Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Galactic Edge Clouds I: Molecular Line Observations and Chemical
  Modelling of Edge Cloud 2   Edge Cloud 2 (EC2) is a molecular cloud, about 35 pc in size, with one of the
largest galactocentric distances known to exist in the Milky Way. We present
observations of a peak CO emission region in the cloud and use these to
determine its physical characteristics. We calculate a gas temperature of 20 K
and a density of n(H2) ~ 10^4 cm^-3. Based on our CO maps, we estimate the mass
of EC2 at around 10^4 M_sun and continuum observations suggest a dust-to-gas
mass ratio as low as 0.001. Chemical models have been developed to reproduce
the abundances in EC2 and they indicate that: heavy element abundances may be
reduced by a factor of five relative to the solar neighbourhood (similar to
dwarf irregular galaxies and damped Lyman alpha systems); very low extinction
(Av < 4 mag) due to a very low dust-to-gas ratio; an enhanced cosmic ray
ionisation rate; and a higher UV field compared to local interstellar values.
The reduced abundances may be attributed to the low level of star formation in
this region and are probably also related to the continuing infall of
primordial (or low metallicity) halo gas since the Milky Way formed. Finally,
we note that shocks from the old supernova remnant GSH 138-01-94 may have
determined the morphology and dynamics of EC2.

31
The categories of this paper are: ao-sci
1
The categories of this paper are: cs
26
The categories of this paper are: bayes-an
24
The categories of this paper are: dg-ga
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The categories of this paper are: chem-ph
5
The categories of this paper are: gr-qc
12
The categories of this paper are: nucl-th
2
The categories of this paper are: math
29
The categories of this paper are: funct-an
10
The categories of this paper are: q-bio
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The categories of this paper are: comp-gas
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The categories of this paper are: solv-int
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The categories of this paper are: q-alg
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The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
18
The categories of this paper are: econ