Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Why a ring of stars at r = 20 kpc?   The recently discovered ring of stars near Galactocentric distance $r = 20$
kpc is interpreted as baryonic matter accreted onto the second caustic ring of
dark matter in our galaxy. Caustic rings of dark matter were predicted in the
Galactic plane at radii $a_n \simeq 40 kpc/n$ where $n=1,2,3 ..$. They can
reveal themselves by their gravitational influence on the distribution of
baryonic matter. There is additional evidence for caustic rings of dark matter
in the Milky Way from a series of sharp rises in the Galactic rotation curve.
The positions of the rises are consistent at the 3% level with the above law
for the caustic ring radii. Also, a triangular feature in the IRAS map of the
galactic plane is consistent with the imprint of the caustic ring of dark
matter nearest to us ($n=5$) upon dust and gas in the Galactic plane. These
observations imply that the dark matter in our neighborhood is dominated by a
single flow whose density and velocity vector are estimated.

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The categories of this paper are: cmp-lg
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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: nucl-ex
4
The categories of this paper are: cond-mat
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The categories of this paper are: chem-ph
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The categories of this paper are: comp-gas
5
The categories of this paper are: gr-qc
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The categories of this paper are: ao-sci
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The categories of this paper are: math
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The categories of this paper are: funct-an
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The categories of this paper are: quant-ph
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The categories of this paper are: nlin
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The categories of this paper are: hep-lat
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The categories of this paper are: mtrl-th
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The categories of this paper are: supr-con