Narrow Ultraviolet Emission Lines from SN 1987A: Evidence for CNO Processing in the Progenitor

UV observations of SN 1987A with the IUE satellite are reported. Spectra after 1987 May 24 show emission lines of He II, C III, N III, N IV, N V, and O III, increasing in strength with time. High-resolution observations show that the widths of the lines are less than ~30 km s^-1^ (FWHM). The line strengths and widths indicate an origin in a photoionized low-density circumstellar gas, lost by the progenitor in its red supergiant phase. The most likely excitation source is the EUV burst at the time of the shock breakout. The time evolution is consistent with that expected from a fluorescent light echo by a circumstellar shell. A nebular analysis reveals a large nitrogen overabundance with N/C = 7.8 +/- 4 and N/O = 1.6 +/- 0.8. These are respectively factors of 37 and 12 higher than the solar values, implying that the gas has undergone substantial CNO processing. To reveal CNO-processed material at the surface, the progenitor of SN 1987A is likely to have lost much of its hydrogen envelope before the explosion. This, and the existence of the shell, are consistent with models where a red supergiant evolves to the blue supergiant stage before exploding.