Late-Time Optical and Ultraviolet Spectra of SN 1979C and SN 1980K

A low-dispersion Keck I spectrum of SN 1980K taken in 1995 August (t = 14.8 yr after explosion) and a spectrum taken in 1997 November (t = 17.0 yr) at the MDM Observatory show broad 5500 km s-1 emission lines of Hα, [O I] 6300, 6364 Å, and [O II] 7319, 7330 Å. Weaker but similarly broad lines detected include [Fe II] 7155 Å, [S II] 4068, 4072 Å, and a blend of [Fe II] lines at 5050–5400 Å. The presence of strong [S II] 4068, 4072 Å emission but a lack of [S II] 6716, 6731 Å emission suggests electron densities of 105–106 cm-3. From the 1997 spectrum, we estimate an Hα flux of (1.3 ± 0.2) × 10-15 ergs cm-2 s-1, indicating a 25% decline from the 1987–1992 levels during the period 1994 to 1997, possibly related to a reported decrease in its nonthermal radio emission. A 1993 May, Multiple Mirror Telescope spectrum of SN 1979C (t = 14.0 yr) shows a somewhat different spectrum from that of SN 1980K. Broad, 6000 km s-1 emission lines are also seen but with weaker Hα, stronger [O III] 4959, 5007 Å, more highly clumped [O I] and [O II] line profiles, no detectable [Fe II] 7155 Å emission, and a faint but very broad emission feature near 5750 Å. A 1997 Hubble Space Telescope Faint Object Spectrograph, near-UV spectrum (2200–4500 Å) shows strong lines of C II] 2324, 2325 Å, [O II] 2470 Å, and Mg II 2796, 2803 Å, along with weak [Ne III] 3969 Å, [S II] 4068, 4072 Å, and [O III] 4363 Å emissions. The UV emission lines show a double-peak profile with the blueward peak substantially stronger than the red, suggesting dust extinction within the expanding ejecta [E(B-V) = 0.11–0.16 mag]. The lack of detectable [O II] 3726, 3729 Å emission, together with [O III] λλ(4959 + 5007)/λ4363 ≃ 4, implies electron densities 106–107 cm-3. These Type II linear supernovae (SNe II-L) spectra show general agreement with the lines expected in a circumstellar interaction model, but the specific models that are available show several differences with the observations. High electron densities (105–107 cm-3) result in stronger collisional de-excitation than assumed in the models, thereby explaining the absence of several moderate to strong predicted lines such as [O II] 3726, 3729 Å, [N II] 6548, 6583 Å, and [S II] 6716, 6731 Å. Interaction models are needed that are specifically suited to these supernovae. We review the overall observed range of late-time SNe II-L properties and briefly discuss their properties relative to young, ejecta-dominated Galactic supernova remnants.