Abstract
Rings and gaps have been observed in a wide range of proto-planetary discs, from young systems like HLTau to older discs like TW Hydra. Recent disc simulations have shown that magnetohydrodynamic (MHD) turbulence (in both the ideal or non-ideal regime) can lead to the formation of rings and be an alternative to the embedded planets scenario. In this paper, we have investigated the way in which these ring form in this context and seek a generic formation process, taking into account the various dissipative regimes and magnetisations probed by the past simulations. We identify the existence of a linear and secular instability, driven by MHD winds, and giving birth to rings of gas that have a width larger than the disc scale height. We show that the linear theory is able to make reliable predictions regarding the growth rates, the contrast and spacing between ring and gap, by comparing these predictions to a series of 2D (axisymmetric) and 3D MHD numerical simulations. In addition, we demonstrate that these rings can act as dust traps provided that the disc is sufficiently magnetised, with plasma beta lower than 104. Given its robustness, the process identified in this paper could have important implications, not only for proto-planetary discs but also for a wide range of accreting systems threaded by large-scale magnetic fields.
Highlights
The radio-interferometer ALMA and the new generation of instruments like SPHERE at the Very Large Telescope have imaged a variety of structures in proto-planetary discs around young stars (Garufi et al 2017)
We show in particular that axisymmetric modes projected into the Fourier space grow exponentially, with welldefined growth rates corresponding to those predicted by the theory
We assumed that mass is replenished locally at a constant rate σi
Summary
The radio-interferometer ALMA and the new generation of instruments like SPHERE at the Very Large Telescope have imaged a variety of structures in proto-planetary discs around young stars (Garufi et al 2017). Several simulations in the local and global configuration, including a mean vertical field, have brought evidence that MHD flows and their winds, self-organise into large-scale axisymmetric structures or “zonal flows” associated with rings of matter (Kunz & Lesur 2013; Bai 2015; Béthune et al 2016, 2017). These features appear predominantly in the presence of non-ideal effects, but were noticed in MRI simulations without any explicit diffusion (Steinacker & Papaloizou 2002; Bai & Stone 2014; Suriano et al 2018).
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