NON-IDEAL MHD EFFECTS AND MAGNETIC BRAKING CATASTROPHE IN PROTOSTELLAR DISK FORMATION

By Zhi-Yun Li, Ruben Krasnopolsky, H. Shang. 2011

156 citations recorded by the source corpus.

Dense, star-forming cores of molecular clouds are observed to be significantly magnetized. A realistic magnetic field of moderate strength has been shown to suppress, through catastrophic magnetic braking, the formation of a rotationally supported disk (RSD) during the protostellar accretion phase of low-mass star formation in the ideal MHD limit. We address, through two-dimensional (axisymmetric) simulations, the question of whether realistic levels of non-ideal effects, computed with a simplified chemical network including dust grains, can weaken the magnetic braking enough to enable an RSD to form. We find that ambipolar diffusion (AD), the dominant non-ideal MHD effect over most of the density range relevant to disk formation, does not enable disk formation, at least in two dimensions. The reason is that AD allows the magnetic flux that would be dragged into the central stellar object in the ideal MHD limit to pile up instead in a small circumstellar region, where the magnetic field strength (and thus the braking efficiency) is greatly enhanced. We also find that, on the scale of tens of AU or more, a realistic level of Ohmic dissipation does not weaken the magnetic braking enough for an RSD to form, either by itself or in combination with AD. The Hall effect, the least explored of these three non-ideal MHD effects, can spin up the material close to the central object to a significant, supersonic rotation speed, even when the core is initially non-rotating, although the spun-up material remains too sub-Keplerian to form an RSD. The problem of catastrophic magnetic braking that prevents disk formation in dense cores magnetized to realistic levels remains unresolved. Possible resolutions of this problem are discussed.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Physical and Theoretical Chemistry

View Paper: https://www.semanticscholar.org/paper/c4fee35586033fc0cc6904c83150229c110f3cae

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Citations in the corpus

RADIATION MAGNETOHYDRODYNAMIC SIMULATIONS OF PROTOSTELLAR COLLAPSE: PROTOSTELLAR CORE FORMATION (2012, background): “Moreover, Mellon & Li (2008, 2009) claimed that rotationally supported circumstellar disks are difficult to form in magnetized molecular cloud cores because the angular momentum transport due to magnetic fields is too efficient (“the magnetic braking catastrophe”; see also Li et al. 2011).”
RADIATION MAGNETOHYDRODYNAMIC SIMULATIONS OF PROTOSTELLAR COLLAPSE: NONIDEAL MAGNETOHYDRODYNAMIC EFFECTS AND EARLY FORMATION OF CIRCUMSTELLAR DISKS (2015, background): “Our results seem to be inconsistent with the earlier nonideal MHD simulations by Li et al. (2011) (see also Mellon & Li 2009), in which no rotationally supported disk was formed even with ambipolar diffusion.”
Protostellar disk formation and transport of angular momentum during magnetized core collapse (2012, background): “In this so-called ambipolar diffusion-induced accretion shock (Mellon & Li 2009; Li et al. 2011), the magnetic braking is greatly enhanced and can efficiently prevent the formation of rotationally supported disks.”
On the diversity and statistical properties of protostellar discs (2018, background): “…by magnetic braking in ideal MHD calculations, the effects of introducing ambipolar diffusion alone are insufficient to allow the formation of large discs (Duffin & Pudritz 2009; Mellon & Li 2009; Li et al. 2011; Dapp et al. 2012; Tomida et al. 2015; Tsukamoto et al. 2015b; Wurster et al. 2016).”
TADPOL: A 1.3 mm SURVEY OF DUST POLARIZATION IN STAR-FORMING CORES AND REGIONS (2013, background): “Objects with misaligned B-fields and rotation axes are less susceptible to the “magnetic braking catastrophe,” where magnetic braking prevents the formation of a rotationally supported Keplerian disk (Allen et al. 2003; Li et al. 2011).”
A Keplerian disk around a Class 0 source: ALMA observations of VLA1623A (2013, background): “Consideration of non-ideal MHD effects and their role in disk formation has been explored, but there is no clear solution (Li et al. 2011).”