Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ionization-bounded and Density-bounded Planetary Nebulae   Planetary nebulae (and in general any photoionized region) can be classified
as ionization-bounded or density-bounded. It is important to determine in which
case is the planetary nebula studied to be able to estimate from nebular
observations, for example, the total rate of ionizing photons produced by the
central star. In this paper we present a simple observational criterion that
uses radio continuum images and that allows to establish a necessary (but not
sufficient) condition for the planetary nebula to be considered as
ionization-bounded. We apply the criterion to two planetary nebulae: NGC 7027
is most possibly ionization-bounded, while Hb~4 is density-bounded, at least in
some directions.

2
The categories of this paper are: math
7
The categories of this paper are: hep-th
12
The categories of this paper are: nucl-th
36
The categories of this paper are: cmp-lg
8
The categories of this paper are: hep-ex
16
The categories of this paper are: stat
0
The categories of this paper are: hep-ph
37
The categories of this paper are: supr-con
10
The categories of this paper are: q-bio
17
The categories of this paper are: q-fin
25
The categories of this paper are: patt-sol
28
The categories of this paper are: alg-geom
29
The categories of this paper are: funct-an
18
The categories of this paper are: econ
32
The categories of this paper are: atom-ph
3
The categories of this paper are: physics