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A series of optical and one near-infrared nebular spectra covering the firstyear of the Type Ia supernova SN 2011fe are presented and modelled. The densityprofile that proved best for the early optical/ultraviolet spectra, "rho-11fe",was extended to lower velocities to include the regions that emit at nebularepochs. Model rho-11fe is intermediate between the fast deflagration model W7and a low-energy delayed-detonation. Good fits to the nebular spectra areobtained if the innermost ejecta are dominated by neutron-rich, stable Fe-groupspecies, which contribute to cooling but not to heating. The correct thermalbalance can thus be reached for the strongest [FeII] and [FeIII] lines to bereproduced with the observed ratio. The 56Ni mass thus obtained is 0.47 +/-0.05 Mo. The bulk of 56Ni has an outermost velocity of ~8500 km/s. The mass ofstable iron is 0.23 +/- 0.03 Mo. Stable Ni has low abundance, ~10^{-2} Mo. Thisis sufficient to reproduce an observed emission line near 7400 A. Asub-Chandrasekhar explosion model with mass 1.02 Mo and no central stable Fedoes not reproduce the observed line ratios. A mock model where neutron-richFe-group species are located above 56Ni following recent suggestions is alsoshown to yield spectra that are less compatible with the observations. Thedensities and abundances in the inner layers obtained from the nebularanalysis, combined with those of the outer layers previously obtained, are usedto compute a synthetic bolometric light curve, which compares favourably withthe light curve of SN 2011fe.

Nebular spectra and abundance tomography of the type Ia supernova SN 2011fe: a normal SN Ia with a stable Fe core