Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Radio Continuum and Recombination Line Study of UC HII Regions with
  Extended Envelopes   We have carried out 21 cm radio continuum observations of 16 UC HII regions
using the VLA (D-array) in search of associated extended emission. We have also
observed H76$_\alpha$ recombination line towards all the sources and
He76$_\alpha$ line at the positions with strong H76$_\alpha$ line emission. The
UC HII regions have simple morphologies and large (>10) ratios of single-dish
to VLA fluxes. Extended emission was detected towards all the sources. The
extended emission consists of one to several compact components and a diffuse
extended envelope. All the UC HII regions but two are located in the compact
components, where the UC HII regions always correspond to their peaks. The
compact components with UC HII regions are usually smaller and denser than
those without UC HII regions. Our recombination line observations indicate that
the ultracompact, compact, and extended components are physically associated.
The UC HII regions and their associated compact components are likely to be
ionized by the same sources on the basis of the morphological relations
mentioned above. This suggests that almost all of the observed UC HII regions
are not `real' UC HII regions and that their actual ages are much greater than
their dynamical age (<10000 yr). We find that most of simple UC HII regions
previously known have large ratios of single-dish to VLA fluxes, similar to our
sources. Therefore, the `age problem' of UC HII regions does not seem to be as
serious as earlier studies argued. We present a simple model that explains
extended emission around UC HII regions. Some individual sources are discussed.

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The categories of this paper are: hep-ex
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The categories of this paper are: chem-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: q-fin
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The categories of this paper are: hep-th
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The categories of this paper are: econ
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The categories of this paper are: comp-gas
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The categories of this paper are: acc-phys
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The categories of this paper are: patt-sol
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The categories of this paper are: hep-ph
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The categories of this paper are: eess
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The categories of this paper are: plasm-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: physics
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The categories of this paper are: dg-ga
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The categories of this paper are: ao-sci