Coagulation Instability in Protoplanetary Disks: A Novel Mechanism Connecting Collisional Growth and Hydrodynamical Clumping of Dust Particles

Abstract

We present a new instability driven by a combination of coagulation and radial drift of dust particles. We refer to this instability as "coagulation instability" and regard it as a promising mechanism to concentrate dust particles and assist planetesimal formation in the very early stages of disk evolution. Because of dust-density dependence of collisional coagulation efficiency, dust particles efficiently (inefficiently) grow in a region of positive (negative) dust density perturbations, which lead to a small radial variation of dust sizes and as a result radial velocity perturbations. The resultant velocity perturbations lead to dust concentration and amplify dust density perturbations. This positive feedback makes a disk unstable. The growth timescale of coagulation instability is a few tens of orbital periods even when dust-to-gas mass ratio is of the order of 10 −3 . In a protoplanetary disk, radial drift and coagulation of dust particles tend to result in dust depletion. The present instability locally concentrates dust particles even in such a dust-depleted region. The resulting concentration provides preferable sites for dust-gas instabilities to develop, which leads to further concentration. Dust diffusion and aerodynamical feedback tend to stabilize short-wavelength modes, but do not completely suppress the growth of coagulation instability. Therefore, coagulation instability is expected to play an important role in setting up the next stage for other instabilities to further develop toward planetesimal formation, such as streaming instability or secular gravitational instability.

Dust Dynamics in Transitional Disks: Clumping and Disk Recession

“…For instance, self-gravitationally collapsed dust clumps could potentially form planetesimals (e.g., Simon et al 2016). The clump formation observed in this paper may interact with other clump formation mechanisms, such as streaming instability (e.g., Youdin & Goodman 2005;Jacquet et al 2011), coagulation instability (Tominaga et al 2021), and some meso-scale instability triggered by the dust feedback in dust rings (Huang et al 2020). If our clumps do tend to coagulate, a recessing disk edge could leave behind a trail of planetesimals, creating an ideal breeding ground for close-in super-Earths.…”

“…However, the origin of those inner cavities is still uncertain. Recent surveys on transitional disks by Atacama Large Millimeter/Submillimeter Array (ALMA) and Australia Telescope Compact Array (ATCA) suggest that the cavities are cleared by planets in the disk (e.g., van der Marel et al 2018;Francis & van der Marel 2020;Norfolk et al 2021). However, evidence shows that planets may not be the only explanation for these cavities:…”

“…2. A complete disk survey in the Lupus star-forming region shows that the fraction of disks with a large cavity (≥20 au) is 11% (van der Marel et al 2018). This fraction is noticeably higher than the fraction of giant planets on such high orbits around main sequence stars (<8% for all stars, <6.8% for FGK stars, and <4.2% for M stars; Bowler 2016).…”