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\nunburned fuel and a product of C burning, C is only occasionally seen at the\nearliest times, and it represents the most direct way of investigating\nprimordial white dwarf material and its relation to SN Ia explosion scenarios\nand mechanisms. In this paper, we search for C absorption features in 188\noptical spectra of 144 low-redshift (z < 0.1) SNe Ia with ages <3.6 d after\nmaximum brightness. These data were obtained as part of the Berkeley SN Ia\nProgram (BSNIP; Silverman et al. 2012) and represent the largest set of SNe Ia\nin which C has ever been searched. We find that ~11 per cent of the SNe studied\nshow definite C absorption features while ~25 per cent show some evidence for C\nII in their spectra. Also, if one obtains a spectrum at t < -5 d, then there is\na better than 30 per cent chance of detecting a distinct absorption feature\nfrom C II. SNe Ia that show C are found to resemble those without C in many\nrespects, but objects with C tend to have bluer optical colours than those\nwithout C. The typical expansion velocity of the C II {\\lambda}6580 feature is\nmeasured to be 12,000-13,000 km/s, and the ratio of the C II {\\lambda}6580 to\nSi II {\\lambda}6355 velocities is remarkably constant with time and among\ndifferent objects with a median value of ~1.05. While the pseudo-equivalent\nwidths (pEWs) of the C II {\\lambda}6580 and C II {\\lambda}7234 features are\nfound mostly to decrease with time, we see evidence of a significant increase\nin pEW between ~12 and 11 d before maximum brightness, which is actually\npredicted by some theoretical models. The range of pEWs measured from the BSNIP\ndata implies a range of C mass in SN Ia ejecta of about (2-30) * 10^-3 M_Sun.\n