Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Low-ionization structures in planetary nebulae: confronting models with
  observations   Around 50 PNe are presently known to possess "small-scale" low-ionization
structures (LISs). We consider here jets, jet-like, symmetrical and
non-symmetrical LISs and present a detailed comparison of the existing model
predictions with the observational morphological and kinematical properties. We
find that nebulae with LISs appear indistinctly spread among all morphological
classes of PNe, indicating that the processes leading to the formation of LISs
are not necessarily related to those responsible for the asphericity of the
large-scale morphological components of PNe. We show that both the observed
velocities and locations of most non-symmetrical LISs can be reasonably well
reproduced assuming either fossil condensations originated in the AGB wind or
in-situ instabilities. The jet models proposed to date (HD and MHD interacting
winds or accretion-disk collimated winds) appear unable to account
simultaneously for the kinematical ages and the angle between the jet and the
symmetry axes of the nebulae. The linear increase in velocity observed in
several jets favors MHD confinement compared to pure HD interacting wind
models. On the other hand, we find that the formation of jet-like systems
characterized by relatively low expansion velocities cannot be explained by any
of the existing models. Finally, the knots which appear in symmetrical and
opposite pairs of low velocity could be understood as the survival of fossil
(symmetrical) condensations formed during the AGB phase or as structures that
have experienced substantial slowing down by the ambient medium.

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The categories of this paper are: patt-sol
16
The categories of this paper are: stat
37
The categories of this paper are: supr-con
18
The categories of this paper are: econ
20
The categories of this paper are: acc-phys
13
The categories of this paper are: quant-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: mtrl-th
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The categories of this paper are: cmp-lg
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The categories of this paper are: atom-ph
3
The categories of this paper are: physics
11
The categories of this paper are: hep-lat
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The categories of this paper are: chao-dyn
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The categories of this paper are: gr-qc
4
The categories of this paper are: cond-mat
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The categories of this paper are: adap-org