Abstract
We have used the Faint Object Spectrograph on the Hubble Space Telescope to observe the spectra of SN 1987A over the wavelength range 2000 -- 8000\ \AA\ on dates 1862 and 2210 days after the supernova outburst. Even these pre-COSTAR observations avoid much of the contamination from the bright stars nearby and provide a very useful set of line strengths and shapes for analysis. The spectrum is formed in an unusual physical setting: cold gas which is excited and ionized by energetic electrons from the radioactive debris of the supernova explosion. The spectra of SN 1987A at this phase are surprisingly similar to those of the nova shells of CP Puppis and T Pyxidis decades after outburst. SN 1987A and the novae are characterized by emission from material with electron temperatures of only a few hundred degrees Kelvin, and show narrow Balmer continuum emission and strong emission lines from O$^+$. The Balmer continuum shape requires the electron temperature in the supernova ejecta to be as low as 500 K on day 1862 and 400 K on day 2210 after outburst. The \OIIUV\ doublet is surprisingly strong and is plausibly powered by collisional ionization of neutral oxygen to excited states of O$^+$. The line intensity ratio of the \OID\ doublet obtained from Gaussian fits of the 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 an artifact 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 is consistent with the data and allows a calculation of the decomposed line profile. All the observed strong lines are found to be blueshifted by a similar amount
Highlights
SN 1987A provides a unique opportunity to observe the debris from a supernova explosion
As part of our Supernova INtensive Study (SINS) of supernovae with HST, we present spectroscopic observations obtained with the Hubble Space Telescope (HST)
The observed emission lines are powered through non–thermal excitations by fast electrons produced by collisions with the gamma rays that result from the decay of radioactive 57Co and 44Ti produced in the supernova explosion
Summary
SN 1987A provides a unique opportunity to observe the debris from a supernova explosion. As part of our Supernova INtensive Study (SINS) of supernovae with HST, we present spectroscopic observations obtained with the Hubble Space Telescope (HST). When these HST observations were taken, in 1992 and 1993, the ejecta were so cold that thermal electrons played a negligible role in the formation of optical and ultraviolet lines. The observed emission lines are powered through non–thermal excitations by fast electrons produced by collisions with the gamma rays that result from the decay of radioactive 57Co and 44Ti produced in the supernova explosion. Late time observations of SN 1987A provide us with an excellent opportunity to test these theories, and to analyze the ejecta
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