Late emission from supernovae - A window on stellar nucleosynthesis
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Abstract
Observations of Type Ib and Type II supernovae show strong evidence that the energy input after 100-200 days is dominated by γ-rays from radioactive decay of ^56^Co. The energy deposition of these γ-rays, and the subsequent thermalization of the nonthermal electrons, are calculated using Monte Carlo techniques. The relative fractions going into heating, ionizations, and direct excitations are studied. Above an electron fraction of ~ 0.1 direct excitations are negligible, and the line emission arises from thermal processes. The temperature and ionization structure of the ejecta and its emission are discussed for general cases, and for detailed nucleosynthesis models. It is found that the emission is dominated by neutral and singly ionized lines, the strongest due to [O I], [Ca II], Mg I], [C I] and [Si I]. Tests of the explosion models and determinations of absolute and relative abundances from the late emission are studied. For a given model, the relative and absolute line strengths depend on both the column densities of the different abundance zones, and the relative abundances within each zone. For reliable modeling, the density structure must be known. It is shown how the emission-line profiles can be used as a powerful diagnostic of this, as well as of the abundance structure of the ejecta. The variation of the composition with the He core mass is shown to lead to substantial differences in the spectra, especially in the [O I]/[Ca II] emission-line ratio. Also, the degree of convective mixing is important for the result. The late spectrum is thus a good diagnostic of the nucleosynthetic structure of the supernova. At late times, a thermal instability is possible, since fine-structure far-IR lines dominate the cooling. This may trigger molecule and dust formation in the ejecta. The emission from a central pulsar may prevent this from occurring. The model is applied to the observations of the Type Ib SN 1985F. From a nebular analysis, including most of the diagnostic lines available, it is concluded that a white dwarf model cannot be excluded, purely on this basis. However, from calculations of the emission from 4-8 M_sun_ He cores, it is found that a core mass of ~ 8 M_sun_ reproduces both the total and the relative line strengths well, while lower mass cores give a considerably poorer agreement. Thus, the spectrum indicates that the Type Ib supernovae come from stars of ~ 25 M_sun_ and above. The density distribution, as inferred from the line profiles, is, however, considerably more extended than in the hydrodynamical models. This may be a result of instabilities. A macroscopic mixing of clumps from the different burning shells is shown to be the most likely explanation for the lack of a velocity correlation from lines of the different elements. Models based on exploding white dwarfs are shown to be inconsistent with the observations, owing to low absolute line luminosities and to the strong [C I] lines.