Spectral Sequences of Type Ia Supernovae. II. Carbon as a Diagnostic Tool for Explosion Mechanisms

How an otherwise inert carbon-oxygen white dwarf can be made to explode as an SN Ia remains unknown. A promising test of theoretical models is to constrain the distribution of material that is left unburned, carbon in particular. So far, most investigations have used line identification codes to detect carbon in the ejecta, a method that cannot be readily compared against model predictions because it requires assumed opacities and temperatures. Here, we instead use tomographic techniques to investigate the amount of carbon in the inner layers of SN?2011fe, starting from the previously published tomographic analysis of Mazzali et al. From the presence of the carbon feature in the optical at early epochs and its disappearance later on, we derive an average carbon mass fraction between 0.001 and 0.05 for velocities in the range 13,500 ? v ? 16,000 km s-1, and an upper limit of 0.005 inside that region. Based on our models and the assumed density profile, only small amounts of carbon should be in the neutral state, too little to be responsible for features seen in near-infrared spectra that were previously identified as due to neutral carbon. We discuss possible reasons for this discrepancy and compare our results against a suite of explosion models, although uncertainties in both the models and our simulations make it difficult to draw definitive conclusions.