Abstract
ABSTRACT In their early stages, protoplanetary discs are sufficiently massive to undergo gravitational instability (GI). This instability is thought to be involved in mass accretion, planet formation via gas fragmentation, the generation of spiral density waves, and outbursts. A key and very recent area of research is the interaction between the GI and magnetic fields in young protoplanetary discs, in particular whether this instability is able to sustain a magnetic field via a dynamo. We conduct 3D, stratified shearing-box simulations using two independent codes, PLUTO and Athena++, to characterize the GI dynamo in poorly ionized protostellar discs subject to ambipolar diffusion. We find that the dynamo operates across a large range of ambipolar Elssaser number Am (which characterizes the strength of ambipolar diffusion) and is particularly strong in the regime Am = 10–100, with typical magnetic to thermal energy ratios of order unity. The dynamo is only weakly dependent on resolution (at least for Am ≲ 100), box size, and cooling law. The magnetic field is produced by the combination of differential rotation and large-scale vertical roll motions associated with spiral density waves. Our results have direct implications for the dynamo process in young protoplanetary discs and possibly some regions of active galactic nucleus discs.
Highlights
Magnetic fields should be ubiquitous in accretion discs and are likely to influence their dynamics and long-term evolution significantly
We find that the gravito-turbulent dynamo in the presence of ambipolar diffusion is very similar to that described by RL19: there is no fundamental difference between the magnetic fields generated in the presence of ambipolar diffusion and those produced subject to Ohmic dissipation
As shown by Riols & Latter (2018b), when 3D dynamics are allowed in simulations of gravito-turbulence, the customary spiral density waves are accompanied by characteristic hydrodynamical motions such as vertical rolls that play an important role in amplifying magnetic fields
Summary
Magnetic fields should be ubiquitous in accretion discs and are likely to influence their dynamics and long-term evolution significantly. Even in the inner regions of protoplanetary discs and in more ionised objects like AGN discs, the viability of the MRI dynamo process is not guaranteed, because of the low magnetic Prandtl number that characterises the plasma in these environments (see Fromang et al 2007; Balbus & Henri 2008; Kapyla & Korpi 2011; Meheut et al 2015; Riols et al 2017b, for more discussions about this issue) Another potential source of turbulence in accretion discs is the gravitational instability (GI). The answer is non-trivial since there are major differences between the two diffusion processes: unlike Ohmic dissipation, ambipolar diffusion is non-linear, anisotropic, and does not allow reconnection of the field Another important question that has been only partially addressed (Riols & Latter 2018a) is the effect of a net vertical field threading the disc on the dynamo.
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