Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Hubble Space Telescope Spectroscopic Observations of the Ejecta of SN
  1987A at 2000 Days   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

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The categories of this paper are: chem-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: nucl-ex
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The categories of this paper are: dg-ga
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The categories of this paper are: bayes-an
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The categories of this paper are: supr-con
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The categories of this paper are: comp-gas
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The categories of this paper are: cs
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The categories of this paper are: atom-ph
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The categories of this paper are: q-bio
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The categories of this paper are: hep-ex
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The categories of this paper are: nucl-th
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The categories of this paper are: ao-sci
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The categories of this paper are: econ
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The categories of this paper are: q-fin
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The categories of this paper are: quant-ph