Hst -cos Observations of Hydrogen, Helium, Carbon, and Nitrogen Emission From the Sn 1987a Reverse Shock

Abstract

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.