Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Formation of Globules in Planetary Nebulae   We discuss the formation of globules in planetary nebulae, typified by those
observed in the Helix Nebula. We show that the properties of the globules,
their number, mass, separation, and overall geometry strongly support a
scenario in which globules are formed by the fragmentation of a swept-up shell
as opposed to models in which the knots form in the AGB wind. We show that the
RT or other instabilities which lead to the break-up of shells formed in the
nebulae by fast winds or ionization fronts can produce arrays of globules with
the overall geometry and within the mass range observed. We also show that the
presence of a magnetic field in the circumstellar gas may play an important
role in controlling the fragmentation process. Using field strengths measured
in the precursor AGB envelopes, we find that close to the central star where
the fields are relatively strong, the wavelengths of unstable MRT modes are
larger than the shell dimensions, and the fragmentation of the shell is
suppressed. The wavelength of the most unstable MRT mode decreases with
increasing distance from the star, and when it becomes comparable to the shell
thickness, it can lead to the sudden, rapid break-up of an accelerating shell.
For typical nebula parameters, the model results in numerous fragments with a
mass scale and a separation scale similar to those observed. Our results
provide a link between global models of PN shaping in which shells form via
winds and ionization fronts, and the formation of small scale structures in the
nebulae.

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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: bayes-an
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The categories of this paper are: patt-sol
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The categories of this paper are: nucl-th
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The categories of this paper are: cond-mat
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The categories of this paper are: eess
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The categories of this paper are: gr-qc
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The categories of this paper are: dg-ga
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The categories of this paper are: quant-ph
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The categories of this paper are: math-ph
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The categories of this paper are: hep-ex
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The categories of this paper are: hep-lat
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The categories of this paper are: supr-con
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The categories of this paper are: adap-org
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The categories of this paper are: math