Supernovae and Radiation-Hydrodynamics
Massive stars end with violent explosions as their final fate (widely known as core-collapse supernovae). Because they are extreme energetic phenomena, they have fascinated not only astronomers but also a variety of scientific fields over several decades. The collaboration between observation and theory allows us to deeply investigate the mechanism of explosions, and also help us what is going on in such extremely high-temperature, high-density and strong-gravitational field environments.
I have worked on the mechanism of core-collapse supernovae (CCSNe) by phenomenological or ab-initio approaches. Each study is rather independently developing but mutually related in a complementary style. In recent years, I have devoted to develop a multi-dimensional (7-dimensional) Boltzmann-neutrino-radiation hydrodynamic code with detailed input-physics; gravity, weak-interactions, nuclear equation-of-state and magnetic field. I still keep polishing up some of these modules while our project acheives successfuly running supernova simulations on Japanse super-computer "K" with high-performance. I currently analyze these numerical data and adjust data-style in order to facilitate comparison with observations.
Animation: Momentum Space of neutrinos in CCSNe
Gamma-Ray Bursts and Jets
Animation: Relativistic hydrodynamical simulations for the jet breakout from a massive star ADS
Gama-Ray-Bursts (GRBs) are extreme transient gamma-ray events and their total releasing energy are comparable or sometimes much exceed supernova explosion. The nascent black hole or highly-magnetized neutron star (proto-magnetar), which are formed via either collapsing of massive stars or mergeing of compact stars, are supposed to play as "the central engine" of GRBs, though there are lots of uncertainties of their physical mechanisms. The theory of GRBs conncects the wide field of astronomy and astrophysics such as core collapse supernovae, Population III stars (first stars), compact stellar mergers and so on. They could be candidates for the electro-magnetic counterparts for the gravitational waves. Now that multi-messenger astronomy kicks in, both theories and observations will progress more and more.
Thus far, I studied the properties of collimate outflows, namely jets, for GRBs by using relativistic hydrodynamic simulations. The jet, which potentially accelerates to 99.99 percent of the speed of light, pushes aside the envelope of massive stars or ejecta of double neutron star mergers. The jet goes through complex interactions with surrounding ambients during drilling the ambient matter, and result in the formation of cocoon and recollimation shocks. I have studies these hydrodynamical properties of jet propagation, and its penetrability of the stellar mantle or ejecta of double neutron stars. I have still kept progressing these work with many collaborators.
Animation: Relativistic hydrodynamical simulations for jets in the ejecta of double neutron star mergers (left: 0.01M ejecta, right: 0.001M ejecta) ADS
Black Hole Accretion Disk
In general, black holes are not isolated but enveloped by matter, whose systems are often denoted as Black Hole Accretion Disks (BHADs). In these disks, the gravity and centrifugal force (with thermal and turbulent pressure) are almost balanced, but some of them advect due to angular momentum loss via magnetic field or turbulence, and eventually they are swallowed into black holes. In our universe, BHADs emergent in multiple scales, such as AGNs (supermassive black holes), X-ray binaries (stellar mass black holes) and CCSNe or double neutron star mergers (nascent stellar mass black hole). Since there are various interesting features in BHADs, astrophysicists have investigated their properties from different angles.
I have studied the stability of BHADs and mainly focus on the case with the existence of shocks. It is interesting to note that the accretion shock is generally unstable to non-radial perturbations, which can be confirmed by our general-relativistic hydrodynamic (GRHD) simulations. We often call these instability as "Standing Accretion Shock Instability" or SASI, which are also supposed to play an important role for CCSNe. By using our GRHD simulation data, we also perform general relativistic radiation (photon) transport to check the detectability of dynamical image of black hole shadows.
Animation: Intensity Distributions obtained by general relativistic radiation tranport simulations for Black Hole SASIADS