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In studies on intermediate- and high-resolution spectra of Type Ia supernovae(SNe Ia), some objects exhibit narrow Na-I D absorptions often blueshifted withrespect to the rest wavelength within the host galaxy. The absence of these inother SNe Ia may reflect that the explosions have different progenitors:blueshifted Na-I D features might be explained by the outflows of'single-degenerate' systems (binaries of a white dwarf with a non-degeneratecompanion). In this work, we search for systematic differences among SNe Ia forwhich the Na-I D characteristics have been clearly established in previousstudies. We perform an analysis of the chemical abundances in the outer ejectaof 13 'spectroscopically normal' SNe Ia (five of which show blueshifted Nalines), modelling time series of photospheric spectra with a radiative-transfercode. We find only moderate differences between 'blueshifted-Na','redshifted-Na' and 'no-Na' SNe Ia, so that we can neither conclusively confirma 'one-scenario' nor a 'two-scenario' theory for normal SNe Ia. Yet, some ofthe trends we see should be further studied using larger observed samples:Models for blueshifted-Na SNe tend to show higher photospheric velocities thanno-Na SNe, corresponding to a higher opacity of the envelope. Consistently,blueshifted-Na SNe show hints of a somewhat larger iron-group content in theouter layers with respect to the no-Na subsample (and also to the redshifted-Nasubsample). This agrees with earlier work where it was found that the lightcurves of no-Na SNe - often appearing in elliptical galaxies - are narrower,i.e. decline more rapidly.

Type Ia supernovae with and without blueshifted narrow Na I D lines - how different is their structure?