[P-PS07] Planetary Sciences

[PPS07-18] Planetesimal formation via coagulation instability in protoplanetary disks

*Ryosuke Tominaga¹, Hidekazu Tanaka², Hiroshi Kobayashi³, Shu-ichiro Inutsuka³ (1.RIKEN, 2.Tohoku University, 3.Nagoya University)

Keywords:protoplanetary disk, dust, planetesimal formation

The formation of planetesimals 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. Two processes have been proposed to explain the planetesimal formation beyond those barriers. One is porous dust aggregation (e.g., Okuzumi et al. 2012; Kobayashi & Tanaka 2021) and the other is hydrodynamical clumping (e.g., Youdin & Goodman 2005; Johansen et al. 2007). In this work, we investigate the latter process due to coagulation instability (Tominaga et al. 2021). Coagulation instability is a new instability that we found in our previous study. The instability operates and causes radial dust concentration even when the dust-to-gas surface density ratio is less than 0.01. We conduct one-dimensional simulations of coagulation instability with taking fragmentation and backreaction to the gas disk into account. The results show that multiple dust rings form and dust grains efficiently grow larger in the rings even at r >10 au. We find that the radial concentration via the instability locally increases the dust-to-gas surface density ratio to ~ 0.01 but stops eventually. We attribute the saturation of the dust concentration to the stabilization of coagulation instability due to the backreaction. Thus, the dust evolution consists of (1) the first radial concentration via the instability and (2) pure coagulation in the resulting rings. The dust concentration and the backreaction reduce the inward drift velocity and the collision velocities. This enables the dust growth beyond the drift barrier and the fragmentation barrier. Thus, the nonlinear development of coagulation instability potentially explains the planetesimal formation. We also find that the resulting rings can be unstable to secular gravitational instability (GI) that was proposed as another mechanism of hydrodynamical clumping (e.g., Ward 2000; Youdin 2011; Takahashi & Inutsuka 2014; Tominaga et al. 2019). The subsequent development of secular GI and/or classical dust GI will trigger top-down planetesimal formation, which might be faster than the bottom-up growth of large dust, e.g., beyond ~1-10 m. Therefore, we propose the successive development of coagulation instability and secular GI/classical GI as one promising mechanism of planetesimal formation.