Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Dual Axisymmetry in Proto-Planetary Nebula Reflection Nebulosities:
  Results from an HST Snapshot Survey of PPN Candidates   We summarize results of our {\it HST} imaging survey of proto-planetary
nebula (PPN) candidates, in which we discovered two types of axisymmetric
reflection nebulosities. The Star-Obvious Low-level-Elongated (SOLE) nebulae
show a bright central star embedded in a smooth, faint extended nebulosity,
whereas the DUst-Prominent Longitudinally-EXtended (DUPLEX) nebulae have
remarkable bipolar structure with a completely or partially obscured central
star. The intrinsic axisymmetry of the PPN reflection nebulosities demonstrates
that the axisymmetry often found in planetary nebulae predates the PPN phase.
We suggest that the major cause for the apparent dual morphology is the optical
depth difference in the circumstellar dust shell rather than the inclination
angle effect.

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The categories of this paper are: math
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The categories of this paper are: atom-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: q-alg
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The categories of this paper are: ao-sci
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The categories of this paper are: cond-mat
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The categories of this paper are: hep-ph
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The categories of this paper are: math-ph
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The categories of this paper are: q-fin
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The categories of this paper are: bayes-an
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The categories of this paper are: q-bio
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The categories of this paper are: chao-dyn
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The categories of this paper are: acc-phys
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The categories of this paper are: stat
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The categories of this paper are: econ