Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On expansion parallax distances for planetary nebulae   The distances to individual wind-driven bubbles such as Planetary Nebulae
(PNe) can be determined using expansion parallaxes: the angular expansion
velocity in the sky is compared to the radial velocity of gas measured
spectroscopically. Since the one is a pattern velocity, and the other a matter
velocity, these are not necessarily the same. Using the jump conditions for
both shocks and ionization fronts, I show that for typical PNe the pattern
velocity is 20 to 30% larger than the material velocity, and the derived
distances are therefore typically 20 to 30% too low. I present some corrected
distances and suggest approaches to be used when deriving distances using
expansion parallaxes

21
The categories of this paper are: adap-org
34
The categories of this paper are: chem-ph
10
The categories of this paper are: q-bio
5
The categories of this paper are: gr-qc
30
The categories of this paper are: q-alg
12
The categories of this paper are: nucl-th
33
The categories of this paper are: plasm-ph
26
The categories of this paper are: bayes-an
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The categories of this paper are: eess
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The categories of this paper are: nucl-ex
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The categories of this paper are: chao-dyn
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The categories of this paper are: quant-ph
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The categories of this paper are: hep-th
27
The categories of this paper are: comp-gas
37
The categories of this paper are: supr-con
3
The categories of this paper are: physics