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We have used the Faint Object Spectrograph on the Hubble Space Telescope toobserve the spectra of SN 1987A over the wavelength range 2000 -- 8000\ \AA\ ondates 1862 and 2210 days after the supernova outburst. Even these pre-COSTARobservations avoid much of the contamination from the bright stars nearby andprovide a very useful set of line strengths and shapes for analysis. Thespectrum is formed in an unusual physical setting: cold gas which is excitedand ionized by energetic electrons from the radioactive debris of the supernovaexplosion. The spectra of SN 1987A at this phase are surprisingly similar tothose of the nova shells of CP Puppis and T Pyxidis decades after outburst. SN1987A and the novae are characterized by emission from material with electrontemperatures of only a few hundred degrees Kelvin, and show narrow Balmercontinuum emission and strong emission lines from O$^+$. The Balmer continuumshape requires the electron temperature in the supernova ejecta to be as low as500 K on day 1862 and 400 K on day 2210 after outburst. The \OIIUV\ doublet issurprisingly strong and is plausibly powered by collisional ionization ofneutral oxygen to excited states of O$^+$.  The line intensity ratio of the \OID\ doublet obtained from Gaussian fits ofthe line profiles is 1.8$\pm0.2$, contrary to the optically thin limit of 3.This ratio is {\it not} due to an optical depth effect, but rather is anartifact of assuming a Gaussian profile to fit the \OID\ doublet profile.Specifying the line ratio $R\, = \, F([{\rm OI}]6300)/F([{\rm OI}]6364)$ = 3 isconsistent with the data and allows a calculation of the decomposed lineprofile.  All the observed strong lines are found to be blueshifted by a similar amountComment: 26 pages, 8 Postcript figures availlable at  ftp://tycho.as.utexas.edu/pub/users/lwang/SN87ASIN

Hubble Space Telescope Spectroscopic Observations of the Ejecta of SN 1987A at 2000 Days