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

February 1999 • 1999AJ....117..725F

Authors • Fesen, Robert A. • Gerardy, Christopher L. • Filippenko, Alexei V. • Matheson, Thomas • Chevalier, Roger A. • Kirshner, Robert P. • Schmidt, Brian P. • Challis, Peter • Fransson, Claes • Leibundgut, Bruno • van Dyk, Schuyler D.

Abstract • 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 10^5-10^6 cm^-3. From the 1997 spectrum, we estimate an Hα flux of (1.3+/-0.2)x10^-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] lambdalambda(4959+5007)/lambda4363~=4, implies electron densities 10^6-10^7 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 (10^5-10^7 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.

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Schuyler Van Dyk

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