Chen & Lin, 2020, ApJ accepted, preprint
The formation of planetesimals - the building blocks of planets - is critical to understanding how planets form. The "streaming instability" (SI), first identified by Youdin & Goodman (2005), is now the leading theory for planetesimal formation. It works by concentrating dust particles to the point of gravitational collapse. Theoretically speaking, the SI is a generic phenomenon: it only requires mutually interacting dust and gas in rotation, which is exactly what constitutes a protoplanetary disk (PPD) - the birth sites of planets.
However, there are several obstacles in a real PPD that make whether or not the SI can actually operate a non-trivial issue. For example, PPDs have finite lifetimes of a few million years, while solid particles can be lost to the star on shorter timescales. These considerations imply a maximum allowed growth timescale for the SI to remain relevant. Furthermore, turbulence expected in PPDs can directly thwart the instability.
In this new paper, lead by my former ASIAA summer student Kan Chen (now an astrophysics masters student at the University of Cambridge), we quantify efficiency of the SI in realistic PPDs in order to evaluate its viability as a planetesimal-formation mechanism. To this end, we incorporate effects such as turbulence and calculate growth timescales of the SI in physical disk models.
The red curve in the figure to the left show growth timescales of the SI as a function of distance from the central star. The black curve show the inward drift timescales of dust particles. This particular disk model assumes a high dust-to-gas mass ratio of 0.1 and consider cm-sized dust grains. From previous studies that do not consider turbulence, these conditions should favor the SI. However, in more realistic disk models we find long growth timescales inside ~40AU, where dust will be lost before the SI can grow significantly. On the other hand, at larger radii (>40AU) the SI can grow before dust is lost. Our results indicates that the SI, and hence planetesimal formation, is favored in the outer disk.
We also performed more theoretical calculations surveying the impact of turbulence on the SI. The figure to the right show growth rates (normalized to the disk rotation frequency) as a function of turbulence strength α and particle size St. The growth "gap" is a special region in parameter space where the midplane dust-to-gas is unity, whence growth rates vanish. The region to the right of the red curve correspond to uninterestingly long growth timescales. The figure shows that large dust grains are more resilient to turbulence.
Our results show that it is rather difficult for small dust grains (say, smaller than mm-sizes) to undergo sufficient growth via the SI to trigger planetesimal formation in a realistic, turbulent PPD.