The evolution of ultraviolet emission lines from circumstellar material surrounding SN 1987a

The presence of narrow high-temperature emission lines from nitrogen-rich gas close to SN 1987A has been a principal observational constraint on the evolutionary status of the supernova's progenitor. A new analysis of the complete 5 year set of low- and high-resolution IUE ultraviolet spectra of SN 1987A (1987.2-1992.3) provides fluxes for the N V lambda 1240, N IV] lambda 1486, He II lambda 1640, O III] lambda 1665, N III] lambda 1751, and C III] lambda 1908 lines with significantly reduced random and systematic errors and reveals significant short-term fluctuations in the light curves. The N V N IV], and N III] lines turn on sequentially over 15-20 days and show a progression from high to low ionization potential, implying an ionization gradient in the emitting region. The line emission turns on suddenly at 83+/-4 days after the explosion, as defined by N IV]. The N III] line reaches peak luminosity at 399+/-15 days. A ring radius of (6.24+/-0.20)x10(17) cm and inclination of 41 degrees.0+/-3 degrees.9 is derived from these times, assuming a circular ring. The probable role of resonant scattering in the N V light curve introduces systematic errors that leads us to exclude this line from the timing analysis. A new nebular analysis yields improved CNO abundance ratios of N/C=6.1+/-1.1 and N/O=1.7+/-0.5, confirming the nitrogen enrichment found in our previous paper. From the late-time behavior of the light curves we find that the emission originates from progressively lower density gas and that the emitting region has a multicomponent density structure. We estimate the emitting mass near maximum (similar to 400 days) to be similar to 4.7x10(-2) M., assuming a filling factor of unity and an electron density of 2.6x10(4) cm(-3). These results are discussed in the context of current models for the emission and hydrodynamics of the ring.