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 λ1240, N IV] λ1486, He II λ1640, O III] λ1665, N III] λ1751, and C III] λ1908 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) × 10 17 cm and inclination of 41 0 ± 3 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 (~400 days) to be ~4.7 × 10 -2 M ☉ , assuming a filling factor of unity and an electron density of 2.6 × 10 4 cm -3 . These results are discussed in the context of current models for the emission and hydrodynamics of the ring.