Issue |
A&A
Volume 463, Number 1, February III 2007
|
|
---|---|---|
Page(s) | 187 - 194 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20065915 | |
Published online | 20 November 2006 |
Trapping of the H II and photodissociation region in a radially stratified molecular cloud
Division of Theoretical Astrophysics, National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan e-mail: hosokawa@th.nao.ac.jp
Received:
27
June
2006
Accepted:
16
October
2006
Aims.We study the expansion of the ionization and dissociation fronts (DFs)
in a radially stratified molecular cloud, whose density distribution
is represented as . We focus on cases
with
, when the ionization front is “trapped” in the
cloud and expands with the preceding shock front.
The simultaneous evolution of the outer photodissociation region
(PDR) is examined in detail.
Methods. We analytically probe the time evolution of the column densities of the shell and envelope outside the HII region, which are key physical quantities for the shielding of dissociating photons. Next, we perform numerical calculations, and study how the thermal/chemical structure of the outer PDR changes with different density gradients. We apply our numerical model to the Galactic HII region, Sharpless 219 (Sh219).
Results. The time evolution of the column densities of the shell and outer
envelope depends on w, and qualitatively changes across .
In the cloud with
, the shell column density increases
as the HII region expands. The DFs are finally trapped in the shell,
and the molecular gas gradually accumulates in the shell.
The molecular shell and envelope surround the HII region.
When
, the shell column density initially
increases, but then decreases. The column density of the outer
envelope also quickly decreases as the HII region increases.
It becomes easier for the dissociating photons to
penetrate the shell and envelope. The PDR
broadly extends around the trapped HII region. A model with
successfully explains the observed properties
of Sh219. Our model suggests that a density-bounded
PDR surrounds the photon-bounded HII region in Sh219.
Key words: stars: early-type / stars: formation / ISM: HII regions / ISM: kinematics and dynamics / ISM: molecules
© ESO, 2007
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