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

A low-dispersion Keck I spectrum of SN 1980K taken in August 1995 (t = 14.8 yr after explosion) and a November 1997 MDM spectrum (t = 17.0 yr) show broad 5500 km s 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 cm. From the 1997 spectra, we estimate an Hα flux of 1.3± 0.2× 10 erg cm s indicating a 25% decline from 1987–1992 levels during the period 1994 to 1997, possibly related to a reported decrease in its nonthermal radio emission. A May 1993 MMT spectrum of SN 1979C (t = 14.0 yr) shows a somewhat different spectrum from that of SN 1980K. Broad, 6000 km s 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 HST 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] (4959+5007)/4363 ≃ 4 imply electron densities 10 cm. These new 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 cm) result in stronger collisional de-excitation than assumed in the models, thereby explaining the absence of 6127 Wilder Laboratory, Physics & Astronomy Department, Dartmouth College, Hanover, NH 03755 2 Department of Astronomy, University of California, Berkeley, CA 94720-3411 Department of Astronomy, University of Virginia, P.O. Box 3818, Charottesville, VA 22903 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 Mt. Stromlo Observatory, The Australian National University, Private Bag, ACT 2611 Weston Creek P.O., Australia Stockholm Observatory, SE-133 36 Saltsjobaden, Sweden European Southern Observatory, Karl-Schwarzschild-Strasse 2, Garching, Germany IPAC, Caltech, Mailcode 100-22, Pasadena, CA 91125