Physical conditions in the UV line emitting region of supernova 1979cin NGC 4321.

Abstract

Ultraviolet observations of SN 1979c are used to derive the physical conditions in the line-forming regions. The emission lines have asymmetric profiles and large expansion velocities and are therefore formed in a narrow shell close to the photosphere having a constant velocity. The most likely mechanism for the excitation of these lines is photoionization of the gas by energetic radiation from the supernova shock wave. Abundance ratios of N/C = 8 and O/C less than 4 are derived from the intensities of the UV lines. It is concluded that the presupernova star was a supergiant which had undergone a long period of mass loss, thereby revealing CNO-processed material. Finally, evidence is found for a narrow, transient feature in the semiforbidden N III 1750 line, which is likely to arise in the circumstellar gas around the supernova.