Global Simulations of Self-gravitating Magnetized Protoplanetary Disks

Abstract

In the early stages of a protoplanetary disk, when its mass is a significant fraction of its star's, turbulence generated by gravitational instability (GI) should feature significantly in the disk's evolution. At the same time, the disk may be sufficiently ionised for magnetic fields to play some role in the dynamics. Though usually neglected, the impact of magnetism on the GI may be critical, with consequences for several processes: the efficiency of accretion, spiral structure formation, fragmentation, and the dynamics of solids. In this paper, we report on global three-dimensional magnetohydrodynamical simulations of a self-gravitating protoplanetary disk using the meshless finite mass (MFM) Lagrangian technique. We confirm that GI spiral waves trigger a dynamo that amplifies an initial magnetic field to nearly thermal amplitudes (plasma β < 10), an order of magnitude greater than that generated by the magnetorotational instability alone. We also determine the dynamo's nonlinear back reaction on the gravitoturbulent flow: the saturated state is substantially hotter, with an associated larger Toomre parameter and weaker, more 'flocculent' spirals. But perhaps of greater import is the dynamo's boosting of accretion via a significant Maxwell stress; mass accretion is enhanced by factors of several relative to either pure GI or pure MRI. Our simulations use ideal MHD, an admittedly poor approximation in protoplanetary disks, and thus future studies should explore the full gamut of non-ideal MHD. In preparation for that, we exhibit a small number of Ohmic runs that reveal that the dynamo, if anything, is stronger in a non-ideal environment. This work confirms that magnetic fields are a potentially critical ingredient in gravitoturbulent young disks, possibly controlling their evolution, especially via their enhancement of (potentially episodic) accretion.

Parametric instability in a free-evolving warped protoplanetary disc

“…In young circumstellar disc, gravitational instability and magnetohydrodynamic turbulence can generate a relatively large α (Deng et al 2020) leading to diffusive evolution of warps. In the diffusive regime with α > H/R one would expect fast warp diffusion on the timescale of α(R/H) 2 Ω −1 (Papaloizou & Pringle 1983).…”

“…It does not employ the artificial viscosity used in previous warped disc simulations with Smoothed Particle Hydrodynamics (SPH) and thus shows better conservation properties than SPH (Hopkins 2015;Deng et al 2017). As mentioned in the Introduction, at sufficiently high resolution, MFM can even capture the subsonic turbulence resulting from the magnetorotational instability (Deng et al 2019(Deng et al , 2020.…”

“…To save computational resources we utilized the particle splitting scheme in the GIZMO code to prepare initial conditions (see also Deng et al 2020). We sample the density (Fig.…”

“…MFM captures shocks without introducing artificial viscosity and thus shows better angular momentum conservation than SPH (Hopkins 2015;Deng et al 2017). At sufficiently high resolution it can even capture turbulence resulting from the magnetorotational instability while SPH fails and leads to unphysical growth of the magnetic field (Deng et al 2019(Deng et al , 2020). MFM appears promising for modelling low-viscosity warped discs.…”

Formation of the First Stars and Black Holes

“…The angular momentum barrier is thus particularly strong for SMSs. Two mechanisms might allow for efficient angular momentum loss in the accretion process: gravitational torques (Begelman et al, 2006;Wise et al, 2008; and magnetic fields (Pandey et al, 2019;Deng et al, 2019Deng et al, , 2020. Supermassive accretion discs are highly gravitationally unstable, and departures from axial symmetry are pronounced.…”