[P-PS07] Planetary Sciences

[PPS07-20] Secular gravitational instability in vertically stratified disks

*Ryosuke Tominaga¹, Shu-ichiro Inutsuka², Sanemichi Z. Takahashi^3,4 (1.Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research, 2.Nagoya University, 3.National Astronomical Observatory of Japan, 4.Kagoshima University)

Keywords:protoplanetary disk, dust, planetesimal formation

Planetesimal formation is the first step of the planet-forming process. It is known that collisional growth of compact dust grains is faced with many barriers including the radial drift barrier before dust grains grow to planetesimals (e.g., Weidenschilling et al. 1977). One of the promising processes for explaining planetesimal formation is disk instability. This includes streaming instability (e.g., Youdin & Goodman 2005) and secular gravitational instability (secular GI; e.g., Ward 2000). Streaming instability has been extensively studied and regarded as the most promising mechanism. However, recent studies showed that even weak turbulence suppresses the growth of streaming instability (Chen & Lin 2020; Umurhan et al. 2020) and regulates the efficiency of dust clumping (Gole et al. 2020), which questions its viability. In this work, we focus on the other instability, i.e., secular GI. Secular GI is also one promising mechanism since the instability can develop even in gravitationally stable disks in contrast to the classical GI of a dust layer (e.g., Safronov 1969; Hayashi et al. 1985). Previous studies on secular GI utilized a razor-thin disk model for both dust and gas disks. However, recent continuum observations found vertically thin dust disks compared to gas disks (e.g., ALMA Partnership et al. 2015), which suggests the importance of multi-dimensional analyses with the vertically stratified structures. We conduct vertically global and radially local linear analyses to investigate the impact of the vertical structure on secular GI. We found that the dust concentration proceeds mostly in the radial direction while gas shows meridional circulation extended beyond the dust layer. Although the obtained motion is significantly different from the previous one-dimensional analyses, the difference in the linear growth rate is only a factor of a few. Besides, there is a parameter set where secular GI can develop in stratified disks but not in razor-thin disks. This emphasizes the importance of multi-dimensional analyses. We empirically derived the growth condition of secular GI and found it analogous to the previously derived condition. If the dust has grown larger and the turbulence is weak enough, the required dust mass for secular GI to operate is on the order of 0.01 percent of a stellar mass.