Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Early Interaction of the Planetary Nebula NGC 40 with the
  Interstellar Medium   We report the detection of an Halpha emission-line structure in the upstream
side of the planetary nebula NGC 40, which is predicted by numerical
simulations, and which is attributed to Rayleigh-Taylor instability. Such a
Rayleigh-Taylor instability is expected to occur at early stages of the
interaction process between the interstellar medium (ISM) and a fast moving
planetary nebula, as is the case for NGC 40. We resolved the Rayleigh-Taylor
instability `tongues', as well as the flatness of the nebula around the
`tongues', which results from the deceleration by the ISM.

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The categories of this paper are: patt-sol
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The categories of this paper are: eess
4
The categories of this paper are: cond-mat
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The categories of this paper are: funct-an
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The categories of this paper are: solv-int
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The categories of this paper are: nucl-ex
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The categories of this paper are: hep-th
36
The categories of this paper are: cmp-lg
31
The categories of this paper are: ao-sci
37
The categories of this paper are: supr-con
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The categories of this paper are: acc-phys
0
The categories of this paper are: hep-ph
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The categories of this paper are: cs
3
The categories of this paper are: physics
35
The categories of this paper are: mtrl-th
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The categories of this paper are: math-ph