Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Very deep spectroscopy of the bright Saturn Nebula NGC 7009 - II.
  Analysis of the rich optical recombination spectrum   [Abridged] We present a critical analysis of the rich optical recombination
spectrum of NGC 7009, in the context of the bi-abundance nebular model proposed
by Liu et al. (2000). The observed relative intensities are compared with the
theoretical predictions based on high quality effective recombination
coefficients, now available for the recombination line spectrum of a number of
heavy element ions. The possibility of plasma diagnostics using the optical
recombination lines (ORLs) of heavy element ions is discussed in detail. Plasma
diagnostics based on the N II and O II recombination spectra both yield
electron temperatures close to 1000 K, which is lower than those derived from
the collisionally excited line (CEL) ratios by nearly one order of magnitude.
The very low temperatures yielded by the O II and N II ORLs indicate that they
originate from very cold regions. The C^{2+}/H^+, N^{2+}/H^+, O^{2+}/H^+ and
Ne^{2+}/H^+ ionic abundance ratios derived from ORLs are consistently higher,
by about a factor of 5, than the corresponding values derived from CELs. In
calculating the ORL ionic abundance ratios, we have used the newly available
high quality effective recombination coefficients, and adopted an electron
temperature of 1000 K, as given by the ORL diagnostics and as a consequence
presumably representing the physical conditions prevailing in the regions where
the heavy element ORLs arise. A comparison of the results of plasma diagnostics
and abundance determinations for NGC 7009 points to the existence of "cold",
metal-rich (i.e. H-deficient) inclusions embedded in the hot, diffuse ionized
gas, first postulated by Liu et al. (2000).

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The categories of this paper are: solv-int
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The categories of this paper are: physics
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The categories of this paper are: acc-phys
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The categories of this paper are: cmp-lg
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The categories of this paper are: adap-org
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The categories of this paper are: math-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: q-bio
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The categories of this paper are: stat
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The categories of this paper are: alg-geom
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The categories of this paper are: cs
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The categories of this paper are: econ
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The categories of this paper are: nucl-th
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The categories of this paper are: chem-ph
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The categories of this paper are: nlin