Abundance stratification in Type Ia supernovae - II. The rapidly declining, spectroscopically normal SN 2004eo

The variation in properties of Type Ia supernovae, the thermonuclear explosions of Chandrasekhar-mass carbon-oxygen white dwarfs, is caused by different nucleosynthetic outcomes of these explosions, which can be traced from the distribution of abundances in the ejecta. The composition stratification of the spectroscopically normal but rapidly declining SN 2004eo is studied by performing spectrum synthesis of a time-series of spectra obtained before and after maximum, and of one nebular spectrum obtained about eight months later. Early-time spectra indicate that the outer ejecta are dominated by oxygen and silicon, and contain other intermediate-mass elements, implying that the outer part of the star was subject only to partial burning. In the inner part, nuclear statistical equilibrium (NSE) material dominates, but the production of (56)Ni was limited to similar to 0.43 +/- 0.05 M(circle dot). An innermost zone containing similar to 0.25 M(circle dot) of stable Fe-group material is also present. The relatively small amount of NSE material synthesized by SN 2004eo explains both the dimness and the rapidly evolving light curve of this supernova.