Research Interest
Supernovae and Radiation-Hydrodynamics
Massive stars end with violent explosions known as a core-collapse supernova. They have fascinated not only astrophysicists but also other physicists including nuclear physics, particle physics, plasma physics and even experimental or computational science over several decades. Both observational and theoretical studies are important for the comprehensive understanding of the mechanism of explosions.
I have worked on studying 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. I have developed a multi-dimensional (7-dimensional) Boltzmann-neutrino-radiation hydrodynamic code with detailed input-physics; gravity, weak-interactions, nuclear equation-of-state and magnetic field. More recently, I recently carried out axisymmetric core-collapse supernova simulations with using more than 3 million node hours on Japanese super-computer "K", and reveal some important characteristics of neutrino radiation fields and hydrodynamics during development of explosion. In addition to my own project, I recently have launched collaboration works in Princeton supernova group which have developed their own supernova code "Fornax". The code is capable of three-dimensional simulations under feasible computational resources. By taking advantage of both merits and compensating for the disadvantages in both codes, I further investigate the physics of core-collapse supernova and also more energetic events such as gamma-ray bursts.
Animation: 2D supernova simulation with full Boltzmann neutrino transport (Left: entropy(left) and velocity(right) contour with focusing on post-shock flows. Right: entropy contour with fluid vector field (left) and neutrino vector field (right) in the vicinity of proto-neutron star) ADS
Neutrino Qunatum kinetics
There are many experimental evidences that neutrinos can undergo flavor conversions during propagation in space, which is known as neutrino oscillations. In dense neutrino-environments such as CCSNe and binary neutron star mergers (BNSMs), neutrino flavor conversions are induced by neutrino-self interactions. This quantum kinetic properties of neutrinos potentially give a significant impact on fluid dynamics, nucleosynthesis, and observable signals including neutrinos, gravitational waves, and electromagnetic waves including kilonova from binary neutron star merger.
I have studied this intriguing quantum kinetic features of neutrinos by using numerical simulations of quantum kinetic neutrino transport. In our recent study, we find many evidences that neutrino flavor conversions occur inside CCSNe and BNSMs. See my publication list to catch the up-to-date result.
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 merging of compact stars, are supposed to play as "the central engine" of GRBs, though there are lots of uncertainties of their physical mechanisms.
The GRBs are associated with various fields of astrophysics such as transient events (e.g., core collapse supernovae and binary neutron star mergers), Population III stars (first stars), and cosmology. They are also the electro-magnetic counterparts for the gravitational waves. Currently on-going and evolving multi-messenger astronomy will shed light on the nature of GRBs.
In my projects, I have studied the properties of collimate outflows, namely jets, for GRBs by using relativistic hydrodynamic simulations. The jet can be accelerated around the 99.99 percent of the speed of light, that can push aside the envelope of massive stars or ejecta of double neutron star mergers. The jet undergoes complex interactions with surrounding ambients during drilling the ambient matter, and then they result in the formation of cocoon and recollimation shocks. I have studies these hydrodynamical properties of fluid dynamics with focusing on its penetrability of the stellar mantle or ejecta of double neutron stars. I keep progressing these in collaborating with both theorists and observers.
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
Standing Accretion Shock Instability in Black Hole Accretion Disk (left: Schwartzschild Black Hole, right:Kerr Black Hole (a/M=0.3))ADS1 ADS2
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