Update (Mar. 9, 2019): the paper has now been accepted by MNRAS. Click here for a preprint.
In a recently submitted paper, I studied the settling of small dust particles in turbulent protoplanetary discs.
The building blocks of planets are planetesimals: km-sized or larger rocks that attract one another through gravity, subsequently coalescing into protoplanets. However, solids in protoplanetary discs -- the birthplace of planets -- begin as much smaller dust grains. How to form planetesimals from these tiny grains is one of the major questions in modern planet formation theory.
One idea is gravitational instability. A population of small grains may undergo a collective collapse due to their cumulative self-gravity. On the other hand, the critical density for this to occur is high: the dust density must be about 30 times larger than the gas density. Comparing this to the typical dust-to-gas ratio of 1% in newly formed protoplanetary discs, it is evident that some other dust-concentrating mechanism must operate first.
In laminar discs, dust particles can sink to the disc midplane. Eventually, the midplane can become sufficiently dense in dust to trigger planetesimal formation. However, protoplanetary discs may not be laminar. Turbulence in the disc can stir and lift particles, thereby hinder particle settling and prevent planetesimal formation. This is demonstrated in the figure above.
It is therefore important to know under what conditions can dust particles settle in turbulent protoplanetary discs. We use numerical simulations to investigate this problem. We explicitly model the turbulence and find its strength is affected by the dust. The more dusty the less turbulent, and the easier for the dust to settle. This is demonstrated in the figure below, where we increased the dust content by a factor of 10 relative to the solar value, and observe the dust settles into a thin layer at the disc midplane.
In previous studies, the degree to which dust can settle is usually attributed to the particle size and the turbulence strength, with the latter often assumed to be independent of the dust properties. Here, we show that this may not be true, and one needs to explicitly simulate the mutual effect of dust and gas on each other.