France, KevinMcCray, RichardPenton, Steven V.Kirshner, Robert P.Challis, PeterMartin Laming, J.Bouchet, PatriceChevalier, RogerGarnavich, Peter M.Fransson, ClaesHeng, KevinLarsson, JosefinLawrence, StephenLundqvist, PeterPanagia, NinoPun, Chun S. J.Smith, NathanSollerman, JesperSonneborn, GeorgeSugerman, BenCraig Wheeler, J.2019-09-252011France, Kevin, Richard McCray, Steven V. Penton, Robert P. Kirshner, Peter Challis, J. Martin Laming, Patrice Bouchet, et al. 2011. “HST-COS OBSERVATIONS OF HYDROGEN, HELIUM, CARBON, AND NITROGEN EMISSION FROM THE SN 1987A REVERSE SHOCK.” The Astrophysical Journal 743 (2): 186. https://doi.org/10.1088/0004-637x/743/2/186.0004-637X1538-4357http://nrs.harvard.edu/urn-3:HUL.InstRepos:41399770We 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.en-USJournal Article2019-09-2510.1088/0004-637X/743/2/186