Trapping solids at the inner edge of the dead zone: 3-D global MHD simulations

Abstract

Context. The poorly-ionized interior of the protoplanetary disk or "dead zone" is the location where dust coagulation processes may be most efficient. However even here, planetesimal formation may be limited by the loss of solid material through radial drift, and by collisional fragmentation of the particles. Both depend on the turbulent properties of the gas. Aims. Our aim here is to investigate the possibility that solid particles are trapped at local pressure maxima in the dynamically evolving disk. We perform the first 3-D global non-ideal magnetohydrodynamical (MHD) calculations of a section of the disk treating the turbulence driven by the magneto-rotational instability (MRI). Methods. We use the ZeusMP code with a fixed Ohmic resistivity distribution. The domain contains an inner MRI-active region near the young star and an outer midplane dead zone, with the transition between the two modeled by a sharp increase in the magnetic diffusivity.Results. The azimuthal magnetic fields generated in the active zone oscillate over time, changing sign about every 150 years. We thus observe the radial structure of the "butterfly pattern" seen previously in local shearing-box simulations. The mean magnetic field diffuses from the active zone into the dead zone, where the Reynolds stress nevertheless dominates, giving a residual α between 10 −4 and 10 −3 . The greater total accretion stress in the active zone leads to a net reduction in the surface density, so that after 800 years an approximate steady state is reached in which a local radial maximum in the midplane pressure lies near the transition radius. We also observe the formation of density ridges within the active zone. Conclusions. The dead zone in our models possesses a mean magnetic field, significant Reynolds stresses and a steady local pressure maximum at the inner edge, where the outward migration of planetary embryos and the efficient trapping of solid material are possible.

High-resolution simulations of planetesimal formation in turbulent protoplanetary discs

“…Pressure bumps can also be launched by a radial variation in resistivity, e.g. at the edges of dead zones (Lyra et al 2008b;Dzyurkevich et al 2010).…”

Gas structure inside dust cavities of transition disks: Ophiuchus IRS 48 observed by ALMA

“…The planet/companion-disk interaction can also induce perturbations in the gas structure leading to local gas pressure maxima. These pressure maxima can prevent large ( mm-sized) dust grains from quickly drifting towards the star before planet formation through dust coagulation and core accretion can take place (e.g., Whipple 1972;Rice et al 2006;Alexander & Armitage 2007;Garaud 2007;Kretke & Lin 2007;Dzyurkevich et al 2010;Pinilla et al 2012;Birnstiel et al 2013;Lyra & Lin 2013). Thus, the gas structure in transition disks is of direct importance for planet formation.…”

Fossil magnetic field of accretion disks of young stars

“…Global numerical investigations of magnetized accretion disks and "dead" zones were performed in the ideal MHD limit (Fromang and Nelson 2006) and in resistive limit (Dzyurkevich et al 2010) with magnetic field strength being free parameter. Bai and Stone (2011), Simon et al (2013a,b) performed calculations with MAD in the frame of the local shearing-box approximation with fixed magnetic field strength and/or geometry.…”