Existing evidence of photometric and spectroscopic diversity among Type Ia supernovae is compared with the predictions from physical modeling of the explosions. Concerning light curves, changes in the central ignition density of massive (M ≃ Mch) C+O white dwarfs alone do not give appreciable variation. Spectroscopic diversity has been found in the nebular phase, the underluminous SN 1991bg providing an extreme case. A range of 0.4–0.8 M⊙ of 56Ni synthesized in the explosions is derived from the nebular spectra of a sample of SNe Ia. For SN 1991bg, however, a 56Ni mass of ∼ 0.1 M⊙ only is obtained. That leads us to explore models based on the detonation of low–mass WDs for this SN. Additionally, a nebular spectrum of SN 1991bg shows narrow Hα emission at the position of the SN. If this emission is confirmed against background contamination from the galaxy, it would be first evidence of a nondegenerate, H–rich companion in a SNIa.

Introduction

Type Ia supernovae (SNIa) are attributed to the thermonuclear explosion of C+O white dwarfs. Explosive ignition would be the outcome of accretion of matter from a close companion in a binary system and it would completely burn the star, leaving no bound remnant. In most models, explosive C burning starts at the center of the WD as a result of the increase in density and temperature induced by quasistatic mass growth.