ar X iv : a st ro - p h / 06 01 61 4 v 1 2 6 Ja n 20 06 Low Carbon Abundance in Type Ia Supernovae
نویسندگان
چکیده
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 µm) 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 O I line at λ rest = 0.7773 µm are observed through a wide range of expansion velocities ≈ 9 − 18 × 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 × 10 3 km s −1. This result favors deflagration to detonation transition (DD) models over pure deflagration models for SNe Ia.
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