Schedule Aug 21, 2009
SN2008S : An Electron Capture SN from a Super AGB Progenitor
Maria Teresa Botticella (Univ. Belfast)

We present comprehensive photometric and spectroscopic observations of the faint transient SN 2008S discovered in the nearby galaxy NGC 6946. SN 2008S exhibited slow photometric evolution and almost no spectral variability during the first nine months, implying a long photon diffusion time and a high density circumstellar medium. Its bolometric luminosity is low with respect to most core collapse supernovae but is comparable to the faintest type II-P events. Our quasi-bolometric lightcurve extends to 300 days and shows a tail phase decay rate consistent with that of {56}Co. We propose that this is evidence for an explosion and formation of {56}Ni. The mass of {56}Ni synthesized by SN 2008S is 0.0014 +/- 0.0003 Msun. Spectra of SN 2008S show intense emission lines of Halpha, [CaII] doublet and Ca II NIR triplet, all without obvious P-Cygni absorption troughs. The large MIR flux detected shortly after explosion can be explained by a light echo from pre-exisiting dust. The late NIR flux excess is plausibly due to a combination of warm newly-formed ejecta dust together with shock-heated dust in the circumstellar environment. We reassess the progenitor object detected previously in Spitzer archive images, supplementing this discussion with a model of the MIR spectral energy distribution. This supports the idea of a dusty, optically thick shell around SN 2008S with an inner radius of nearly 90AU and outer radius of 450AU, and an inferred heating source of 3000 K. The luminosity of the central star is L ~10^{4.6} Lsun. All the nearby progenitor dust was likely evaporated in the explosion leaving only the much older dust lying further out in the circumstellar environment. The combination of our long term multi-wavelength monitoring data and the evidence from the progenitor analysis leads us to support the scenario of a weak electron capture supernova explosion in a super-AGB progenitor star (of initial mass 6-8 Msun) embedded within a thick circumstellar gaseous envelope. We suggest that all of main properties of the electron capture SN phenomenon are observed in SN 2008S and future observations may allow a definitive answer.

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