Ring and Gap Formation in Wind-Launching Disks: Ambipolar Diffusion and Reconnection

Radial substructures in circumstellar discs are now routinely observed by Atacama Large Millimeter/submillimeter Array (ALMA). There is also growing evidence that disc winds drive accretion in such discs. We show through 2D (axisymmetric) simulations that rings and gaps develop naturally in magnetically coupled disc-wind systems on the scale of tens of au, where ambipolar diffusion (AD) is the dominant non-ideal magnetohydrodynamic effect. In simulations where the magnetic field and matter are moderately coupled, the disc remains relatively laminar with the radial electric current steepened by AD into a thin layer near the mid-plane. The toroidal magnetic field sharply reverses polarity in this layer, generating a large magnetic torque that drives fast accretion, which drags the poloidal field into a highly pinched radial configuration. The reconnection of this pinched field creates magnetic loops where the net poloidal magnetic flux (and thus the accretion rate) is reduced, yielding dense rings. Neighbouring regions with stronger poloidal magnetic fields accrete faster, forming gaps. In better magnetically coupled simulations, the so-called avalanche accretion streams develop continuously near the disc surface, rendering the disc-wind system more chaotic. Nevertheless, prominent rings and gaps are still produced, at least in part, by reconnection, which again enables the segregation of the poloidal field and the disc material similar to the more diffusive discs. However, the reconnection is now driven by the non-linear growth of magnetorotational instability channel flows. The formation of rings and gaps in rapidly accreting yet laminar discs has interesting implications for dust settling and trapping, grain growth, and planet formation.

Animation of the reference model. The color contours show (a) the logarithm of poloidal velocity (cm s−1), (b) the logarithm of plasma-β, (c) the ratio of the toroidal to the poloidal magnetic field components, Bϕ/Bp, (d) the logarithm of density (g cm−3), (e) an axisymmetric, face-on view of the disc surface density distribution normalized to the initial power-law distribution, Σi = Σ0(r/r0)−1/2, and (f) the mass flux per unit polar angle, d/dθ=2πr2ρvrsinθ⁠, normalized to 0=r02ρ0cs,0⁠. Poloidal magnetic field lines (i.e. magnetic flux contours) are shown in grey in panel (c). Panel (d) shows two specific poloidal magnetic field lines with mid-plane footpoints at r = 8 au (magenta) and r = 7 au (black). Poloidal velocity unit vectors are plotted in black in panels (a), (c), (d), and (f).

Related Publications

  • Suriano SS; Li Z-Y; Krasnopolsky RShang H“The formation of rings and gaps in magnetically coupled disk-wind systems: ambipolar diffusion and reconnection”, MNRAS: 477(1), 1239-1257, June, 2018 [SCI] ADS | Fulltext )
  • Suriano SS; Li Z-Y; Krasnopolsky RShang H“Rings and gaps produced by variable magnetic disc winds and avalanche accretion streams – I. Axisymmetric resistive MHD simulations”, MNRAS: 468(4), 3850-3868, July 11, 2017 [SCI] ADS | Fulltext )