Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen
  in the Diffuse Interstellar Medium   We have used archival far-ultraviolet spectra from observations made by
HST/STIS and FUSE to determine the column densities and rotational excitation
temperatures for CO and H2, respectively, along the lines of sight to 23
Galactic O and B stars. The sightlines have reddening values in the range
E(B-V)= 0.07-0.62, sampling the diffuse to translucent interstellar medium. We
find that the H2 column densities range from 5x10^18-8x10^20 cm^-2 and the CO
from upper limits around 2x10^12 cm^-2 to detections as high as 1.4x10^16
cm^-2. CO increases with increasing H2, roughly following a power law of factor
\~2. The CO/H2 column density ratio is thus not constant, and ranges from 10^-7
- 10^-5, with a mean value of 3x10^-6. The sample segregates into "diffuse" and
"translucent" regimes, the former having a molecular fraction less than ~0.25
and A_V/d<1 mag kpc^-1. The mean CO/H2 for these two regimes are 3.6x10^-7 and
9.3x10^-6, respectively, significantly lower than the canonical dark cloud
value of 10^-4. In six of the sightlines, 13CO is observed, and the isotopic
ratio we observe (~50-70) is consistent with, if perhaps a little below, the
average 12C/13C for the ISM at large. The average H2 rotational excitation
temperature is 74+/-24 K, in good agreement with previous studies, and the
average CO temperature is 4.1 K, with some sightlines as high as 6.4 K. The
higher excitation CO is observed with higher column densities, consistent with
the effects of photon trapping in clouds with densities in the 20-100 cm^-3
range. We discuss the implications for the structure of the diffuse/translucent
regimes of the interstellar medium and the estimation of molecular mass in
galaxies.

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The categories of this paper are: alg-geom
31
The categories of this paper are: ao-sci
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The categories of this paper are: hep-ex
35
The categories of this paper are: mtrl-th
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The categories of this paper are: plasm-ph
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The categories of this paper are: adap-org
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The categories of this paper are: q-alg
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The categories of this paper are: hep-lat
0
The categories of this paper are: hep-ph
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The categories of this paper are: q-bio
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The categories of this paper are: bayes-an
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The categories of this paper are: atom-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: comp-gas
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The categories of this paper are: cs