Gravitoturbulent dynamos in astrophysical discs

Abstract

The origin of large-scale and coherent magnetic fields in astrophysical discs is an important and long standing problem. Researchers commonly appeal to a turbulent dynamo, sustained by the magneto-rotational instability (MRI), to supply the large-scale field. But research over the last decade in particular has demonstrated that various non-ideal MHD effects can impede or extinguish the MRI, especially in protoplanetary disks. In this paper we propose a new scenario, by which the magnetic field is generated and sustained via the gravitational instability (GI). We use 3D stratified shearing box simulations to characterise the dynamo and find that it works at low magnetic Reynolds number (from unity to ∼ 100) for a wide range of cooling times and boundary conditions. The process is kinematic, with a relatively fast growth rate ( 0.1Ω), and it shares some properties of mean field dynamos. The magnetic field is generated via the combination of differential rotation and spiral density waves, the latter providing compressible horizontal motions and large-scale vertical rolls. At greater magnetic Reynolds numbers the build up of large-scale field is diminished and instead small-scale structures emerge from the breakdown of twisted flux ropes. We propose that GI may be key to the dynamo engine not only in young protoplanetary discs but also in some AGN and galaxies.

Evidence for supernova feedback sustaining gas turbulence in nearby star-forming galaxies

“…Ohmic diffusion, ambipolar diffusion, and Hall effect) can suppress the MRI instability (e.g. Wardle 1999;Kunz & Balbus 2004;Korpi et al 2010;Riols & Latter 2019). In addition, there are indications that SN driven-turbulence can counteract the MRI in the star-forming regions (e.g.…”

Formation of intermediate-mass planets via magnetically-controlled disk fragmentation

“…As the first attempt, we apply R m = 20 throughout the disk because full non-ideal MHD with coupled ionization chemistry is still challenging even for non-self-gravitating disk 32 . Nevertheless, we note that the large scale dynamo in gravito-turbulent disk thrives at low R m in contrast to MRI dynamo 8 . In the meantime, the fields are amplified by the local circulation near the Hill sphere (Fig.…”

“…Previously, fragmentation in magnetized protoplanetary disks has only been considered when the magneto-rotational instability is the driving mechanism for magnetic field growth 7 . Yet, this instability is naturally superseded by the spiral-driven dynamo when more realistic, nonideal MHD conditions are considered 8,9 . Here we report on MHD simulations of disk fragmentation in the presence of a spiral-driven dynamo.…”

“…The MHD simulation leads to more clumps due to the different power spectrum of the density fluctuations relative to the HD simulations, which is skewed to higher order modes with short wavelength 9 . The clumps, despite their small masses, begin to stir rotational flow around them locally, and the magnetic fields aligned with the spiral arms 8 are curved and amplified by the flow (Figure 1). In this initial phase the magnetic energy is less than 1% of the gas pressure (plasma beta β p > 100) in the inner region of the clumps (Figure 1, upper right).…”

Spiral structures in gravito-turbulent gaseous disks

“…GI turbulence may thus affect the vertical segregation of different dust sizes (Pinte et al 2016;Villenave et al 2020) or induce local enhancements of the dust disk thickness (Benisty et al 2015;Dong et al 2015). These 3D flows also open a new route of magnetic field amplification via a spiral dynamo in sufficiently ionized disks (Riols & Latter 2019;Deng et al 2020). 3.…”