Kenichi Nomoto

Presupernova models and supernovae

Present status of the theories for presupernova evolution and triggering mechanisms of supernova explosions are summarized and discussed from the standpoint of the theory of stellar structure and evolution. It is not intended to collect every detail of numerical results thus far obtained, but to extract physically clear-cut understanding from complexities of the numerical stellar models. For this purpose the evolution of stellar cores is discussed in a generalized fashion. The following types of the supernova explosions are discussed. The carbon deflagration supernova of intermediate mass star which results in the total disruption of the star. Massive star evolves into a supernova triggered by photo-dissociation of iron nuclei which results in a formation of a neutron star or a black hole depending on its mass. These two are typical types of the sueprnovae. Between them there remains a range of mass for which collapse of the stellar core is triggered by electron captures, which has been recently shown to leave a neutron star despite oxygen deflagration competing with the electron captures. Also discussed are combustion and detonation of helium or carbon which take place in accreting white dwarfs, and the collapse which is triggered by electron-pair creation in very massive stars.

Explosive Nucleosynthesis in Type I and Type II Supernovae

There exist many original and review articles about the mechanisms of type I and type II supernovae (SNI and SNII, e.g. Nomoto, Thielemann, Yokoi 1984; Nomoto 1986; Bruenn 1989; Cooperstein and Baron 1989; Wilson et al. 1986; Woosley and Weaver 1986) and a number of contributions to this conference (Nomoto, Woosley, Cooperstein and Baron, Mayle and Wilson), so that we do not intend to repeat this discussion here. We rather want to concentrate on the accompanying nucleosynthesis processes. This introduction contains a general presentation of the nucleosynthesis processes, while the application to both types of supernova events is given in sections 2 and 3. Section 4 includes a comparison of both contributions to the enrichment of heavy elements in the interstellar medium and a general conclusion. For a discussion of the nuclear physics input in the present nucleosynthesis calculations see Thielemann (1989) and Thielemann, Hashimoto, Nomoto (1990). One of the major free parameters in stellar evolution is the still uncertain 12C(α,γ)16O reaction (see Filippone, Humblet, Langanke 1989; Caughlan et al. 1985; Caughlan and Fowler 1988). The present calculations were performed with the rate of Caughlan et al. (1985).

Core-Collapse Supernovae in the Early Universe: Radiation Hydrodynamics Simulations of Multicolor Light Curves

The properties of the first generation of stars and their supernova (SN) explosions remain unknown due to the lack of their actual observations. Pop III stars may have been very massive and predicted to be exploded as pair-instability SNe, but the observed metal-poor stars show the abundance patterns which are more consistent with yields of core-collapse SNe. We study the multicolor light curves for a metal-free core-collapse SN models (massive stars of 25100 solar mass range) to determine the indicators for the detection and identification of first generation SNe. We use mixing-fallback supernova explosion models which explain the observed abundance patterns of metal poor stars. Numerical calculations of the multicolor light curves are performed using the multigroup radiation hydrodynamic code STELLA. The calculated light curves of metal-free SNe are compared with our calculations of non-zero metallicity models and observed SNe.

Formation of Luminous Contact Binaries by Rapid Accretion onto White Dwarfs

During the evolution of a close binary system, there is a phase of mass exchange between its component stars. When the mass-losing star is an evolved star off the main sequence, the mass exchange is very rapid. Its rate may well exceed the critical accretion rate corresponding to the Eddington limit of the mass accreting star. Effects of such a rapid accretion were studied for the mass-accreting main-sequence stars (Flannery and Ulrich 1977; Kippenhahn and Mayer-Hofmeister 1977; Neo et al. 1977). The mass-accreting stars become quickly overluminous and their radii increase even to a red-giant size as mass is accumulated. Then the both component stars will fill their Roche lobes to form a contact binary system.

Recurrence of Nova Explosions

It has been shown by many computations that a nova explosion is triggered by mass accretion onto a white dwarf in a close binary system. Such nova explosions will recur many times in the following way. When a certain amount of hydrogen-rich gas has been accreted on the white dwarf, a hydrogen-shell flash is ignited. Then the hydrogen-rich envelope expands greatly, which, in some cases, grows into a nova explosion. Almost all envelope mass is ejected into space or at least overflows its Roche lobe. After that the mass of the envelope increases again by accretion and the shell flash is ignited again. The period of such recurrence is given by τ rec = ΔMH/(dM/dt), where dM/dt and ΔMH are the rate of accretion and the mass contained in the hydrogen-rich envelope at the ignition point.