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tokens: but relatively insensitive to temperature. From fig. 1 of Keenan et al. ( 1995 ), the observed λ1483.3/λ1486.5 ratio corresponds to an electron density of (2.0–3.6) × 10 5 cm −3 in the temperature range (1–2) × 10 4 K. We conclude that the CSM density should be safely lower than the critical densities of the semiforbidden lines. 3.2.4 Line profiles Fig. 7 compares the velocity profile of several UV and optical lines at ∼3100 d. The Hα line extends to ∼2600 km s −1 on the blue side and only ∼1000 km s −1 on the red. Hβ, He i λ5876, and Mg ii λλ2796, 2803 have similar line profiles. While the blue side can be well fitted with an electron-scattering wing, the line centroid is shifted to the blue, with a ‘shoulder’ at ∼−800 km s −1 . As shown by Taddia et al. ( 2020 ) and Dessart et al. ( 2015 ), this type of profile can be explained by a combination of emission from the shock wave and emission from the pre-shock CSM. The emission from the shock is responsible for the blueshifted shoulder, while the central component is coming from pre-ionized gas in the CSM. Both components are affected by electron scattering in the low-velocity CSM. This gives rise to the smooth, blue wing shortward of the ‘shoulder.’ Because of the smoothing by the electron scattering, the velocity of the ‘shoulder’ is lower than the shock velocity. Figure 7. Line profiles of Hα (black), Hβ (green), He i λ5876 (red), Lyα (magenta), and N iv ] λλ1483.3, 1486.5 (blue). Note the narrower line profile of the N iv ] lines, as well as the contribution of the semiforbidden λ1483.3 component on the blue side. Both Hβ and He i λ5876 have line profiles similar to that of Hα, as expected if these arise from similar regions (Fig. 7 ). The Lyα line has a profile broadly consistent with Hα, although strongly distorted by a central absorption feature from both the Galactic and host-galaxy ISM. The C iii ], N iii ], N iv ], and N v UV lines are considerably narrower than Hα, and consistent with being unresolved at the ∼300 km s −1 instrumental resolution of STIS. These lines do not show the velocity shift of the Balmer and He i lines, and most likely come from the same optically thin, highly ionized region of the CSM as the high-ionization optical lines. 3.2.5 Spectral evolution The optical spectra in this paper span one of the longest periods of any SN. Fig. 8 highlights only a very slow evolution of the spectrum over three epochs: 736, 3099, and 5105 d. Unlike most other SNe, there is no obvious transition to a nebular spectrum even at this extremely late stage. However, the line ratios do exhibit some interesting quantitative changes that are consistent with the shock wave propagating into a medium of decreasing density. Figure 8. Evolution of the optical spectrum from day 736 to 5105 with identification of the high-ionization lines at the top and colour coding and identification of the low-ionization lines for the day 3099 spectrum. We start with a look at some with the most