Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: MSX Mid Infrared Imaging of Massive Star Birth Environments. I:
  Ultracompact HII Regions   We present mid-IR 21micron images of a sample of radio selected Ultracompact
HII (UCHII) regions, obtained with the Midcourse Space Experiment (MSX). All,
with one possible exception, are detected at mid-IR wavelengths, sampling the
warm dust emission of the cocoons of the OB star central exciting sources. Many
of the UCHII regions have nearby (up to a few pc distant) companion dust
emission sources, which represent other potential star birth sites. In some
objects the companion dominates the IRAS point source catalogue entry for the
UCHII region. We compare the mid- and far-IR dust emission, measuring the
embedded hot star luminosity, with published UCHII radio emission, measuring
the Lyman continuum luminosity. We find a spectral type dependence, as
predicted by the standard model of an ultracompact ionized hydrogen region,
surrounded by a natal dust shell, with some scatter, which can be understood by
consideration of: 1) dust absorption of some fraction of the emitted Lyman
continuum photons; 2) fainter companion stars within the UCHII region; 3) the
structure of the UCHII regions differing from star to star. Overall, the higher
spatial resolution offered by MSX alleviates difficulties often encountered by
comparison of IRAS far-IR fluxes with radio derived ionizing fluxes for UCHII
regions.

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The categories of this paper are: atom-ph
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The categories of this paper are: physics
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The categories of this paper are: math-ph
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The categories of this paper are: q-alg
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The categories of this paper are: q-fin
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The categories of this paper are: patt-sol
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The categories of this paper are: chem-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: q-bio
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The categories of this paper are: adap-org
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The categories of this paper are: cs
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The categories of this paper are: chao-dyn
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The categories of this paper are: comp-gas
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The categories of this paper are: bayes-an
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The categories of this paper are: cmp-lg