Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Searching for O$_2$ in the SMC:Constraints on Oxygen Chemistry at Low
  Metallicities   We present a 39 h integration with the Odin satellite on the ground-state
118.75 GHz line of O2 towards the region of strongest molecular emission in the
Small Magellanic Cloud. Our 3sigma upper limit to the O2 integrated intensity
of <0.049 K km/s in a 9'(160 pc) diameter beam corresponds to an upper limit on
the O2/H2 abundance ratio of <1.3E-6. Although a factor of 20 above the best
limit on the O2 abundance obtained for a Galactic source, our result has
interesting implications for understanding oxygen chemistry at sub-solar metal
abundances. We compare our abundance limit to a variety of astrochemical models
and find that, at low metallicities, the low O2 abundance is most likely
produced by the effects of photo-dissociation on molecular cloud structure.
Freeze-out of molecules onto dust grains may also be consistent with the
observed abundance limit, although such models have not yet been run at
sub-solar initial metallicities.

30
The categories of this paper are: q-alg
2
The categories of this paper are: math
13
The categories of this paper are: quant-ph
22
The categories of this paper are: chao-dyn
32
The categories of this paper are: atom-ph
18
The categories of this paper are: econ
1
The categories of this paper are: cs
9
The categories of this paper are: nlin
35
The categories of this paper are: mtrl-th
17
The categories of this paper are: q-fin
34
The categories of this paper are: chem-ph
19
The categories of this paper are: eess
20
The categories of this paper are: acc-phys
4
The categories of this paper are: cond-mat
36
The categories of this paper are: cmp-lg
3
The categories of this paper are: physics