Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Systematic variation of the 12CO/13CO ratio as a function of
  star-formation rate surface density   We show that the12CO/13CO intensity ratio in nearby galaxies varies
systematically as a function of the star formation rate surface density and gas
surface density. The same effect is observed in different transitions, and in
the 12CO/C18O ratio, while the 13CO/C18O ratio appears to remain constant as a
function of the star formation rate surface density. We discuss the cause of
these variations, considering both changes in the physical state of the gas,
and chemical changes that lead to abundance variations. We used the observed
correlations with C18O to suggest that abundance variations are unlikely to be
causing the systematic trend observed with the star formation rate surface
density, and thus that the mean gas temperature and/or velocity dispersion are
systematically higher in higher star-formation rate surface density regions. We
present the best fitting relations between the star formation rate surface
density and the 12CO/13CO and 12CO/C18O ratios, and discuss how this effect can
help us predict CO isotope emission from galaxies across the known universe.

17
The categories of this paper are: q-fin
31
The categories of this paper are: ao-sci
34
The categories of this paper are: chem-ph
18
The categories of this paper are: econ
28
The categories of this paper are: alg-geom
15
The categories of this paper are: nucl-ex
13
The categories of this paper are: quant-ph
14
The categories of this paper are: math-ph
26
The categories of this paper are: bayes-an
32
The categories of this paper are: atom-ph
8
The categories of this paper are: hep-ex
21
The categories of this paper are: adap-org
12
The categories of this paper are: nucl-th
3
The categories of this paper are: physics
30
The categories of this paper are: q-alg
25
The categories of this paper are: patt-sol