Toshikazu Shigeyama

Theoretical light curve of SN 1987A and mixing of hydrogen and nickel in the ejecta

Earlier light curve models for SN 1987A have been improved by adopting more realistic presupernova models and adopting a postexplosion elemental distribution consistent with X-ray and gamma-ray observations. The light curve shape is sensitive to the distributions of abundance and expansion velocity in the ejecta. It is found that the prepeak smooth rise and the broad peak of the light curve, respectively, are well modeled by assuming some mixing of Ni-56 into the hydrogen-rich envelope and of hydrogen into the core. The existence of the hydrogen recombination front is responsible for the formation of the plateaulike peak. For explosion energy, good agreement with the observed light curve is obtained for E/M(env) = (1.1 + or - 0./3) x 10 to the 50th ergs/solar mass. Further improvements on the early light curve, the dependence of the light curve on the distribution of Ni-56 and hydrogen, and the relation with other type II supernovae are discussed. 72 refs.

The Origin of Carbon Enhancement and the Initial Mass Function of Extremely Metal-poor Stars in the Galactic Halo

It is known that the carbon-enhanced, extremely metal-poor (CEMP) stars constitute a substantial proportion of the extremely metal-poor (EMP) stars of the Galactic halo, and a by far larger proportion than CH stars among Population II stars. We investigate their origin by taking into account an additional evolutionary path to the surface carbon enrichment, triggered by hydrogen engulfment by the helium flash convection, in EMP stars with [Fe/H] -2.5. This process is distinct from the third dredge-up operating in more metal-rich stars and in EMP stars. In binary systems of EMP stars, the secondary stars become CEMP stars through mass transfer from the low- and intermediate- mass primary stars that have developed the surface carbon enhancement. Our binary scenario can predict the variations in the abundances not only for carbon but also for nitrogen and s-process elements and can reasonably explain the observed properties such as the stellar distributions of the carbon abundances, the binary periods, and the evolutionary stages. Furthermore, from the observed frequencies of CEMP stars with and without s-process element enhancement, we demonstrate that the initial mass function of EMP stars needed gives the mean mass ~ 10 M☉ under the reasonable assumptions for the distributions of orbital separations and mass ratios of the binary components. This also indicates that the currently observed EMP stars were exclusively born as the secondary members of binaries, making up ~10% of EMP binary systems, with mass ~ 108 M☉ in total; in addition to CEMP stars with white dwarf companions, a significant fraction of them have experienced supernova explosions of their companions. We discuss the implications of the present results for the formation of the Galactic halo.

Late detonation models for the type Ia supernovae SN 1991T and SN 1990N

We present hydrodynamical models of exploding white dwarfs to account for the newly observed premaximum features of Type Ia supernovae 1990N and 1991T in a unified manner. In these models, a carbon deflagration, producing a central Fe/Co/Ni core and an intermediate Si/S/Ca layer, is later transformed into a detonation in the outermost layer. Depending on the transition density, the nucleosynthesis in such late detonations results in the formation of Fe-dominated surface layers or Si/Fe-rich surface layers, which are qualitatively consistent with the inferred composition of SN 1991T and 1990N, respectively

Low-mass helium star models for Type Ib supernovae : light curves, mixing, and nucleosynthesis

The applicability of theoretical models of He-star explosions to type Ib SN explosions is explored. Particular attention is given to light curves and mixing, Rayleigh-Taylor instabilities and mixing, and nucleosynthesis and the mass of Ni-56. Typical numerical results are presented in graphs, and it is concluded that the explosions of SN 1983N and SN 1983I can be accurately represented in terms of explosions of He stars with M(alpha) of 3-4 solar mass. A strong M(alpha) dependence of light-curve shape, photospheric velocity, and Ni-56 mass is found. 44 refs.

Enrichment history of r-process elements shaped by a merger of neutron star pairs

The origin of r-process elements remains unidentified and still puzzles us. The recent discovery of evidence for the ejection of r-process elements from a short-duration γ-ray burst singled out neutron star mergers (NSMs) as their origin. In contrast, core-collapse supernovae are ruled out as the main origin of heavy r-process elements (A > 110) by recent numerical simulations. However, the properties characterizing NSM events ‐ their rarity and high yield of r-process elements per event ‐ have been claimed to be incompatible with the observed stellar records on r-process elements in the Galaxy. We add to this picture with our results, which show that the observed constant [r-process/H] ratio in faint dwarf galaxies and one star unusually rich in r-process in the Sculptor galaxy agree well with this rarity of NSM events. Furthermore, we found that a large scatter in the abundance ratios of r-process elements to iron in the Galactic halo can be reproduced by a scheme that incorporates an assembly of various protogalactic fragments, in each of which r-process elements supplied by NSMs pervade the whole fragment while supernovae distribute heavy elements only inside the regions swept up by the blast waves. Our results demonstrate that NSMs occurring at Galactic rate of 12‐23 Myr −1 are the main site of r-process elements, and we predict the detection of gravitational waves from NSMs at a high rate with upcoming advanced detectors.

Possible models for the Type Ia supernova 1990N

Possible interpretations of new spectral and photometric features of Type Ia supernova 1990N with carbon deflagration/detonation models are presented. To explain the presence of Si and Ca at high expansion velocities in SN 1990N, we suggest four possible models, among which is a carbon detonation in a white dwarf with a mass ∼1 M ⊙ . The slow rise of the light curve of SN 1990N can be reproduced by both the deflagration and detonation models if the optical opacity is somewhat larger than that previously adopted

Mixing in ejecta of supernovae. I. General properties of two-dimensional Rayleigh-Taylor instabilities and mixing width in ejecta of supernovae

Nonlinear growth of two-dimensional Rayleigh-Taylor (R-T) instabilities are numerically studied to apply to the mixing in the supernova ejecta. First, much refined calculations of mixing in the realistic model of SN 1987A ar epresented with a better code and various mesh resolutions. The results show that the mixing width (or the extent of mixing) due to Rayleigh-Taylor instabilities is still too small to account for the observations even with relatively large initial perturbation. The mixing width is found to depend only slightly on the mesh resolution when the initial amplitude is larger than ∼5% of the expansion speed

Instabilities and Mixing in SN 1993J

Rayleigh-Taylor (R-T) instabilities in the explosion of SN 1993J are investigated by means of two-dimensional hydrodynamical simulations. It is found that the extent of mixing is sensitive to the progenitors core mass and the envelope mass. Because the helium core mass (3-4 M☉) is smaller than that of SN 1987A, R-T instabilities at the He/C+O interface develop to induce a large-scale mixing in the helium core, while the instability is relatively weak at the H/He interface as a result of the small envelope mass. The predicted abundance distributions, in particular the extent of the 56Ni mixing, are compared with those required in modeling the bolometric light curve and the late-time optical spectra. These comparisons provide significant constraints on the masses of the helium core and the envelope of the progenitor of SN 1993J.

Rayleigh-Taylor instability and mixing in SN 1987A

The stability of the supernova ejecta is compared with the Rayleigh-Taylor instability for a realistic model of SN 1987A. A linear analysis indicates that the layers around the composition interface between the hydrogen-rich and helium zones, and become Rayleigh-Taylor unstable between the helium and metal zones. In these layers, the pressure increases outward because of deceleration due to the reverse shock which forms when the blast shock hits the massive hydrogen-rich envelope. On the contrary, the density steeply decreases outward because of the preexisting nuclear burning shell. Then, these layers undergo the Raleigh-Taylor instability because of the opposite signs of the pressure and density gradients. The estimated growth rate is larger than the expansion rate of the supernova. The Rayleigh-Taylor instability near the composition interface is likely to induce mixing, which has been strongly suggested from observations of SN 1987A. 25 refs.

Gamma rays, X-rays, and optical light from the cobalt and the neutron star in SN 1987A

Recent developments in modeling the X-ray and gamma-ray emission from SN 1987A are discussed by taking into account both the decaying cobalt and the buried neutron star. The light curve and the spectra evolution of X-rays and gamma-rays are well modeled up to day of about 300 if mixing of Co-56 into hydrogen-rich envelope is assumed. However, the 16-28 keV flux observed by Ginga declines very slowly, whereas the spherical mixing model predicts that the flux should have decreased by a large factor at t greater than 300d. It is shown that this problem can be solved if the photoelectric absorption of X-rays is effectively reduced as a result of the formation of chemically inhomogeneous clumps. Based on the adopted hydrodynamical model and the abundance distribution, predictions are offered for future optical, X-ray, and gamma-ray light curves by taking into account other radioactive sources and various types of the central source, e.g., a buried neutron star accreting the reinfalling material or an isolated pulsar.