The optical recovery of SN 1979C in NGC 4321 (M100)

Abstract

Optical images and spectra of the Type II-L SN 1979C taken in 1990 and 1991 are presented. SN 1979C occurred in a faint H II region located just south of a prominent spiral arm in NGC 4321. Low-dispersion spectra show faint, broad Hα emission (V_exp_ = 6300 km s^-1^) with an asymmetric profile brighter toward the blue. Somewhat stronger emissions of [O I] λ6300, [O II] λ7325, and [O III] λ5007 show broad line profiles with two emission maxima at V_r_ = - 5700 km s^-1^ and ~0 km s^-1^, possibly corresponding to ejecta in the front portion of the facing expanding hemisphere and in the plane of the sky, respectively. No redshifted oxygen line emission was detected. The blueshifts of SN 1979C's line emission are more extreme than seen in other old, radio bright SNs and suggest clumped O-rich ejecta with substantial dust formation. Despite the absence of redshifted emission, SN 1979C's [O I] luminosity is more than 5 times larger than seen in the comparable object SN 1980K, possibly suggesting strong shock interactions with the progenitor's circumstellar material.