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THE EVOLUTION OF LATE-TIME OPTICAL EMISSION FROM SN 1979C

Optical spectra of the bright Type II-L supernova SN 1979C obtained in April 2008 with the 6.5 m Multiple Mirror Telescope are compared with archival late-time spectra to follow the evolution of its optical emission over the age range of 11–29 years. We estimate an Hα flux decrease of around 35% from 1993 to 2008 but noticeable increases in the strength of blueshifted emission of forbidden oxygen lines. While the maximum expansion of the broad ∼6700 km s−1 Hα emission appears largely unchanged from 1993, we find a significant narrowing of the double-peaked emission profiles in the [O i] λλ6300, 6364 and [O ii] λλ7319, 7330 lines. A comparison of late-time optical spectra of a few other Type II SNe which, like SN 1979C, exhibit bright late-time X-ray, optical, and radio emissions, suggests that blueshifted double-peaked oxygen emission profiles may be a common phenomenon. Finally, detection of a faint, broad emission bump centered around 5800 Å suggests the presence of WC-type Wolf–Rayet stars in the SN's host star cluster.