2001MNRAS.321..341M

The spectrum of the normal Type Ia SN 1990N observed very early on (14 days before B maximum) was analysed by Fisher et al., who showed that the large width and the unusual profile of the strong line near 6000Å can be reproduced if the line is assumed to be due to Cii 6578, 6583Å and if carbon is located in a high-velocity shell. This line is one of the characterizing features of SNe Ia, and is usually thought to be due to Siii. A Monte Carlo spectrum synthesis code is used to investigate this suggestion further. The result is that if a standard explosion model is used, the mass enclosed in the shell at the required high velocity (25000-35000km.s^–1) is too small to give rise to a strong Cii line. At the same time, removing silicon has a negative effect on the synthetic spectrum at other wavelengths, and removing carbon from the lower velocity regions near the photosphere makes it difficult to reproduce two weak lines which are naturally explained as Cii, one of them being the line which Fisher et al. suggested is responsible for the strong 6000-Å feature. However, synthetic spectra confirm that although Siii can reproduce most of the observed 6000-Å line, the red wing of the line extends too far to be compatible with a Siii origin, and that the flat bottom of the line is also not easy to reproduce. The best fit is obtained for a normal SN Ia abundance mix at velocities near the photosphere (15500-19000km.s^–1) and an outer carbon-silicon shell beyond 20000km.s^–1. This suggests that mixing is not complete in the outer ejecta of an SN Ia. Observations at even earlier epochs might reveal to what extent a carbon shell is unmixed.