Oral Presentation
Coagulation instability and planetesimal formation in protoplanetary disks
Presenter: Ryosuke Tominaga (RIKEN Cluster for Pioneering Research)
Planetesimal formation is the first step of the planet-forming process. It is known that radial drift of dust grains and collisional fragmentation potentially prevent dust growth toward planetesimals. Some processes have been proposed to explain the planetesimal formation beyond those barriers. One of the processes is hydrodynamical clumping via disk instabilities (e.g., Youdin & Goodman 2005; Johansen et al. 2007; Youdin 2011; Takahashi & Inutsuka 2014). In most cases, such instabilities require (1) large dust grains and (2) relatively large dust-to-gas ratio (e.g., Carrera et al. 2015). However, dust is known to deplete after the size growth (e.g., Brauer et al. 2008), and thus some retention mechanisms such as dust trap at a pressure bump are necessary for the planetesimal formation via the instabilities. In this work, we propose a new instability driven by dust coagulation as a mechanism of the prerequisite dust retention. We name the new instability ``coagulation instability” (Tominaga et al. 2021). Our local linear analysis shows that coagulation instability operates and causes radial dust concentration even when the dust-to-gas surface density ratio is less than 0.01. We also conduct one-dimensional simulations of coagulation instability in a radially extended disk. In the radially global simulations, we take effects of fragmentation and backreaction into account. The results show that multiple dust rings form and dust-to-gas surface density ratio increases from ~0.001 to ~0.01. We also find that dust grains efficiently grow larger beyond the fragmentation barrier in the rings, and the dimensionless stopping time reaches unity. This is because the backreaction in the resulting dust rings reduces the collision velocities. Consequently, the nonlinear development of coagulation instability sets up preferable regions for the previous instabilities to develop. Indeed, some of the formed rings are found unstable to secular gravitational instability (GI; e.g.,Tominaga et al. 2019). We expect that the successive development of coagulation instability and the other dust-gas instabilities such as secular GI is one promising mechanism of planetesimal formation.
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