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tokens: fit the absorption trough. In Fig. 8 , it is clear that the P Cygni model profile matches the observed spectrum in this region. The model profile is unable to reproduce the emission region of the observed spectrum. This is a result of the feature not being produced purely by scattering; clearly the feature is in net emission, and as a result we underestimate the amount of flux in this region. However, this is not important, as we are only interested in obtaining velocity ranges for the line-forming region. We obtained a minimum ejecta velocity, v phot ∼ 20 000 , and a maximum ejecta velocity, v max ∼ 26 000 . These velocity values are, surprisingly, ∼2 times higher than those obtained from our tardis models. This is quite strong evidence that there is an additional high-velocity component of the ejecta that is not observed in the early, photospheric spectra. The computation for the P Cygni Ca ii line produces a velocity from the absorption trough that is fairly robust, as is the velocity estimates from our tardis models. Therefore, we believe the difference to be a real physical effect, and is an indication of additional high-velocity material, likely of low mass, which has been ejected beyond the emitting photospheric region (which is travelling at ∼12 000–14 500 ). We note that the level of the observed continuum to the red of the P Cygni peak falls significantly below that on the blue side. We attribute this to some oversubtraction of the host galaxy flux model as described in Appendix A , and do not attach any line forming significance to the discrepancy. Our best-fitting model required a Sobolev optical depth, τ ∼ 1.3, which tells us that the P Cygni feature originates from a moderately optically thick region. Even though the +7 d spectrum appears to be nebular, the region forming the P Cygni feature is optically thick, at least for the wavelengths of the Ca ii triplet. The optical depth for these lines in all three of our tardis models is 1. Hence, the envelope is optically thin at these wavelengths in our tardis models. However, the optical depth in the tardis models becomes progressively larger with time. Since a P Cygni line requires an optically thick transition, we have a consistent picture of the Ca ii triplet line being optically thin at early epochs (probably due to high temperature), after which the material develops an optical depth around τ ∼ 1.3, providing the opacity for the absorption trough to form. We will see the next spectrum is consistent with the idea that the opacity in this line has dropped further, due to expansion, and a pure emission component has formed. 4.2 Interpreting the +14 d spectrum In the +14 d spectrum, we observe the Ca ii feature to be purely in emission. We fit a Gaussian profile to the emission feature, to further estimate the velocity of the ejecta material. By assuming the feature is solely produced by the three transitions of the Ca ii triplet, we can estimate the ejecta velocity, and the