Comparative direct analysis of Type Ia supernova spectra. I. SN 1994D

As the first step in a comprehensive, comparative, direct analysis of the spectra of Type Ia supernovae ( SNe Ia), we use the parameterized supernova synthetic-spectrum code SYNOW to interpret 26 spectra of the well-observed SN 1994D. Our results are consistent with the traditional view that the composition structure ( element abundance fractions versus ejection velocity) is radially stratified. We find that resonance-scattering features due to permitted lines of Ca II, Na I, and Fe II persist to more than 100 days after explosion. The fitting parameters for SN 1994D, together with those to be determined for other SNe Ia, will provide an internally consistent quantification of the spectroscopic diversity among SNe Ia and shed light on how the various manifestations of observational diversity are related to their physical causes.