Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Peculiar Nebula Simeis 57: I. Ionized Gas and Dust Extinction   We present high resolution radio continuum maps of the Galactic nebula Simeis
57 (= HS 191 = G 80.3+4.7) made with the WSRT and the DRAObservatory at
frequencies of 609, 1412 and 1420 MHz. At optical and at radio wavelengths, the
nebula has a peculiar ``S'' shape, crossed by long, thin and straight
filaments. The radio maps, combined with other maps from existing databases,
show essentially all radio emission from the peculiar and complex nebula to be
thermal and optically thin. Although neither the distance nor the source of
excitation of Simeis 57 are known, the nebula can only have a moderate electron
density of typically n(e) = 100 cm-3. Its mass is also low, not exceeding some
tens of solar masses. Peak emission measures are 5000 pc cm-6.Obscuring dust is
closely associated with the nebula, but seems to occur mostly in front of it.
Extinctions vary from A(V) = 1.0 mag to A(V) = 2.8 mag with a mean of about 2
mag. The extinction and the far-infrared emission at 100 microns are
well-correlated.

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The categories of this paper are: patt-sol
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The categories of this paper are: alg-geom
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The categories of this paper are: hep-lat
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The categories of this paper are: cs
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The categories of this paper are: math
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The categories of this paper are: nucl-th
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The categories of this paper are: physics
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The categories of this paper are: nlin
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The categories of this paper are: bayes-an
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The categories of this paper are: eess
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The categories of this paper are: quant-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: solv-int
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The categories of this paper are: dg-ga
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The categories of this paper are: gr-qc
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The categories of this paper are: chem-ph