Abstract
ABSTRACTA comparative study of optical spectra of Type Ia supernovae (SNe Ia) is extended, in light of new data. The discussion is framed in terms of the four groups defined in previous papers of this series: core normal (CN), broad line (BL), cool (CL), and shallow silicon (SS). Emerging features of the SN Ia spectroscopic diversity include evidence (1) that extreme CL SN 1991bg–likes are not a physically distinct subgroup and (2) for the existence of a substantial number of SN 1999aa–like SSs that are very similar to each other and distinguishable from CN even as late as 3 weeks after maximum light. SN 1999aa–likes may be relatively numerous yet not a physically distinct subgroup. The efficacy of quantitative spectroscopic subclassification of SNe Ia based on the equivalent widths of absorption features near 5750 Å and 6100 Å near maximum light is discussed. The absolute-magnitude dispersion of a small sample of CNs is no larger than the characteristic absolute-magnitude uncertainty.
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