Illustration of the flux contribution from the continuum only (top row), and then He I, O I, Mg II, Ca II, Fe I, and Fe II bound-bound transitions (and continuum) to the total flux (black) in our interaction model based on the helium-star explosion model he4p0. We assume that all the model mass has been compressed into a dense shell. The total power from the interaction is set at 2 × 10⁴² erg s⁻¹, the radius of the dense shell is at 3 × 10¹⁵ cm, and its velocity is 2000 km s⁻¹. All spectra have been convolved with a Gaussian kernel (with a FWHM of 23.5 Å– standard deviation of 10 Å).