5:15 PM - 7:15 PM
[PPS06-P23] Mixing of Solid Particles in Molecular Clouds: Diffusion Coefficients Due to Turbulence
Keywords:turbulent molecular cloud, Diffusion coefficient, Isotopic anomaly, Numerical calculations
The materials contained in the planets and meteorites of the present solar system provide valuable information in the search for the origin of the solar system. A typical example is the isotope ratios of elements. Stable isotopes of chromium with masses of 50, 51, 52, and 54 exist, and their abundance ratios have been found to vary slightly from planet to planet and meteorite to meteorite. Such isotopic anomalies have been observed not only for chromium but also for other elements such as titanium.
Isotopic ratios vary among solar system bodies despite stable nuclide numbers remaining constant, suggesting different origin materials. While material mixing occurs at each stage from molecular clouds to cores to protoplanetary disks to planets the persistent isotopic heterogeneity indicates incomplete mixing. This heterogeneity likely originated from diverse stellar sources (supernovae and AGB stars) producing materials with distinct isotopic signatures. The degree of heterogeneity preserved in molecular cloud cores depends on the balance between turbulent mixing and core formation rates, though our understanding of these mixing processes remains limited.
The purpose of this study is to analyze the motion of solid particles in turbulent molecular clouds and to derive the diffusion coefficient when this is considered as a diffusion due to turbulence. This will clarify the rate of diffusion and mixing by turbulence and is expected to provide clues to elucidate the conditions under which the molecular cloud core that formed the present solar system was formed. In this study, hydrodynamics simulations are performed by varying the turbulence intensity to obtain the diffusion coefficients in accordance with the turbulence intensity. Based on astronomical observations of molecular clouds, we also derive diffusion coefficients under conditions of supersonic turbulence. On the other hand, the effects of magnetic field and gas self-gravity are neglected in this study, but these effects should be considered in future studies.
For the motion of solid particles, it is assumed that the gas and solid particles move in unison because the particle size is small (0.1 μm or less) and the stopping time due to gas drag is extremely short. Under this assumption, the motion of solid particles is calculated in parallel with the fluid calculation. The motion of the fine particles is tracked starting from multiple points in the calculation domain, and based on the results, the average distance traveled by the particles is calculated as a function of time, and the diffusion coefficient is obtained.
The Athena++ code is used for the hydrodynamics calculations. In this study, the effects of magnetic field and self-gravity are not considered, and a periodic boundary is set as the boundary condition. Turbulence is generated using the turbulence generation method provided by Athena++. By applying a force of appropriate strength, a steady turbulence is created in which the kinetic energy is constant in the entire space. By adjusting the intensity of the force, steady turbulence of different intensities is generated.
Numerical simulations with various parameters have been performed to obtain diffusion coefficients for turbulent flows with average velocities ranging from subsonic to supersonic. According to the results, the diffusion coefficients seem to be expressed using parameters such as the mean velocity of the turbulence and the spatial scale of the system.
In the future, we aim to determine the turbulence intensity in actual molecular clouds from observed data of molecular clouds and to estimate the diffusion coefficient in the molecular clouds. By using the diffusion coefficient, we would like to clarify the initial conditions of molecular clouds and the conditions for the formation of molecular cloud cores necessary to create isotopic heterogeneity in the current solar system.
Isotopic ratios vary among solar system bodies despite stable nuclide numbers remaining constant, suggesting different origin materials. While material mixing occurs at each stage from molecular clouds to cores to protoplanetary disks to planets the persistent isotopic heterogeneity indicates incomplete mixing. This heterogeneity likely originated from diverse stellar sources (supernovae and AGB stars) producing materials with distinct isotopic signatures. The degree of heterogeneity preserved in molecular cloud cores depends on the balance between turbulent mixing and core formation rates, though our understanding of these mixing processes remains limited.
The purpose of this study is to analyze the motion of solid particles in turbulent molecular clouds and to derive the diffusion coefficient when this is considered as a diffusion due to turbulence. This will clarify the rate of diffusion and mixing by turbulence and is expected to provide clues to elucidate the conditions under which the molecular cloud core that formed the present solar system was formed. In this study, hydrodynamics simulations are performed by varying the turbulence intensity to obtain the diffusion coefficients in accordance with the turbulence intensity. Based on astronomical observations of molecular clouds, we also derive diffusion coefficients under conditions of supersonic turbulence. On the other hand, the effects of magnetic field and gas self-gravity are neglected in this study, but these effects should be considered in future studies.
For the motion of solid particles, it is assumed that the gas and solid particles move in unison because the particle size is small (0.1 μm or less) and the stopping time due to gas drag is extremely short. Under this assumption, the motion of solid particles is calculated in parallel with the fluid calculation. The motion of the fine particles is tracked starting from multiple points in the calculation domain, and based on the results, the average distance traveled by the particles is calculated as a function of time, and the diffusion coefficient is obtained.
The Athena++ code is used for the hydrodynamics calculations. In this study, the effects of magnetic field and self-gravity are not considered, and a periodic boundary is set as the boundary condition. Turbulence is generated using the turbulence generation method provided by Athena++. By applying a force of appropriate strength, a steady turbulence is created in which the kinetic energy is constant in the entire space. By adjusting the intensity of the force, steady turbulence of different intensities is generated.
Numerical simulations with various parameters have been performed to obtain diffusion coefficients for turbulent flows with average velocities ranging from subsonic to supersonic. According to the results, the diffusion coefficients seem to be expressed using parameters such as the mean velocity of the turbulence and the spatial scale of the system.
In the future, we aim to determine the turbulence intensity in actual molecular clouds from observed data of molecular clouds and to estimate the diffusion coefficient in the molecular clouds. By using the diffusion coefficient, we would like to clarify the initial conditions of molecular clouds and the conditions for the formation of molecular cloud cores necessary to create isotopic heterogeneity in the current solar system.