Issue |
A&A
Volume 496, Number 3, March IV 2009
|
|
---|---|---|
Page(s) | 597 - 608 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361/200811220 | |
Published online | 09 February 2009 |
Global MHD simulations of stratified and turbulent protoplanetary discs
II. Dust settling
1
CEA, Irfu, SAp, Centre de Saclay, 91191 Gif-sur-Yvette, France e-mail: sebastien.fromang@cea.fr
2
UMR AIM, CEA-CNRS-Univ. Paris VII, Centre de Saclay, 91191 Gif-sur-Yvette, France
3
Astronomy Unit, Queen Mary, University of London, Mile End Road, London E1 4NS, UK
Received:
24
October
2008
Accepted:
15
January
2009
Aims. The aim of this paper is to study the vertical profile of small dust particles in protoplanetary discs in which angular momentum transport is due to MHD turbulence driven by the magnetorotational instability. We consider particle sizes that range from approximately 1 micron up to a few millimeters.
Methods. We use a grid-based MHD code to perform global two-fluid simulations of turbulent protoplanetary discs which contain dust grains of various sizes.
Results. In quasi-steady state, the gravitational settling of dust particles is
balanced by turbulent diffusion. Simple and standard models of this
process fail to describe accurately the vertical profile of the
dust density. The disagreement is larger for small dust particles
(of a few microns in size), especially in the disc upper layers
(, where H is the scale-height).
Here there can be orders of magnitude in the disagreement
between the simple model predictions and the simulation results.
This is because MHD turbulence is not homogeneous in accretion
discs, since velocity fluctuations increase significantly in the disc
upper layer where a strongly magnetized corona develops.
We provide an alternative model that gives a better fit to the
simulations. In this model, dust particles are diffused away from
the midplane by MHD turbulence, but the diffusion coefficient
varies vertically and is everywhere proportional to the square of the
local turbulent vertical velocity fluctuations.
Conclusions. The spatial distribution of dust particles can be used to trace the properties of MHD turbulence in protoplanetary discs, such as the amplitude of the velocity fluctuations. In the future, detailed and direct comparison between numerical simulations and observations should prove a useful tool for constraining the properties of turbulence in protoplanetary discs.
Key words: accretion, accretion disks / Magnetohydrodynamics (MHD) / methods: numerical / turbulence
© ESO, 2009
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