10:45 AM - 12:15 PM
[PPS07-P20] Dust settling and diffusion under vertical shear instability turbulence in protoplanetary disks
Keywords:protoplanetary disks, turbulence, dust evolution
Planets form in disks of gas and dust particles (protoplanetary disks) around a young star. The initial stage of planet formation is the formation of kilometer-sized planetesimals from micron-sized dust grains. To clarify these dust growth and planetesimal formation processes, it is important to understand the vertical profile of dust particles. If the dust settles on the disk midplane, it can concentrate and lead to planetesimal formation through gravitational contraction of dust. Understanding the dust particle's vertical profile is also important for interpreting the degree of dust ring settling suggested by recent high-resolution radio observations.
In protoplanetary disks, the vertical profile of dust particles depends on disk gas turbulence. One of the candidate hydrodynamic mechanisms that can affect vertical dust diffusion in the outer disk regions is the vertical shear instability (VSI), which is caused by radiative cooling. Turbulence driven by the VSI has a predominant vertical motion that can prevent dust settling. However, dust diffusion can alter the disk cooling rate distribution and VSI-driven turbulence intensity. The outcome of this interaction remains unclear regarding the resulting dust vertical profile.
The purpose of this study is to assess if an equilibrium condition between vertical dust diffusion and settling can exist in VSI-driven turbulence. First, we assume a certain dust vertical profile and determine the spatial distribution of the disk cooling rate. Next, we estimate the intensity of VSI-driven turbulence using the results of hydrodynamical simulations. Finally, we compare the estimated turbulence intensity with the assumed dust distribution and examine the possibility of an equilibrium state.
We identify two types of states: the diffusion states where the dust strongly diffuses vertically by the VSI-driven turbulence, and the runaway settling states where the VSI-driven turbulence fails to stop the dust settling and the dust continues to settle on the midplane. These two states depend on the dust size. When the maximum dust size is less than 0.1 mm, VSI-driven turbulence leads to the diffusion states with a diffusion coefficient of 10^(-3). On the other hand, the runaway settling states are realized when the maximum dust is larger than 0.1 mm. We also find that the maximum dust size permitting the diffusion states decreases as the distance from the central star increases.
Our results suggest that the VSI can result in two contrasting states depending on the size of the dust, either with strong vertical diffusion or settling. Therefore, the observed settling of dust rings may be explained by these differences in the vertical profile of dust particles caused by VSI-driven turbulence.
In protoplanetary disks, the vertical profile of dust particles depends on disk gas turbulence. One of the candidate hydrodynamic mechanisms that can affect vertical dust diffusion in the outer disk regions is the vertical shear instability (VSI), which is caused by radiative cooling. Turbulence driven by the VSI has a predominant vertical motion that can prevent dust settling. However, dust diffusion can alter the disk cooling rate distribution and VSI-driven turbulence intensity. The outcome of this interaction remains unclear regarding the resulting dust vertical profile.
The purpose of this study is to assess if an equilibrium condition between vertical dust diffusion and settling can exist in VSI-driven turbulence. First, we assume a certain dust vertical profile and determine the spatial distribution of the disk cooling rate. Next, we estimate the intensity of VSI-driven turbulence using the results of hydrodynamical simulations. Finally, we compare the estimated turbulence intensity with the assumed dust distribution and examine the possibility of an equilibrium state.
We identify two types of states: the diffusion states where the dust strongly diffuses vertically by the VSI-driven turbulence, and the runaway settling states where the VSI-driven turbulence fails to stop the dust settling and the dust continues to settle on the midplane. These two states depend on the dust size. When the maximum dust size is less than 0.1 mm, VSI-driven turbulence leads to the diffusion states with a diffusion coefficient of 10^(-3). On the other hand, the runaway settling states are realized when the maximum dust is larger than 0.1 mm. We also find that the maximum dust size permitting the diffusion states decreases as the distance from the central star increases.
Our results suggest that the VSI can result in two contrasting states depending on the size of the dust, either with strong vertical diffusion or settling. Therefore, the observed settling of dust rings may be explained by these differences in the vertical profile of dust particles caused by VSI-driven turbulence.