Berkeley Supernova Ia Program - IV. Carbon detection in early-time optical spectra of Type Ia supernovae

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 ≲3.6 d 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 ∼11 per cent of the SNe studied show definite C absorption features, while ∼25 per cent show some evidence for C II in their spectra. Also, if one obtains a spectrum at ≲ -5 d, then there is a better than 30 per cent chance of detecting a distinct absorption feature from C II. 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 C II λ6580 feature is measured to be 12 000-13 000 km s^-1, and the ratio of the C II λ6580 to Si II 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 C II λλ6580 and 7234 features are found mostly to decrease with time, we see evidence of a significant increase in pEW between ∼12 and 11 d 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) × 10^-3 M_⊙.