Radiation magnetohydrodynamic simulations of star formation processes 原始星形成過程の輻射磁気流体シミュレーション
Radiation magnetohydrodynamic simulations of star formation processes
This thesis is devoted to theoretical studies of star formation processes using computational simulations. We develop a three-dimensional nested-grid radiation magneto-hydrodynamic (RMHD) simulation code and investigate collapse from a molecular cloud core to a protostellar core. We also predict the observational properties of star forming clouds, particularly of first cores, directly from the RMHD simulations. Our RMHD simulations include necessary physical processes in star formation processes, and will significantly contribute our understanding in this field.Stars are most fundamental elements in our universe, and understanding their formation is one of the most highlighted topics in astrophysics. Therefore star formation processes have been extensively studied so far. Because star formation is quite a complicated and highly non-linear process involving many physical processes, and because the immediate sites of star formation are difficult to observe, numerical simulations have played crucial roles in expanding our knowledge on star formation. Despite the long history of the studies, computational simulations of star formation including complicated physical processes are still being developed, and multi-dimensional RMHD simulations are surely most important topic in this field. Because the Atacama Large Millimeter/submillimeter Array (ALMA) recently started its early-science operations, such realistic simulations are highly demanded.During star formation, the energetics of the system is dominated by the release of gravitational energy in the very central region around the formed protostar. Both magnetic and radiation feedback are of major importance in this region, but there has been no work resolving this small scale (< 1 AU) including required physics. To tackle this problem, we develop a new high-resolution simulation code including required physical processes such as magnetohydrodynamics, self-gravity, chemical reactions and radiation transfer.In this thesis, we present our works in five parts. In the first part, we describe the development of the three dimensional nested-grid RMHD simulation code including many physical processes in detail. For radiation transfer, we adopt flux limited diffusion approximation and implicit time-integration which significantly reduce the computational load and enable stable simulations. We also implement additional physics required in star formation simulations such as non-ideal MHD effects and realistic equation-of-state.The other chapters are devoted to the results and applications of the RMHD code. First, we show the evolution in the early phase of protostellar collapse, especially focusing on the formation and properties of so-called first hydrostatic cores. In comparison with previous simulations without proper treatment of radiation transfer, radiation transfer does not seem to drastically change the global scenario of low-mass star formation. However, quantitatively there is non-negligible difference; for example, the temperature distribution is significantly changed by introducing radiation transfer. Realistic treatment of gas thermodynamics alters some properties and structure of the core quantitatively. The mass and size of the first core at a certain central density become larger because of higher entropy, and the lifetime becomes slightly longer. We also find two components of bipolar outflow are driven from the first core via respectively different mechanisms. Next, we show observational properties predicted directly from the results of RMHD simulations using post-processing radiation transfer calculations. We calculate Spectral Energy Distributions and Visibility Amplitude Distributions in thermal dust continuum emissions. We propose a strategy to identify first cores distinguishing from young stellar objects and starless molecular cloud cores. We also perform non-LTE molecular line transfer simulations and predict future observations with ALMA such as channel maps and position-velocity diagrams. Our results can be directly compared to observations and are useful for planning and interpreting observations.We describe our novel theoretical model of first cores in the next chapter; the Exposed Long-lived First-core. We find that first cores formed in very low mass cloud cores can be significantly long-lived. Their evolution is strongly affected by radiation cooling and is qualitatively different from ordinary first cores. We also calculate the observational properties of such first cores and show that they can be observed with current instruments such as ALMA and $Herschel$. Our results suggest that such first cores can be observed more frequently than those in molecular cloud cores of ordinary masses.Finally, we report the results of the RMHD simulations of the formation of protostellar cores with and without Ohmic dissipation. In the ideal MHD models, the evolution of the protostellar cores are very similar to that in spherically symmetric non-rotating models due to efficient angular momentum transport. However, if the resistivity presents, rotationally-supported circumstellar disks are rapidly built up in the vicinity of the protostellar cores. Magnetic fields are amplified by rotation and fast outflows are launched from the protostellar cores via magnetic pressure gradient force. These are the first 3D RMHD simulations resolving the protostellar cores in the world.