Publication Date:
2015-11-28
Description:
If very massive stars ( M 100 M ) can form and avoid too strong mass-loss during their evolution, they are predicted to explode as pair-instability supernovae (PISNe). One critical test for candidate events is whether their nucleosynthesis yields and internal ejecta structure, being revealed through nebular-phase spectra at t 1 yr, match those of model predictions. Here, we compute theoretical spectra based on model PISN ejecta at 1–3 yr post-explosion to allow quantitative comparison with observations. The high column densities of PISNe lead to complete gamma-ray trapping for t 2 yr which, combined with fulfilled conditions of steady state, leads to bolometric supernova luminosities matching the 56 Co decay. Most of the gamma-rays are absorbed by the deep-lying iron and silicon/sulphur layers. The ionization balance shows a predominantly neutral gas state, which leads to emission lines of Fe i , Si i , and S i . For low-mass PISNe, the metal core expands slowly enough to produce a forest of distinct lines, whereas high-mass PISNe expand faster and produce more featureless spectra. Line blocking is complete below ~5000 Å for several years, and the model spectra are red. The strongest line is typically [Ca ii ] 7291, 7323, one of few lines from ionized species. We compare our models with proposed PISN candidates SN 2007bi and PTF12dam, finding discrepancies for several key observables and thus no support for a PISN interpretation. We discuss distinct spectral features predicted by the models, and the possibility of detecting pair-instability explosions among non-superluminous supernovae.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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