Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: HI Shells Behind the Coalsack   We report the discovery of two new large HI shells in the direction of the
Coalsack nebula. Both shells were observed with the Parkes Radio Telescope as
part of the Southern Galactic Plane Survey. The largest shell, GSH 304-00-12,
is at a distance of ~1.2 kpc and has derived physical dimensions of 280 x 200
pc. The second shell, GSH 305+01-24, is at a distance of ~2.2 kpc and has
derived dimensions of 280 x 440 pc. We present a simple numerical model to show
that GSH 305+01-24 most likely formed from stellar winds in the Centaurus OB1
stellar association. There is associated radio, infrared and H-alpha continuum
emission. Both shells are situated in the Sagittarius-Carina arm, with GSH
305+01-24 more distant. The far edge of GSH 304-00-12 is at the near side of
the arm and opens into the interarm region. We find no evidence for closure at
the near side of the shell and therefore describe the geometry as conical.
Emission from the near side of the shell may be lost in absorption by the
Coalsack Nebula.

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The categories of this paper are: supr-con
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The categories of this paper are: physics
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The categories of this paper are: nucl-th
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The categories of this paper are: nlin
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The categories of this paper are: econ
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The categories of this paper are: acc-phys
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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: funct-an
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The categories of this paper are: cond-mat
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The categories of this paper are: q-bio
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The categories of this paper are: mtrl-th
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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: adap-org
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The categories of this paper are: hep-ph