Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Interactions of Massive Stars with Their Parental Clouds   Here we discuss the interaction of massive stars with their parental
molecular clouds. A summary of the dynamical evolution of HII regions and
wind-driven bubbles in high-pressure cloud cores is given. Both ultracompact
HII regions and ultracompact wind-driven bubbles can reach pressure equilibrium
with their surrounding medium. The structures stall their expansion and become
static and, as long as the ionization sources and the ambient densities remain
about constant, the resulting regions are stable and long lived. For cases with
negative density gradients, and depending on the density distribution, some
regions never reach the static equilibrium condition. For power-law density
stratifications, $\rho \propto r^{-w}$, the properties of the evolution depend
on a critical exponent, $w_{crit}$, above which the ionization front cannot be
slowed down by recombinations or new ionizations, and the cloud becomes fully
ionized. This critical exponent is $w_{crit}=3/2$ during the expansion phase.
For $w>3/2 $ the gas expands supersonically into the surrounding ionized
medium, and there are two regimes separated by $w=3$. For $3/2<w\leq 3$, the
slow regime, the inner region drives a weak shock moving with almost constant
velocity through the cloud. For $w>3$, the fast regime, the shock becomes
strong and accelerates with time. Finally, the evolution of slow winds in
highly pressurized region is described briefly.

9
The categories of this paper are: nlin
32
The categories of this paper are: atom-ph
26
The categories of this paper are: bayes-an
0
The categories of this paper are: hep-ph
22
The categories of this paper are: chao-dyn
8
The categories of this paper are: hep-ex
3
The categories of this paper are: physics
13
The categories of this paper are: quant-ph
12
The categories of this paper are: nucl-th
35
The categories of this paper are: mtrl-th
36
The categories of this paper are: cmp-lg
18
The categories of this paper are: econ
5
The categories of this paper are: gr-qc
23
The categories of this paper are: solv-int
16
The categories of this paper are: stat
14
The categories of this paper are: math-ph