The effect of MHD turbulence on massive protoplanetary disk fragmentation

Abstract

Abstract. Massive disk fragmentation has been suggested to be one of the mechanisms leading to the formation of giant planets. While it has been heavily studied in quiescent hydrodynamic disks, the effect of MHD turbulence arising from the magnetorotational instability (MRI) has never been investigated. This paper fills this gap and presents 3D numerical simulations of the evolution of locally isothermal, massive and magnetized disks. In the absence of magnetic fields, a laminar disk fragments and clumps are formed due to the effect of self-gravity. Although they disapear in less than a dynamical timescale in the simulations because of the limited numerical resolution, various diagnostics suggest that they should survive and form giant planets in real disks. When the disk is magnetized, it becomes turbulent at the same time as gravitational instabilities develop. At intermediate resolution, no fragmentation is observed in these turbulent models, while a large number of fragments appear in the equivalent hydrodynamical runs. This is because MHD turbulence reduces the strength of the gravitational instability. As the resolution is increased, the most unstable wavelengths of the MRI are better resolved and small scale angular momentum transport starts to play a role: fragments are found to form in massive and turbulent disks in that case. All of these results indicate that there is a complicated interaction between gravitational instabilities and MHD turbulence that influences disk fragmentation processes.

Magnetic Fields of Nondegenerate Stars

“…In particular, the discs fields and associated MHD turbulence can potentially inhibit disc fragmentation through gravitational instabilities and the subsequent formation of giant planets (Fromang 2005); they can modify as well the migration rate and angular momentum of protoplanets Machida et al 2006). By disrupting the inner regions of accretion discs, magnetic fields of cTTSs may stop the inward migration of giant planets formed earlier in the outer disc (which would no longer experience the gravitational torque from the disc once they enter the magnetospheric gap, Romanova & Lovelace 2006); orbital distances of most close-in giant planets discovered in the last decade around main sequence stars (smaller than 0.1 AU) are indeed compatible with typical sizes of magnetospheric gaps in cTTSs.…”

Circumstellar disks and planets

“…As a consequence, they are increasingly susceptible to gravitational instabilities. Numerical simulations, including realistic cooling conditions (e.g., Durisen et al, 2001;Johnson and Gammie, 2003) and MHD turbulence (e.g., Fromang et al, 2004;Fromang, 2005) show that sufficiently massive disks tend to form dense spiral arms, arclets, ridges and similar kinds of surface distortions. For disk-star systems with a mass ratio M disk /M * > 0.3, the disk might even begin to fragment into distinct blobs.…”

A Class of Self‐Gravitating, Magnetized Accretion Disks

“…This situation becomes more complicated when we consider magnetic fields. It seems that there is a complicated interaction between gravitational instability and MHD turbulence that influences disk structure, but MHD turbulence reduces the strength of the gravitational instability (Fromang 2005).…”