EVIDENCE FOR A SPECTROSCOPIC SEQUENCE AMONG TYPE IA SUPERNOVAE

In this Letter we present evidence for a spectral sequence among Type Ia supernovae (SN Ia's). The sequence is based on the systematic variation of several features seen in the near-maximum light spectrum. This sequence is analogous to the recently noted photometric sequence among SN Ia's which shows a relationship between the peak brightness of a SN Ia and the shape of its light curve. In addition to the observational evidence we present a partial theoretical explanation for the sequence. This has been achieved by producing a series of non-LTE synthetic spectra in which only the effective temperature is varied. The synthetic sequence nicely reproduces most of the differences seen in the observed one and presumably corresponds to the amount of Ni-56 produced in the explosion.