​Supernovae
(Maeda)

Supernovae are exposions of stars at their demise, showing up suddenly on the sky. They have extremely high luminosity, which is a billion times, or even more, brighter than the Sun. These "new stars"have been known from the ancient era, but they still stand out as one of the biggest unresolved problems in astrophysics for many aspects. My approach is a combination of theory and observation. The theoretical study is primary on hydrodynamics, nuclear reactions, and radiation transport, done eitehr through analytic approach or by computer simulations. It is also a great pleasure to test the theory by observational data. Along with the theoretical investigations, I propose observing projects for ground and space telescopes at various wavelengths, covering radio, optical, infrared, X-ray and gamma-rays, and conduct observations.

​Spectra of Novae in their Initial Brightening Stages
(Taguchi & Maeda)

A (classical) nova is a transient event occurring in a binary system composed of a white dwarf (WD, the primary star) and a late-type star (the secondary star). In the binary system, hydrogen-rich material from the secondary star accretes onto the WD to create a hydrogen-rich envelope on the surface of WD. Once the gas has sufficiently accreted, the envelope would be hot and dense enough to ignite itself. Once the hydrogen ignites, the energy released by the nuclear burning would power the nova eruption. We are trying to research the nature of novae in their initial rapid brightening stages, which typically end within ~ 1 day or so, by optical spectra and spectral analyses. Though optical spectra of novae in such early stages had been almost unobtainable in the past, we are sure that recent progress in transient surveys and our Seimei telescope enable us to obtain such spectra.
Our research is based on the following three:
1. Establish a network for quick discovery of nova candidates.
2. Quick follow-up spectroscopy of nova candidates by Seimei telescope.
3. Modelling optical spectra of novae in their initial brightening stages.

​Nebular spectroscopy and envelope-stripping mechanisms
(Fang & Maeda)

When nuclear fuel in the core of a massive star with a zero-age main-sequence mass M_{\rm ZAMS} ≳ 8 M⊙ is exhausted, the central part (the iron or oxygen–neon–magnesium core) collapses and forms a neutron star or a black hole. The material above the collapsing core is rapidly ejected, leading to an SN explosion. As the ejecta expands, it becomes more transparent and enters so-called nebular phase, which enables us to "see" the most inner part of the supernova. Recently my research interest focuses on using the nebular spectrum of core-collapse supernova to study the physical properties (including the composition and geometry) of the ejecta, and hopefully reveal the explosion mechanism and progenitor of these energetic events.

Modeling and Observations for Peculiar Transients
(Uno & Maeda)

In recent years, thanks to new-generation surveys, peculiar transients that are unlike typical supernovae (SNe) have been discovered. The new-type transients give us valuable insights into the endpoints of the stellar evolution. Among them, I am particularly interested in fast blue optical transients (FBOTs) and tidal disruption events (TDEs). FBOTs are one of the most enigmatic events, which are characterized by high luminosity and fast-evolving light curves. Astronomers around the world are trying to clarify their origin, but it still remains unclear. TDEs are phenomena in which a star is destroyed by a massive black hole. Their radiation mechanisms in UV/Optical are unknown. With analytical modeling and radiation hydrodynamics, I am trying to reveal the enigmatic origin of the peculiar transients.Of course, I am also curious to understand SNe. I have been carrying out observations for SNe with Japanese telescopes (especially, Seimei and Kanata) and the Subaru telescope.

Spectra Synthesis Calculation of Type Ia Supernovae
(Ogawa & Maeda)

It is widely accepted that Type Ia supernovae (SNe Ia) are thermonuclear explosions of a CO white dwarf in a binary system, but it is still unknown how the explosive nucleosynthesis proceeds during the explosion. Thanks to the recent technological development of the transient observations, many supernovae are now detected shortly after the explosion, followed by quick spectroscopic observations. In this study, we focus on very early-phase spectra of SNe Ia and try to constrain the explosion models of SNe Ia. By using one-dimensional Monte Carlo radiation transfer code, TARDIS, we estimate the density and the abundance structure of the outermost ejecta of SNe Ia. Applying the method for a sample of SNe Ia, we systematically investigate whether and how the outermost ejecta structure is different for different subclasses of SNe Ia.

Calculation of Early Light Curve of SNe IIn
(Inutsuka & Maeda) 

Type IIn supernovae(SNe IIn) have narrow H emission lines in their spectra, and their light curves varies in peak luminosity and rise time. These are caused from SN ejecta interacting with dense circumstellar matter(CSM), which is formed before progenitor star’s death. Progenitor of SNe IIn have mass-loss rate more than 0.01 solarmass, although source of such high mass-loss is yet unknown. We focus on early light curve to find out properties of ejecta and CSM, such as ejecta mass, energy, and mass-loss late from progenitor star. By using 1D Lagrangian hydrodynamic & radiation transfer code, SNEC, we are now doing simulations to reproduce early IIn light curves of actual SNe.