Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Off-center HII regions in power-law density distributions   The expansion of ionization fronts in uniform and spherically symmetric
power-law density distributions is a well-studied topic. However, in many
situations, such as a star formed at the edge of a molecular cloud core, an
offset power-law density distribution would be more appropriate. In this paper
a few of the main issues of the formation and expansion of HII regions in such
media are outlined and results are presented for the particular cases where the
underlying power laws are 1/r^2 and 1/r^3. A simple criterion is developed for
determining whether the initial photoionized region will be unbounded, which
depends on the power-law exponent and the ratio of the equivalent Stroemgren
radius produced by the star in a uniform medium to the stellar offset distance.
In the expansion stage, the ionized volumes will eventually become unbounded
unless pressure balance with the external medium is reached before the
ionization front velocity becomes supersonic with respect to the ionized gas.

22
The categories of this paper are: chao-dyn
36
The categories of this paper are: cmp-lg
10
The categories of this paper are: q-bio
29
The categories of this paper are: funct-an
26
The categories of this paper are: bayes-an
23
The categories of this paper are: solv-int
8
The categories of this paper are: hep-ex
31
The categories of this paper are: ao-sci
13
The categories of this paper are: quant-ph
14
The categories of this paper are: math-ph
17
The categories of this paper are: q-fin
37
The categories of this paper are: supr-con
11
The categories of this paper are: hep-lat
0
The categories of this paper are: hep-ph
15
The categories of this paper are: nucl-ex
20
The categories of this paper are: acc-phys