Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Arecibo-Green Bank-LOFAR Carbon Radio Recombination Line observations
  toward cold HI Clouds   We present results from a search for radio recombination lines in three HI
self-absorbing (HISA) clouds at 750 MHz and 321 MHz with the Robert C. Byrd
Green Bank Telescope (GBT), and in three Galactic Plane positions at 327 MHz
with the Arecibo Telescope. We detect Carbon Recombination Lines (CRRLs) in the
direction of DR4 and DR21, as well as in the galactic plane position
G34.94+0.0. We additionally detect Hydrogen Recombination Lines (HRRLs) in
emission in five of the six sightlines, and a Helium line at 750 MHz towards
DR21. Combining our new data with 150 MHz LOFAR detections of CRRL absorption
towards DR4 and DR21, we estimate the electron densities of the line forming
regions by modeling the line width as a function of frequency. The estimated
densities are in the range 1.4 $\rightarrow$ 6.5 cm$^{-3}$ towards DR4, for
electron temperatures 200 $\rightarrow$ 20 K. A dual line forming region with
densities between 3.5 $\rightarrow$ 24 cm$^{-3}$ and 0.008 $\rightarrow$ 0.3
cm$^{-3}$ could plausibly explain the observed line width as a function of
frequency on the DR21 sightline. The central velocities of the CRRLs compare
well with CO emission and HISA lines in these directions. The cloud densities
estimated from the CO lines are smaller (at least a factor of 5) than those of
the CRRL forming regions. It is likely that the CRRL forming and HI
self-absorbing gas is located in a denser, shocked region either at the
boundary of or within the CO emitting cloud.

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The categories of this paper are: chao-dyn
35
The categories of this paper are: mtrl-th
2
The categories of this paper are: math
11
The categories of this paper are: hep-lat
29
The categories of this paper are: funct-an
18
The categories of this paper are: econ
7
The categories of this paper are: hep-th
20
The categories of this paper are: acc-phys
31
The categories of this paper are: ao-sci
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The categories of this paper are: q-fin
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The categories of this paper are: nucl-ex
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The categories of this paper are: math-ph
34
The categories of this paper are: chem-ph
1
The categories of this paper are: cs
10
The categories of this paper are: q-bio
33
The categories of this paper are: plasm-ph