2012MNRAS.425.1917S

While O is often seen in spectra of Type Ia supernovae (SNe Ia) as both unburned fuel and a product of C burning, C is only occasionally seen at the earliest times and represents the most direct way of investigating primordial white dwarf material and its relation to SN Ia explosion scenarios and mechanisms. In this paper, we search for C absorption features in 188 optical spectra of 144 low-redshift (z < 0.1) SNe Ia with ages ≲ 3.6d after maximum brightness. These data were obtained as part of the Berkeley Supernova Ia Program (BSNIP) and represent the largest set of SNe Ia in which C has ever been searched. We find that ∼ 11per cent of the SNe studied show definite C absorption features, while ∼ 25per cent show some evidence for CII in their spectra. Also, if one obtains a spectrum at ≲ -5d, then there is a better than 30per cent chance of detecting a distinct absorption feature from CII. SNe Ia that show C are found to resemble those without C in many respects, but objects with C tend to have bluer optical colours than those without C. The typical expansion velocity of the CII λ6580 feature is measured to be 12000–13000 km/s, and the ratio of the CII λ6580 to SiII λ6355 velocities is remarkably constant with time and among different objects with a median value of ∼ 1.05. While the pseudo-equivalent widths (pEWs) of the CII λλ6580 and 7234 features are found mostly to decrease with time, we see evidence of a significant increase in pEW between ∼ 12 and 11d before maximum brightness, which is actually predicted by some theoretical models. The range of pEWs measured from the BSNIP data implies a range of C masses in SN Ia ejecta of about (2–30) x 10^–3 M_☉.

Marc J. Staley Fellow.