Abstract
Large-scale vertical magnetic fields are believed to play a key role in the evolution of protoplanetary discs. Associated with non-ideal effects, such as ambipolar diffusion, they are known to launch a wind that could drive accretion in the outer part of the disc (R > 1 AU). They also potentially lead to self-organisation of the disc into large-scale axisymmetric structures, similar to the rings recently imaged by sub-millimetre or near-infrared instruments (ALMA and SPHERE). The aim of this paper is to investigate the mechanism behind the formation of these gaseous rings, but also to understand the dust dynamics and its emission in discs threaded by a large-scale magnetic field. To this end, we performed global magneto-hydrodynamics (MHD) axisymmetric simulations with ambipolar diffusion using a modified version of the PLUTO code. We explored different magnetisations with the midplane β parameter ranging from 105 to 103 and included dust grains -treated in the fluid approximation- ranging from 100 μm to 1 cm in size. We first show that the gaseous rings (associated with zonal flows) are tightly linked to the existence of MHD winds. Secondly, we find that millimetre-size dust is highly sedimented, with a typical scale height of 1 AU at R = 100 AU for β = 104, compatible with recent ALMA observations. We also show that these grains concentrate into pressure maxima associated with zonal flows, leading to the formation of dusty rings. Using the radiative transfer code MCFOST, we computed the dust emission and make predictions on the ring-gap contrast and the spectral index that one might observe with interferometers like ALMA.
Highlights
Protoplanetary discs, the nurseries of planets, are known to dissipate over a few million years
To take into account the resolution of instruments like ALMA, in the lower panels we show the same images convolved with a Gaussian kernel with a full width half maximum (FWHM) of 3 AU
In summary, we investigated the gas and dust dynamics of protoplanetary discs threaded by a large-scale magnetic field and subject to ambipolar diffusion
Summary
Protoplanetary discs, the nurseries of planets, are known to dissipate over a few million years. Some examples of discs showing rings are HL tau (ALMA Partnership 2015), TW Hydra (Andrews et al 2016), or the disc around Herbig Ae star HD 163296 (Isella et al 2016) These structures are possibly altering the long-term disc evolution and could be privileged locations of dust accumulation (Pinilla et al 2012), a key step towards planetary core formation. Simulations including a large-scale poloidal field and non-ideal effects (Hall effect, ambipolar diffusion, mainly) have shown that discs are able to launch a wind from their surface removing angular momentum and driving accretion in the process (Bai 2013; Lesur et al 2014; Simon et al 2015; Gressel & Pessah 2015; Béthune et al 2017). We adopt a multi-fluid approximation in which the ionized gas and the dust interact and exchange momentum through drag forces
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