On the spectroscopic diversity of Type Ia supernovae

A comparison of the ratio of the depths of two absorption features in the spectra of Type Ia supernovae (SNe Ia) near the time of maximum brightness with the blueshift of the deep red Si II absorption feature 10 days after maximum shows that the spectroscopic diversity of SNe Ia is multidimensional. There is a substantial range of blueshifts at a given value of the depth ratio. We also find that the spectra of a sample of SNe Ia obtained a week before maximum brightness can be arranged in a "blueshift sequence" that mimics the time evolution of the pre-maximum-light spectra of an individual SN Ia, the well-observed SN 1994D. Within the context of current SN Ia explosion models, we suggest that some of the SNe Ia in our sample were delayed detonations while others were plain deflagrations.