2:45 PM - 3:00 PM
[PPS06-05] Suppression of the vertical shear instability in protoplanetary disks by dust growth : Implications for disk observations
Keywords:protoplanetary disks, dust evolution, turbulence, vertical shear instability, HL Tau disk
Planet formation depends on dust evolution and distribution in protoplanetary disks. Among them, strong dust settling to the midplane in disks promotes dust growth and planetesimal formation. One of the observational evidence of this dust settling is the protoplanetary disk around HL Tau. Recent radio interferometric observations have provided us with evidence that the HL Tau disk has rings and gaps of dust. Because these gaps have well separated and showing a high degree of contrast with the rings, the dust in outer regions in the HL Tau disk settles in the vertical direction.
This dust settling to the midplane means that the vertical turbulent diffusion is weak in outer disk regions. However, the vertical shear instability (VSI), which is the most robust turbulent origin in outer disk regions, drives turbulence with efficiently preventing vertical dust settling. This is about 10 times stronger than the intensity of vertical diffusion estimated from observations of the HL Tau disk. If the VSI drives turbulence in the HL Tau, the strong settling of dust may not occur.
On the other hand, One candidate of factors that weaken the turbulence of this VSI is dust growth and settling. The VSI requires rapid gas cooling and has been suggested to operate in outer disk regions. Dust growth and settling to the midplane affect the cooling rate distribution of disks and make the cooling above the midplane inefficient.
In this study, we develop a model to estimate the diffusion coefficient of the VSI-driven turbulence based on the linear analysis, taking into account dust evolution. Furthermore, by applying this model to the HL Tau disk, we discuss whether the observed dust settling is feasible or not. The model consists of the following two steps. In the first step, we calculate the region where the VSI can be driven and the VSI growth rate by linear analysis, taking into the dust distribution with settling in the disk. In the second step, we estimate the vertical diffusion coefficient of the VSI-driven turbulence from them. We construct the model to be consistent with the hydrodynamical simulations of the VSI. Comparing the estimated diffusion coefficient and the assumed one that is necessary to calculate the dust distribution given initially, we investigate how the VSI-driven turbulence evolves.
We find that dust diffusion due to the VSI-driven turbulence is weakened and dust settling is possible consistent with the HL Tau observations. As the dust particles grow up to 1 mm, the vertical diffusion coefficient of the VSI-driven turbulence is expected to settle down to about 10^(-4), which is consistent with the maximum value of the vertical diffusion coefficient ~10^(-4) assumed from observations. Even if the dust diffuses strongly temporarily due to the VSI, the dust cannot sustain this strong diffusion and may settle on the midplane.
Our results suggest that dust evolution may suppress the VSI-driven turbulence, which causes strong dust diffusion. Given that turbulence generally prevents dust coagulation and planetesimal formation, this may promote dust growth and planetesimal formation under the VSI-driven turbulence.
This dust settling to the midplane means that the vertical turbulent diffusion is weak in outer disk regions. However, the vertical shear instability (VSI), which is the most robust turbulent origin in outer disk regions, drives turbulence with efficiently preventing vertical dust settling. This is about 10 times stronger than the intensity of vertical diffusion estimated from observations of the HL Tau disk. If the VSI drives turbulence in the HL Tau, the strong settling of dust may not occur.
On the other hand, One candidate of factors that weaken the turbulence of this VSI is dust growth and settling. The VSI requires rapid gas cooling and has been suggested to operate in outer disk regions. Dust growth and settling to the midplane affect the cooling rate distribution of disks and make the cooling above the midplane inefficient.
In this study, we develop a model to estimate the diffusion coefficient of the VSI-driven turbulence based on the linear analysis, taking into account dust evolution. Furthermore, by applying this model to the HL Tau disk, we discuss whether the observed dust settling is feasible or not. The model consists of the following two steps. In the first step, we calculate the region where the VSI can be driven and the VSI growth rate by linear analysis, taking into the dust distribution with settling in the disk. In the second step, we estimate the vertical diffusion coefficient of the VSI-driven turbulence from them. We construct the model to be consistent with the hydrodynamical simulations of the VSI. Comparing the estimated diffusion coefficient and the assumed one that is necessary to calculate the dust distribution given initially, we investigate how the VSI-driven turbulence evolves.
We find that dust diffusion due to the VSI-driven turbulence is weakened and dust settling is possible consistent with the HL Tau observations. As the dust particles grow up to 1 mm, the vertical diffusion coefficient of the VSI-driven turbulence is expected to settle down to about 10^(-4), which is consistent with the maximum value of the vertical diffusion coefficient ~10^(-4) assumed from observations. Even if the dust diffuses strongly temporarily due to the VSI, the dust cannot sustain this strong diffusion and may settle on the midplane.
Our results suggest that dust evolution may suppress the VSI-driven turbulence, which causes strong dust diffusion. Given that turbulence generally prevents dust coagulation and planetesimal formation, this may promote dust growth and planetesimal formation under the VSI-driven turbulence.