Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Using CO line ratios to trace the physical properties of molecular
  clouds   The carbon monoxide (CO) rotational transition lines are the most common
tracers of molecular gas within giant molecular clouds (MCs). We study the
ratio ($R_{2-1/1-0}$) between CO's first two emission lines and examine what
information it provides about the physical properties of the cloud. To study
$R_{2-1/1-0}$ we perform smooth particle hydrodynamic simulations with time
dependent chemistry (using GADGET-2), along with post-process radiative
transfer calculations on an adaptive grid (using RADMC-3D) to create synthetic
emission maps of a MC. $R_{2-1/1-0}$ has a bimodal distribution that is a
consequence of the excitation properties of each line, given that $J=1$ reaches
local thermal equilibrium (LTE) while $J=2$ is still sub-thermally excited in
the considered clouds. The bimodality of $R_{2-1/1-0}$ serves as a tracer of
the physical properties of different regions of the cloud and it helps
constrain local temperatures, densities and opacities. Additionally this
bimodal structure shows an important portion of the CO emission comes from
diffuse regions of the cloud, suggesting that the commonly used conversion
factor of $R_{2-1/1-0}\sim 0.7$ between both lines may need to be studied
further.

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The categories of this paper are: hep-lat
5
The categories of this paper are: gr-qc
10
The categories of this paper are: q-bio
19
The categories of this paper are: eess
30
The categories of this paper are: q-alg
7
The categories of this paper are: hep-th
34
The categories of this paper are: chem-ph
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The categories of this paper are: adap-org
22
The categories of this paper are: chao-dyn
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The categories of this paper are: dg-ga
8
The categories of this paper are: hep-ex
33
The categories of this paper are: plasm-ph
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The categories of this paper are: quant-ph
35
The categories of this paper are: mtrl-th
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The categories of this paper are: nucl-th
9
The categories of this paper are: nlin