Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Evolution of NGC 7027 at Radio Frequencies: A New Determination of
  the Distance and Core Mass   We present the results of a 25-year program to monitor the radio flux
evolution of the planetary nebula NGC7027. We find significant evolution of the
spectral flux densities. The flux density at 1465 MHz, where the nebula is
optically thick, is increasing at a rate of 0.251+-0.015 % per year, caused by
the expansion of the ionized nebula. At frequencies where the emission is
optically thin, the spectral flux density is changing at a rate of
-0.145+-0.005 % per year, caused by a decrease in the number of ionizing
photons coming from the central star. A distance of 980+-100 pc is derived. By
fitting interpolated models of post-AGB evolution to the observed changes, we
find that over the 25-yr monitoring period, the stellar temperature has
increased by 3900+-900 K and the stellar bolometric luminosity has decreased by
1.75+-0.38 %. We derive a distance-independent stellar mass of 0.655+-0.01
solar masses adopting the Bloecker stellar evolution models, or about 0.04
solar masses higher when using models of Vassiliadis & Wood which may provide a
better fit. A Cloudy photoionization model is used to fit all epochs at all
frequencies simultaneously. The differences between the radio flux density
predictions and the observed values show some time-independent residuals of
typically 1 %. A possible explanation is inaccuracies in the radio flux scale
of Baars et al. We propose an adjustment to the flux density scale of the
primary radio flux calibrator 3C286, based on the Cloudy model of NGC7027. We
also calculate precise flux densities for NGC7027 for all standard continuum
bands used at the VLA, as well as for some new 30GHz experiments.

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The categories of this paper are: econ
7
The categories of this paper are: hep-th
26
The categories of this paper are: bayes-an
25
The categories of this paper are: patt-sol
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The categories of this paper are: dg-ga
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The categories of this paper are: adap-org
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The categories of this paper are: funct-an
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The categories of this paper are: nucl-th
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The categories of this paper are: quant-ph
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The categories of this paper are: q-fin
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The categories of this paper are: cs
30
The categories of this paper are: q-alg
3
The categories of this paper are: physics
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The categories of this paper are: stat
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The categories of this paper are: supr-con
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The categories of this paper are: alg-geom