Low carbon abundance in type Ia supernovae

We investigate the quantity and composition of unburned material in the outer layers of three normal Type Ia supernovae (SNe Ia): 2000dn, 2002cr, and 2004bw. 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 mu m) 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 lambda(rest) 0: 7773 mu m are observed through awide range of expansion velocities approximate to (9-18) x 10(3) km s(-1). 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. C I lines in the NIR are expected to be 7 - 50 times stronger than those from O I, but there is only marginal evidence of C I in the spectra and none of C II. We deduce that for these three normal SNe Ia, oxygen is more abundant than carbon by factors of 10(2)-10(3). Mg II is also detected in a velocity range similar to that of O I. 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 x 10(3) km s(-1). This result favors deflagration to detonation transition (DD) models over pure deflagration models for SNe Ia.