Signature of Electron Capture in Iron-rich Ejecta of SN 2003du^*

Late-time near-infrared and optical spectra of the normal-bright Type Ia supernova 2003du about 300 days after the explosion are presented. At this late epoch, the emission profiles of well-isolated [Fe II] lines (in particular that of the strong 1.644 μm feature) trace out the global kinematic distribution of radioactive material in the expanding supernova ejecta. In SN 2003du, the 1.644 μm [Fe II] line seems to show a flat-topped profile, indicative of a thick but hollow-centered expanding shell, rather than a strongly peaked profile that would be expected from a "center-filled" distribution. Based on detailed models for exploding Chandrasekhar-mass white dwarfs, we show that the feature is consistent with spherical explosion models. Our model predicts a central region of nonradioactive electron capture elements up to 2500-3000 km s^-1 as a consequence of burning under high density and an extended region of radioactive ⁵⁶Ni up to 9000-10,000 km s^-1. Furthermore, our analysis indicates that the 1.644 μm [Fe II] line profile is not consistent with strong mixing between the regions of electron-capture isotopes and the ⁵⁶Ni layers, as is predicted by detailed three-dimensional models for nuclear deflagration fronts. We discuss the possibility that the flat-topped profile could be produced as a result of an infrared catastrophe and conclude that such an explanation is unlikely. We discuss the limitations of our analysis and place our results into context by comparison with constraints on the distribution of radioactive ⁵⁶Ni in other SNe Ia and briefly discuss the potential implications of our result for the use of SNe Ia as cosmological standard candles.