HST-COS Observations of Hydrogen, Helium, Carbon, and Nitrogen Emissions From the SN 1987A Reverse Shock
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We present the most sensitive ultraviolet observations of Supernova 1987A to date. Imaging spectroscopy from the Hubble Space Telescope-Cosmic Origins Spectrograph shows many narrow (Delta upsilon similar to 300 km s(-1)) emission lines from the circumstellar ring, broad (Delta upsilon similar to 10-20 x 103 km s(-1)) emission lines from the reverse shock, and ultraviolet continuum emission. The high signal-to-noise ratio (>40 per resolution element) broad Ly alpha emission is excited by soft X-ray and EUV heating of mostly neutral gas in the circumstellar ring and outer supernova debris. The ultraviolet continuum at lambda > 1350 angstrom can be explained by H I two-photon (2s (2)S(1/2)-1s (2)S(1/2)) emission from the same region. We confirm our earlier, tentative detection of N V lambda 1240 emission from the reverse shock and present the first detections of broad He II lambda 1640, C IV lambda 1550, and N IV] lambda 1486 emission lines from the reverse shock. The helium abundance in the high-velocity material is He/H = 0.14 +/- 0.06. The N V/H alpha line ratio requires partial ion-electron equilibration (T(e)/T(p) approximate to 0.14-0.35). We find that the N/C abundance ratio in the gas crossing the reverse shock is significantly higher than that in the circumstellar ring, a result that may be attributed to chemical stratification in the outer envelope of the supernova progenitor. The N/C abundance may have been stratified prior to the ring expulsion, or this result may indicate continued CNO processing in the progenitor subsequent to the expulsion of the circumstellar ring.