Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...306..527g&link_type=abstract
Astronomy and Astrophysics, v.306, p.527
Astronomy and Astrophysics
Astrophysics
2
Stars: Circumstellar Matter, Stars: Emission-Line, Stars: Mass Loss, Ism: Hii Regions
Scientific paper
In the framework of spherical pure hydrogen models of stellar envelopes photoionized by photospheric (or chromospheric) radiation we study the dependence of their ionization structure on an outward density decrease. To avoid the problem of radiative transfer we make use of the two classical approximations of optically thin (A) and thick (B) HII regions. We show that all density decreases can be classified, by means of some simple criterion, as slow and fast ones. For slow density decreases the radius of HII region can be estimated in the way which gives a straightforward generalization of Stroemgren radius to inhomogeneous envelopes. But for fast density decreases this generalization can fail, and the radius of the HII region crucially depends on the ionization parameter p near some critical value p_l_. Namely, it grows sharply when p increases up to p_l_ and it becomes very large (essentially infinite), in comparison with the inner radius r_0_, for p>p_l_. This means that in the case of fast density decreases an HII region is essentially a threshold phenomenon: there is no significant HII region when p
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