Self-Consistent Dynamic Models of Steady Ionization Fronts:
I. Weak-D and Weak-R Fronts
Authors
W. J. Henney, S. J. Arthur, R. J. R. Williams, & G. J. Ferland
Journal
ApJ, Submitted August 06, 2004, Accepted 28 Oct 2004
In press 1 March 2005, v621 (tentative)
astro-ph/0501034
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Abstract
We present a method for including steady-state gas flows in the
plasma physics code Cloudy, which was previously restricted to
modeling static configurations. The numerical algorithms are
described in detail, together with an example application to
plane-parallel ionization-bounded HII regions. As well as
providing the foundation for future applications to more complex
flows, we find the following specific results regarding the effect
of advection upon ionization fronts in HII regions:
- Significant direct effects of advection on the global emission
properties occur only when the ionization parameter is lower than
is typical for HII regions. For higher ionization parameters,
advective effects are indirect and largely confined to the
immediate vicinity of the ionization front.
- The overheating of partially ionized gas in the front is not
large, even for supersonic (R-type) fronts. For subsonic (D-type)
fronts we do not find the temperature spike that has been
previously claimed.
- The most significant morphological signature of advective
fronts is an electron density spike that occurs at the ionization
front whenever the relative velocity between the ionized gas and
the front exceeds about one half the ionized isothermal sound
speed. Observational evidence for such a spike is found in
[NII] 6584 Å images of the Orion bar.
- Plane-parallel, weak-D fronts are found to show at best a
shallow correlation between mean velocity and ionization potential
for optical emission lines even when the flow velocity closely
approaches the ionized sound speed. Steep gradients in velocity
versus ionization, such as those observed in the Orion nebula,
seem to require transonic flows, which are only possible in a
diverging geometry when radiation forces are included.