Detectability of mixed unburnt C+O in Type Ia supernova spectra

Motivated by recent three-dimensional calculations of the explosion of Type Ia supernovae via a pure deflagration, we calculate the observed spectra at 15-25 days past maximum light of a parameterized model that has a considerable fraction of unburnt C + O in the central regions. Rather than attempting a self-consistent three-dimensional calculation, which is beyond the scope of current computer codes, we modify the composition structure of the one-dimensional deflagration model W7. In our exploratory parameterized calculations, we find that a central concentration of C + O is not ruled out by observations for the epochs we study. We briefly examine whether nebular phase spectra could be incompatible with observations.