Keiichi Maeda

Hydrodynamics and explosive nucleosynthesis in bipolar supernova explosions are examined to account for some peculiar properties of hypernovae as well as peculiar abundance patterns of metal-poor stars. The explosion is supposed to be driven by bipolar jets which are powered by accretion onto a central remnant. We explore the features of the explosions with varying progenitors' masses and jet properties. The outcomes are different from conventional spherical models. (1) In the bipolar models, Fe

Aspherical explosion models for the hypernova (hyperenergetic supernova) SN 1998bw are presented. Nucleosynthesis in aspherical explosions is examined with a two-dimensional hydrodynamical code and a detailed nuclear reaction network. Aspherical explosions lead to a strong α-rich freezeout, thus enhancing the abundance ratios [44Ca, 48Ti, 64Zn/Fe] in the ejecta. The nebular line profiles of the Fe-dominated blend near 5200 Å and of [O I] 6300, 6363 Å are calculated and compared with the observed

Core-collapse supernovae (CC-SNe) are the explosions that announce the death of massive stars. Some CC-SNe are linked to long-duration gamma-ray bursts (GRBs) and are highly aspherical. One important question is to what extent asphericity is common to all CC-SNe. Here we present late-time spectra for a number of CC-SNe from stripped-envelope stars and use them to explore any asphericity generated in the inner part of the exploding star, near the site of collapse. A range of oxygen emission-line

Late phase nebular spectra and photometry of Type Ib Supernova (SN) 2005bf taken by the Subaru telescope at ~ 270 and ~ 310 days since the explosion are presented. Emission lines ([OI]6300, 6363, [CaII]7291, 7324, [FeII]7155) show the blueshift of ~ 1,500 - 2,000 km s-1. The [OI] doublet shows a doubly-peaked profile. The line luminosities can be interpreted as coming from a blob or jet containing only ~ 0.1 - 0.4 Msun, in which ~ 0.02 - 0.06 Msun is 56Ni synthesized at the explosion. To explain

Massive stars having a CO core of ∼40–60 M⊙ experience pulsational pair-instability (PPI) after carbon-burning. This instability induces strong pulsations of the whole star and a part of outer envelope is ejected. We investigate the evolution and mass ejection of metal-poor very massive stars which experience PPI. We use stellar models with initial masses of 140, 200, and 250 M⊙ and the metallicity Z = 0.004. Their masses decrease to 54.09, 58.65, and 61.03 M⊙ before the neon-burning owing to ma

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Optical spectroscopy and photometry of SN 2006aj have been performed with the Subaru telescope at t > 200 days after GRB060218, the X-ray Flash with which it was associated. Strong nebular emission-lines with an expansion velocity of v ~ 7,300 km/s were detected. The peaked but relatively broad [OI]6300,6363 suggests the existence of ~ 2 Msun of materials in which ~1.3 Msun is oxygen. The core might be produced by a mildly asymmetric explosion. The spectra are unique among SNe Ic in (1) the abse

We report the late-time evolution of Type IIb supernova (SN IIb) 2013df. SN 2013df showed a dramatic change in its spectral features at ∼1 yr after the explosion. Early on it showed typical characteristics shared by SNe IIb/Ib/Ic dominated by metal emission lines, while later on it was dominated by broad and flat-topped Hα and He i emissions. The late-time spectra are strikingly similar to SN IIb 1993J, which is the only previous example clearly showing the same transition. This late-time evolut