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 five-year set of low and high resolution IUE ultraviolet spectra of SN 1987A ( 1987.2–1992.3 ) provide fluxes for the \ion N5 \lambda 1240 , \ion N4 ] \lambda 1486 , \ion He2 \lambda 1640 , \ion O3 ] \lambda 1665 , \ion N3 ] \lambda 1751 , and \ion C3 ] \lambda 1908 lines with significantly reduced random and systematic errors and reveals significant short-term fluctuations in the light curves .
The \ion N5 , \ion N4 ] , and \ion N3 ] lines turn on sequentially over 15 to 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 \pm 4 days after the explosion , as defined by \ion N4 ] .
The \ion N3 ] line reaches peak luminosity at 399 \pm 15 days .
A ring radius of ( 6.24 \pm 0.20 ) \times 10 ^ { \small 17 } cm and inclination of 41 \fdg 0 \pm 3 \fdg 9 is derived from these times , assuming a circular ring .
The probable role of resonant scattering in the \ion N5 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 \pm 1.1 and N/O = 1.7 \pm 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 multi-component density structure .
We estimate the emitting mass near maximum ( \sim 400 days ) to be \sim 4.7 \times 10 ^ { \small - 2 } M _ { \odot } , assuming a filling factor of unity and an electron density of 2.6 \times 10 ^ { \small 4 } ~ { } cm ^ { \small - 3 } .
These results are discussed in the context of current models for the emission and hydrodynamics of the ring .