic field orientation to tend toward a random distribution. We investigated the specific angular momentum components, and the alignment between this vector and the large-scale magnetic field (along the x-axis) as a function of the Mach number and the magnetic field strength.…”