Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Chemical telemetry of OH observed to measure interstellar magnetic
  fields   We present models for the chemistry in gas moving towards the ionization
front of an HII region. When it is far from the ionization front, the gas is
highly depleted of elements more massive than helium. However, as it approaches
the ionization front, ices are destroyed and species formed on the grain
surfaces are injected into the gas phase. Photodissociation removes gas phase
molecular species as the gas flows towards the ionization front. We identify
models for which the OH column densities are comparable to those measured in
observations undertaken to study the magnetic fields in star forming regions
and give results for the column densities of other species that should be
abundant if the observed OH arises through a combination of the liberation of
H2O from surfaces and photodissociation. They include CH3OH, H2CO, and H2S.
Observations of these other species may help establish the nature of the OH
spatial distribution in the clouds, which is important for the interpretation
of the magnetic field results.

16
The categories of this paper are: stat
35
The categories of this paper are: mtrl-th
37
The categories of this paper are: supr-con
31
The categories of this paper are: ao-sci
18
The categories of this paper are: econ
33
The categories of this paper are: plasm-ph
30
The categories of this paper are: q-alg
3
The categories of this paper are: physics
12
The categories of this paper are: nucl-th
27
The categories of this paper are: comp-gas
28
The categories of this paper are: alg-geom
19
The categories of this paper are: eess
22
The categories of this paper are: chao-dyn
2
The categories of this paper are: math
24
The categories of this paper are: dg-ga
32
The categories of this paper are: atom-ph