Unburned Material in the Ejecta of Type Ia Supernovae
  (Abridged) The presence of unburned material in the ejecta of normal Type Ia
supernovae (SNeIa) is investigated using early-time spectroscopy obtained by
the CSP. The tell-tale signature of pristine material from a C+O white dwarf
progenitor star is the presence of carbon. The most prominent carbon lines in
optical spectra of SNeIa are due to C II. We find that at least 30% of the
objects in the sample show an absorption at ~6300 AA which is attributed to C
II 6580. An association to H_alpha is considered to be unlikely. These findings
imply a larger incidence of carbon in SNeIa ejecta than previously noted. We
show how observational biases and physical conditions may hide these C II
lines, and account for the scarcity of previous carbon detections. This
relatively large frequency of carbon detections has crucial implications on our
understanding of the explosive process. The observations indicate that carbon
is present merely ~1000 km s^-1 above the bulk of Si II. Spectral modeling
shows the detections are consistent with a mass of carbon of 10^-3--10^-2
M_sun. This would imply substantial mixing, which may be related to asymmetries
of the flame propagation or clumps of unburned material along the line of
sight. However, the uniformity of the relation between C II and Si II
velocities is not consistent with the latter scenario. The observational
properties of SNeIa with and without carbon signatures are compared. A trend
toward bluer color and lower luminosity is found for objects with carbon.
