The Evolution of Late-time Optical Emission From Sn 1979c

Abstract

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 Ha 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 similar to 6700 km s(-1) Ha emission appears largely unchanged from 1993, we find a significant narrowing of the double-peaked emission profiles in the [OI] lambda lambda 6300, 6364 and [OI] lambda lambda 7319, 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 similar to suggests the presence of WC-type Wolf-Rayet stars in the SN's host star cluster.