Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Hierarchy of Interstellar and Stellar Structures and the Case of the
  Orion Star-Forming Region   Hierarchical structure from stellar clusters, to subgroups, to associations
and star complexes is discussed in the context of the Orion stellar grouping
and its origin. The analogous structure in gas clouds is also reviewed, with an
emphasis on general fractal properties and mass distribution functions. The
distinction between the cloud, cluster, and stellar mass functions is discussed
in terms of different sampling statistics for hierarchically structured clumps
in clouds with an interclump medium.

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The categories of this paper are: physics
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The categories of this paper are: hep-ph
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The categories of this paper are: hep-th
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The categories of this paper are: quant-ph
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The categories of this paper are: chem-ph
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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: hep-ex
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The categories of this paper are: gr-qc
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The categories of this paper are: bayes-an
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The categories of this paper are: solv-int
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The categories of this paper are: hep-lat
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The categories of this paper are: comp-gas
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The categories of this paper are: math-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: eess