Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The volume densities of GMCs in M81   HI features near young star clusters in M81 are identified as the
photodissociated surfaces of Giant Molecular Clouds (GMCs) from which the young
stars have recently formed. The HI column densities of these features show a
weak trend, from undetectable values inside R = 3.7 kpc and increasing rapidly
to values around 3 x 10^21 cm^-2 near R ~ 7.5 kpc. This trend is similar to
that of the radially-averaged HI distribution in this galaxy, and implies a
constant area covering factor of ~ 0.21 for GMCs throughout M81. The incident
UV fluxes G0 of our sample of candidate PDRs decrease radially.
  A simple equilibrium model of the photodissociation-reformation process
connects the observed values of the incident UV flux, the HI column density,
and the relative dust content, permitting an independent estimate to be made of
the total gas density in the GMC. Within the GMC this gas will be predominantly
molecular hydrogen. Volume densities of 1 < n < 200 cm^-3 are derived, with a
geometric mean of 17 cm^-3. These values are similar to the densities of GMCs
in the Galaxy, but somewhat lower than those found earlier for M101 with
similar methods. Low values of molecular density in the GMCs of M81 will result
in low levels of collisional excitation of the CO(1-0) transition, and are
consistent with the very low surface brightness of CO(1-0) emission observed in
the disk of M81.

31
The categories of this paper are: ao-sci
27
The categories of this paper are: comp-gas
8
The categories of this paper are: hep-ex
36
The categories of this paper are: cmp-lg
13
The categories of this paper are: quant-ph
9
The categories of this paper are: nlin
14
The categories of this paper are: math-ph
12
The categories of this paper are: nucl-th
28
The categories of this paper are: alg-geom
18
The categories of this paper are: econ
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The categories of this paper are: patt-sol
34
The categories of this paper are: chem-ph
21
The categories of this paper are: adap-org
16
The categories of this paper are: stat
15
The categories of this paper are: nucl-ex
0
The categories of this paper are: hep-ph