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RESEARCH PROGRAM

There is a wide variety in the research activities in this division, and it covers a considerable part of modern astronomy. Here we describe activities covered by faculty staff members. Of course, there are a lot of PDs doing independent research in addition to the contents below. There are many interactions beyond the sub-categories in astrophysics in this one of the largest group of theoretical astrophysics in Japan. Ongoing research areas are mainly categorized as follows;

  • Formation Theories of Stars and Planets, and Dynamics of Galaxies and Interstellar Medium via Numerical Simulations (Tomisaka, Kokubo, Nomura, Kawabe, Nakamura, Kataoka)
  • Cosmology, Formation Theories of Galaxies or the Large Scale Structure, and related areas, such as Particle Physics and Nuclear Physics (Kajino, Hamana)
  • High Energy Astrophysics, including Solar Physics, Cosmic Rays (Kajino, Takiwaki, Moriya)
  • Development of High Accuracy Numerical Hydro Simulation Method (Tomisaka, Kokubo)

The following description is a little old and should be updated. So you should only use them as reference.

Formation of Stars, Planets, and Galaxies

Star Formation (Tomisaka)

Star formation has attracted many astronomers, observers and theoreticians. Here, in the Theoretical Astrophysics Division, dynamical process of star formation is studied using hydro- and magnetohydrodynamical simulations. How the fundamental properties of new born stars, such as masses, angular momenta, magnetic fluxes, multiplicity of binaries, and so on, are determined – this is our goal.

Formation of Planetary Systems (Kokubo)

Satellite-ring systems are formed around planets as a by-product of planet formation. In the solar system, there is a variety of satellite-ring systems from the single large Moon of the Earth to Saturnian large ring systems with multiple satellites. To clarify the origin and the evolution of these satellite-ring systems is our main goal. We are now studying the origin of a single giant satellite such as the Moon and Charon.

Formation of Satellite-Ring Systems (Kokubo)

Satellite-ring systems are formed around planets as a by-product of planet formation. In the solar system, there is a variety of satellite-ring systems from the single large Moon of the Earth to Saturnian large ring systems with multiple satellites. To clarify the origin and the evolution of these satellite-ring systems is our main goal. We are now studying the origin of a single giant satellite such as the Moon and Charon.

High Density Objects, High Energy Astrophysics, Nuclear Astrophysics

Explosion Mechanisms of Core-Collapse Supernovae (Takiwaki)

Typical profile of the central region of massive star at a supernova explosion. The yellow and red region has higher temperature compared to black and blue region. Model is given by Takiwaki et al. 2016.

Core-collapse supernovae are among the most energetic explosions in the universe making the catastrophic end of massive stars. Since they are related to the formation of compact objects such as neutron stars, black holes, and magnetars, understanding of their physics has been of wide interest in the astrophysical society. In spite of the importance, one can not still tell the explosion mechanism clearly. By performing the numerical simulations, we study the explosion mechanism and the formation mechanisms of the relevant compact objects, paying particular attention to the effects of stellar rotation and magnetic fields on the neutrino-heating mechanism.

Multidimensional neutrino transport simulations coupled to the magnetohydrodynamics are inevitable to investigate the formation mechanisms of compact objects mentioned above. The fact that neutrinos are fermion with their cross sections being dependent on their energies makes it impossible to find the solution of the Boltzmann neutrino transport equations without the help of the numerical computations. We are developing the formalisms and codes to solve the multidimensional Boltzmann neutrino transfer equation.

Multi-Messenger Astronomy of Core-Collapse Supernovae (Takiwaki)

Light curves of neutrinos, gravitational waves and photon. See Nakamura et al 2016 for the details.

Core-Collapse supernovae emit neutrinos and gravitational waves as well as photons. The accumulation of the all information provides smoking-gun evidences for the nature of the explosion. Here we explain importance of neutrino and gravitational wave emissions.

Neutrino astronomy is becoming reality. In fact, the detection of neutrinos from SN1987A by the Kamioka group was a historical event. More advanced neutrino detectors are running currently. The importance of the study of supernova neutrinos is that one may determine the fundamental properties of neutrinos themselves (such as the mixing angles, mass-hierarchy) by comparing the theoretical predictions with the observations. By studying the neutrino oscillations in the realistic supernova models, we try to put the severe constraint on the unknown neutrino parameters. We also investigate the neutrino interactions in the dense and highly magnetized stellar cores, aiming to study the flare events of the magnetars.

When the collapse dynamics in massive stars deviates from the spherical symmetry, gravitational waves (GWs) can be emitted. As for the causes of the asymmetry, stellar rotation has been considered to be most promising. In addition, recent studies suggest that convections, magnetic fields, and anisotropic neutrino radiations can also produce the GWs. By performing the realistic numerical simulations, which take into account the new ingredients, we calculate the waveforms of GWs and discuss their detectability by the currently running laser interferometers, such as TAMA300 and LIGO. Furthermore, we study the gravitational wave background from gamma-ray bursts and the first stars aiming to understand what information about their central engines can be obtained from the GWs.

Neutrino Astrophysics and Nucleosynthesis in Supernovae and Collapsars (Kajino)

Where is the origin of the rapid-neutron capture elements such as Thorium (half life is 14.05 Gy) and Uranium (4.5 Gy), which are of great use for cosmochronometer, the p-elements, and the super-heavy elements? We have not yet identified astrophysical sites for the r- nor p-elements uniquely although there are several proposed candidate sites such as supernovae, neutron-star mergers, or collapsars (i.e. central engines of gamma-ray bursts). We study the nucleosynthetic origin of these heavy elements by modeling the explosion dynamics of neutrino-driven winds and shock propagation from core-collapse Type II supernovae or accretion disks in collapsars. Both neutral and charge current interactions of neutrinos with nuclei and thereby the induced neutrino oscillation (MSW) effects play the critical roles in the explosive nucleosynthesis in outer layers. We try to determine the neutrino-oscillation parameters in terms of nucleosynthesis of light-to-heavy elements in the neutrino-processes.

Galactic Cosmic Rays and Rare Elements (Kajino)

Rare light-to-heavy mass elements have several different origins; the Big-Bang nucleosynthesis, the Galactic cosmic-ray interactions with interstellar medium, the supernova gamma- and neutrino-processes, and the AGN jet interactions with gas clouds. We first study the Big-Bang nucleosynthesis in many different ranks of the standard and non-standard cosmological models. We second study the production mechanism of these elements in high-energy Galactic cosmic-rays or AGN jets by modeling realistic source spectra and their propagation through the cold gas clouds. The latter processes are related to the geometrical structure of high-energy AGN activities. We aim to limit these origins quantitatively in order to constrain the most complicated and fundamental physical processes of supernova gamma- and neutrino-nucleosynthesis.

Ultra-High-Energy Cosmic Rays (Kajino)

What is the realistic picture of the central engines of gamma-ray bursts? Strongly magnetized compact stellar objects such as rapidly rotating proto-neutron stars or black holes are the viable candidates. We study the interactions of ultra-high-energy charged particles (hadrons) with strong magnetic fields on these relativistic compact objects in quark models. Quantizing the meson field in the constituent quark model, we calculate the meson synchrotron radiation probabilities in strong magnetic field and study the implication in the ultra-high-energy cosmic rays whose energies exceed 1020 eV.

Cosmology and Large Scale Structure

Big-Bang Cosmology (Kajino)

There occurred many physical processes in the early Universe which affect the later cosmic evolution. They are the inflation and subsequent creation of elementary particles, the cosmic phase transitions including quark-hadron transition, the symmetry breaking of four kinds of fundamental forces associated with baryo- and lepto-genesis, the weak decoupling of light neutrino families, and so on. We study these fundamental processes and their effects on the primordial nucleosynthesis, cosmic microwave background anisotropies, and structure formation of various scales theoretically. We aim to construct a universal view of cosmic evolution based on fundamental physics of particles and nuclei so that our theoretical models are established through cosmological observations and physics experiments.

Cosmological Parameters and Particle Cosmology (Kajino)

Recent observations of cosmic microwave background anisotropies and Type Ia supernova magnitude-redshift relation have suggested a hypothesis that a flat cosmology of accelerating universal expansion best explains the observed data. What is the nature of dark energy, which manifests a mysterious property of negative pressure and positive energy density? What is the unseen cold dark matter made of, which is still undiscovered experimentally? We try to establish the physical origin of the dark energy, for example as time varying quintessential scalar field, and also identify the dark matter as quantum massive particles required for the spontaneous symmetry breaking or anomaly restoration theoretically.

Extra-Dimensions and Space-Time Structure (Kajino)

Brane world cosmology with extra dimensions is motivated by a unified M-theory or superstring theory which manifests Einstein Universe as a hyper-surface (i.e. brane) in higher dimensional manifold. We explore the quantum properties of cold dark matter particles such as the lightest super-symmetric particles in brane world cosmology, and look for their observational signature in cosmic gravitational wave background, cosmic microwave background radiation, Type Ia supernova magnitude-redshift relation, baryon-to-mass ratio of rich clusters, cluster mass-to-light ratio, etc. We also try to parameterize the dark energy as an inflow energy density of scalar field in the bulk. We aim to understand the space-time structure of the brane world cosmology both theoretically and observationally.

Cosmological Magnetic Field and Structure Formation (Kajino)

Magnetic field, in addition to gravity, plays a critical role in the formation processes of various stellar objects and their dynamical evolution. We study the effects of cosmic magnetic field on the large scale structure. We calculate theoretically the cosmic microwave background anisotropies and their polarization by taking account of the primordial magnetic field in order to constrain the strength and power spectrum at the photon last scattering surface. We then try to model the creation mechanism of the primordial magnetic field in the quark-hadron phase transition or in the inhomogeneous lepton distribution before or after the weak-decoupling of light neutrino flavors.

Cosmic Chemical Evolution and Cosmochronology (Kajino)

Even should the cosmic expansion age be determined from observed cosmological parameters, we still need a cosmological clock to date the origin and evolution of hierarchical cosmic structure. Evolution in physical conditions of the early Universe, clusters, galaxies, and stars are represented by the growth or decline in chemical composition of metals which originate from the Big-Bang nucleosynthesis and the later evolutionary cyclic chain of star formation \s nucleosynthesis \s supernova explosion. We first study the nucleosynthesis of the light-to-heavy metals, especially from the rapid- and slow-neutron capture processes and p-process in AGB stars or supernovae. We then apply to describe the metal-enrichment in the Universe in order to construct the best model of the Galactic chemical evolution. We also try to construct the best nucleo-cosmochronometer using long lived radioactive nuclei of heavy metals.

Cosmic shear statistics (Hamana)

Cosmic shear refers to angular correlations in the apparent shape of distant galaxies arising from the gravitational lensing effect of foreground structures. Via lensing, correlations in the mass distribution are imprinted in the coherent distortions of the galaxy images. Cosmic shear statistics thus provide unique information on the matter distribution and its evolution.

Development of High Accuracy Numerical Hydro Simulation Method

Development of High Accuracy Numerical Hydro Simulation Method (Tomisaka)

Numerical simulations in astrophysics have to have both high accuracy and wide dynamic range. For example, in the dynamical contraction from a molecular cloud core to a protostar, the density increases ~1013 times while the size decreases ~10-6 times. This comes from the self-gravity. We are developing numerical methods to overcome the above difficulty, which are called Nested-Grid method and Adaptive Mesh Refinement.

Integrator for N-body Problems (Kokubo)

High-speed and high-accuracy integrators are the essential tool for the study of N-body problems. We are developing time-symmetric and symplectic integrators suitable for planetary N-body problems.

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