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 mum feature) trace out the global kinematic distribution of radioactive material in the expanding supernova ejecta. In SN 2003du, the 1.644 mum [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 Ni-56 up to 9000-10,000 km s(-1). Furthermore, our analysis indicates that the 1.644 mum [Fe II] line profile is not consistent with strong mixing between the regions of electron-capture isotopes and the Ni-56 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 Ni-56 in other SNe Ia and briefly discuss the potential implications of our result for the use of SNe Ia as cosmological standard candles.