A New Instability Driven by Dust Coagulation in Protoplanetary Disks

Collisional growth of dust grains toward planetesimals is the first step of planet formation. There are some barriers that potentially inhibit planetesimal formation. One of them is called the radial drift barrier: dust grains drift faster through gas as they grow, and the drift becomes too fast for dust to grow substantially before they reach a central star. Another potential barrier is collisional fragmentation, which depends on surface energy of materials. To circumvent these barriers, previous studies proposed dust-gas instabilities, which include streaming instability (e.g., Youdin & Goodman 2005; Simon et al. 2016) and secular gravitational instability (secular GI; e.g., Youdin 2011; Takahashi & Inutsuka 2014). These instabilities accumulate dust grains locally. If the resulting dust-rich regions are massive enough, they collapse self-gravitationally and form planetesimals (e.g., Johansen et al. 2007, 2009). Efficiency of the dust concentration is higher for a higher dust-to-gas ratio and larger dust grains. However, previous models of dust growth found that dust grains get depleted as they grow larger and drift faster (e.g., Brauer et al. 2008). Thus, some dust retention mechanisms are necessary to trigger the above instabilities, for example, dust trap at a pressure bump (e.g., Whipple 1972). We propose a new mechanism of the prerequisite dust retention: an instability driven by dust coagulation. We first conduct linear analyses treating dust coagulation and demonstrate the existence of an unstable mode, which we name coagulation instability (Tominaga et al. 2021). We then perform numerical simulations to investigate its nonlinear development and find that dust grains are concentrated into multiple rings (Tominaga et al. 2022a). In the rings, dust grains grow to sizes that are large enough to trigger secular GI since the aerodynamical feedback reduces dust-collision velocities and makes fragmentation insignificant (Tominaga et al. 2022b). The feedback also reduces the drift velocity, which will save dust grains in a disk. Therefore, coagulation instability is one promising mechanism to bridge the gap between the dust growth and planetesimal formation via the previous instabilities.