Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Structure and Expansion Law of HII Regions in structured Molecular
  Clouds   We present radiation-magnetohydrodynamic simulations aimed at studying
evolutionary properties of H\,{\normalsize II} regions in turbulent,
magnetised, and collapsing molecular clouds formed by converging flows in the
warm neutral medium. We focus on the structure, dynamics and expansion laws of
these regions. Once a massive star forms in our highly structured clouds, its
ionising radiation eventually stops the accretion (through filaments) toward
the massive star-forming regions. The new over-pressured H\,{\normalsize II}
regions push away the dense gas, thus disrupting the more massive collapse
centres. Also, because of the complex density structure in the cloud, the
H\,{\normalsize II} regions expand in a hybrid manner: they virtually do not
expand toward the densest regions (cores), while they expand according to the
classical analytical result towards the rest of the cloud, and in an
accelerated way, as a blister region, towards the diffuse medium. Thus, the
ionised regions grow anisotropically, and the ionising stars generally appear
off-centre of the regions. Finally, we find that the hypotheses assumed in
standard H\,{\normalsize II}-region expansion models (fully embedded region,
blister-type, or expansion in a density gradient) apply simultaneously in
different parts of our simulated H\,{\normalsize II} regions, producing a net
expansion law ($R \propto t^\alpha$, with $\alpha$ in the range of 0.93-1.47
and a mean value of $1.2 \pm 0.17$) that differs from any of those of the
standard models.

22
The categories of this paper are: chao-dyn
27
The categories of this paper are: comp-gas
15
The categories of this paper are: nucl-ex
17
The categories of this paper are: q-fin
9
The categories of this paper are: nlin
1
The categories of this paper are: cs
13
The categories of this paper are: quant-ph
12
The categories of this paper are: nucl-th
30
The categories of this paper are: q-alg
33
The categories of this paper are: plasm-ph
23
The categories of this paper are: solv-int
18
The categories of this paper are: econ
3
The categories of this paper are: physics
0
The categories of this paper are: hep-ph
36
The categories of this paper are: cmp-lg
25
The categories of this paper are: patt-sol