UNBURNED MATERIAL IN THE EJECTA OF TYPE Ia SUPERNOVAE

The presence of unburned material in the ejecta of normal Type Ia supernovae (SNe Ia) is investigated using early-time spectroscopy obtained by the Carnegie Supernova Project. The tell-tale signature of pristine material from a C+O white dwarf progenitor star is the presence of carbon, as oxygen is also a product of carbon burning. The most prominent carbon lines in optical spectra of SNe Ia are expected to arise from CII. We find that at least 30% of the objects in the sample show an absorption at =6300A which is attributed to C II [lambda]6580. An alternative identification of this absorption as H alpha is considered to be unlikely. These findings imply a larger incidence of carbon in SNe Ia ejecta than previously noted. The spectroscopic and photometric properties of SNe Ia with and without carbon signatures are compared. A trend toward bluer color and lower luminosity at maximum light is found for objects which show carbon.