Global MHD simulations of stratified and turbulent protoplanetary discs. II. Dust settling - Astrophysics > Earth and Planetary AstrophysicsReportar como inadecuado




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Abstract: The aim of this paper is to study the vertical profile of small dustparticles in protoplanetary discs in which angular momentum transport is due toMHD turbulence driven by the magnetorotational instability. We considerparticle sizes that range from approximately 1 micron up to a fewmillimeters.We use a grid-based MHD code to perform global two-fluidsimulations of turbulent protoplanetary discs which contain dust grains ofvarious sizes. In quasi-steady state, the gravitational settling of dustparticles is balanced by turbulent diffusion. Simple and standard models ofthis process fail to describe accurately the vertical profile of the dustdensity. The disagreement is larger for small dust particles of a few micronsin size, especially in the disc upper layers $Z>3H$, where $H$ is thescale-height. Here there can be orders of magnitude in the disagreementbetween the simple model predictions and the simulation results. This isbecause MHD turbulence is not homogeneous in accretion discs, since velocityfluctuations increase significantly in the disc upper layer where a stronglymagnetized corona develops. We provide an alternative model that gives a betterfit to the simulations. In this model, dust particles are diffused away fromthe midplane by MHD turbulence, but the diffusion coefficient varies verticallyand is everywhere proportional to the square of the local turbulent verticalvelocity fluctuations. The spatial distribution of dust particles can be usedto trace the properties of MHD turbulence in protoplanetary discs, such as theamplitude of the velocity fluctuations. In the future, detailed and directcomparison between numerical simulations and observations should prove a usefultool for constraining the properties of turbulence in protoplanetary discs.



Autor: Sebastien Fromang, Richard P. Nelson

Fuente: https://arxiv.org/







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