We investigate the quantity and composition of unburned material in the outer
layers of three normal Type Ia supernovae (SNe Ia): 2000dn, 2002cr and 20 04bw.
Pristine matter from a white dwarf progenitor is expected to be a mixture of
oxygen and carbon in approximately equal abundance. Using near-infrared (NIR,
0.7-2.5 microns) spectra, we find that oxygen is abundant while carbon is
severely depleted with low upper limits in the outer third of the ejected mass.
Strong features from the OI line at rest wavelength = 0.7773 microns are
observed through a wide range of expansion velocities approx. 9,000 - 18,000
km/s. This large velocity domain corresponds to a physical region of the
supernova with a large radial depth. We show that the ionization of C and O
will be substantially the same in this region. CI lines in the NIR are expected
to be 7-50 times stronger than those from OI but there is only marginal
evidence of CI in the spectra and none of CII. We deduce that for these three
normal SNe Ia, oxygen is more abundant than carbon by factors of 100 - 1,000.
MgII is also detected in a velocity range similar to that of OI. The presence
of O and Mg combined with the absence of C indicates that for these SNe Ia,
nuclear burning has reached all but the extreme outer layers; any unburned
material must have expansion velocities greater than 18,000 km/s. This result
favors deflagration to detonation transition (DD) models over pure deflagration
models for SNe Ia.Comment: accepted for publication in Ap