Title:
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The Evolution of Ultraviolet Emission Lines From Circumstellar Material
Surrounding SN 1987A.
(Stockholm Observatory preprint No. 82)

Authors:
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George Sonneborn (Goddard), Claes Fransson & Peter Lundqvist (Stockholm)
Angelo Cassatella (Frascati), Roberto Gilmozzi (ESO Garching),
Robert P. Kirshner (CfA), Nino Panagia (STScI) & Willem Wamsteker (ESA-VILSPA)

Abstract:
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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 N V 1240, N IV] 1486,
He II 1640, O III] 1665, N III] 1751, and C III] 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 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 +/- 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)E17 cm and inclination of 41.0 +/- 3.9 degrees
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 multi-component density structure. We estimate the emitting mass
near maximum (around 400 days) to be roughly 4.7E-2 solar masses, assuming a
filling factor of unity and an electron density of 2.6E4 cm^{-3}. These
results are discussed in the context of current models for the emission and
hydrodynamics of the ring.

To appear in:
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ApJ