Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Polarized CO emission from molecular clouds   Linearly polarized, non-masing, rotational lines have been detected for the
first time in the interstellar medium. This effect occurs in molecular clouds
with a magnetic field, and traces the field direction, offering an alternative
technique to dust emission polarimetry. The line polarization mechanism is
similar to that in masers, but with lower degrees of polarization (~1%). We
have detected the effect towards the Galactic Centre and its surrounding `2 pc
ring', and in the molecular clouds S140 and DR21 (tentatively), in the lines CO
J=2-1 and J=3-2, and 13CO J=2-1. The deduced magnetic field directions agree
well with previous dust polarimetry results, confirming that the line
polarization is a real effect. This new technique will be useful in sources
that are too faint for dust polarimetry, and can also be used to investigate
3-dimensional morphology of magnetic fields, where the velocity structure of
the clouds is known.

23
The categories of this paper are: solv-int
7
The categories of this paper are: hep-th
14
The categories of this paper are: math-ph
12
The categories of this paper are: nucl-th
31
The categories of this paper are: ao-sci
18
The categories of this paper are: econ
20
The categories of this paper are: acc-phys
36
The categories of this paper are: cmp-lg
5
The categories of this paper are: gr-qc
30
The categories of this paper are: q-alg
28
The categories of this paper are: alg-geom
16
The categories of this paper are: stat
25
The categories of this paper are: patt-sol
8
The categories of this paper are: hep-ex
9
The categories of this paper are: nlin
21
The categories of this paper are: adap-org