NEBULAR SPECTRA OF TYPE IA SUPERNOVAE

Nebular spectra of Type Ia supernovae are modeled by including importa nt atomic processes and reliable atomic data for a representative set of Type Ia explosion models. The calculated spectra are compared with the observed optical spectra of spectroscopically normal Type Ia super novae, leading to strong constraints on the explosion models. It is co ncluded that sub-Chandrasekhar-mass models are favored, but Chandrasek har-mass models might not be entirely ruled out.