On the ionization structure of HII regions in stellar envelopes with an outward density decrease.

Details On the ionization structure of HII regions in stellar envelopes with an outward density decrease. On the ionization structure of HII regions in stellar envelopes with an outward density decrease.

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 pp_l_, the value of {DELTA}p being much smaller than p_l_. It is shown also that for sufficiently fast density decrease and p in the vicinity of p_l_ there is the possibility of a nonmonotonous distribution of ionization degree throughout the envelope. We present analytical and numerical results for the power law density distribution n(r)=n_0_ (r_0_/r)^k^ in which case the slow and fast decreases correspond to k<3/2 and k>3/2. The case of a stationary flow with the Lamers velocity law is also briefly discussed and numerical estimates are given in the context of the Catala and Kunasz (1987) models of outflowing envelope around Herbig Ae/Be star AB Aur.

Affiliated with

Also associated with

No associations

Rating

On the ionization structure of HII regions in stellar envelopes with an outward density decrease. does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.

If you have personal experience with On the ionization structure of HII regions in stellar envelopes with an outward density decrease., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and On the ionization structure of HII regions in stellar envelopes with an outward density decrease. will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFWR-SCP-O-1153181