Detection of preshock dense circumstellar material of SN 1978K

The supernova SN 1978K has been noted for its lack of emission lines broader than a few thousand kilometers per second since its discovery in 1990. Modeling of the radio spectrum of the peculiar SN 1978K indicates the existence of H II absorption along the line of sight. To determine the nature of this absorbing region, we have obtained a high-dispersion spectrum of SN 1978K at the wavelength range 6530-6610 Angstrom. The spectrum shows not only the moderately broad H alpha emission of the supernova ejecta but also narrow nebular H alpha and [N II] emission. The high [N II] lambda 6583/H alpha ratio, 0.8-1.3, suggests that this radio-absorbing region is a stellar ejecta nebula. The expansion velocity and emission measure of the nebula are consistent with those seen in ejecta nebulae of luminous blue variables. Previous low-dispersion spectra have detected a strong [N II] lambda 5755 line, indicating an electron density of (3-12) x 10(5) cm(-3). We argue that this stellar ejecta nebula is probably part of the preshock dense circumstellar envelope of SN 1978K, We further suggest that SN 1997ab may represent a young version of SN 1978K.