Nebular spectra of type Ia supernovae

Nebular spectra of Type Ia supernovae are modeled by including important 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 supernovae, leading to strong constraints on the explosion models. It is concluded that sub-Chandrasekhar-mass models are favored, but Chandrasekhar-mass models might not be entirely ruled out.