Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: An analysis of the observed radio emission from planetary nebulae   We have analysed the radio fluxes for 264 planetary nebulae for which
reliable measurements of fluxes at 1.4 and 5 GHz, and of nebular diameters are
available. For many of the investigated nebulae the optical thickness is
important, especially at 1.4 GHz. Simple models like the one specified only by
a single optical thickness or spherical, constant density shells do not account
satisfactorily for the observations. Also an r^{-2} density distribution is
ruled out. A reasonable representation of the observations can be obtained by a
two-component model having regions of two different values of optical
thickness. We show that the nebular diameters smaller than 10 arcsec are
uncertain, particularly if they come from photographic plates or gaussian
fitting to the radio profile. While determining the interstellar extinction
from an optical to radio flux ratio caution should be paid to optical thickness
effects in the radio. We have developed a method for estimating the value of
self absorption. At 1.4 GHz self absorption of the flux is usually important
and can exceed a factor of 10. At 5 GHz self absorption is negligible for most
of the objects although in some cases it can reach a factor of 2. The Galactic
bulge planetary nebulae when used to calibrate the Shklovsky method give the
mean nebular mass of 0.14 solar mass. The statistical uncertainty of the
Shklovsky distances is smaller than factor 1.5.

24
The categories of this paper are: dg-ga
0
The categories of this paper are: hep-ph
19
The categories of this paper are: eess
12
The categories of this paper are: nucl-th
4
The categories of this paper are: cond-mat
14
The categories of this paper are: math-ph
10
The categories of this paper are: q-bio
23
The categories of this paper are: solv-int
22
The categories of this paper are: chao-dyn
7
The categories of this paper are: hep-th
30
The categories of this paper are: q-alg
5
The categories of this paper are: gr-qc
32
The categories of this paper are: atom-ph
8
The categories of this paper are: hep-ex
35
The categories of this paper are: mtrl-th
9
The categories of this paper are: nlin