TY - JOUR

T1 - On the ionization structure of HII regions in stellar envelopes with an outward density decrease

AU - Grachev, S. I.

N1 - Copyright: Copyright 2005 Elsevier Science B.V., Amsterdam. All rights reserved.

PY - 1996/2/10

Y1 - 1996/2/10

N2 - 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 Strömgren 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 Pl. Namely, it grows sharply when p increases up to pl and it becomes very large (essentially infinite), in comparison with the inner radius r0, for p > pl. 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 < pl - Δp while the envelope is almost completely ionized when p > pl, the value of Δp being much smaller than pl. It is shown also that for sufficiently fast density decrease and p in the vicinity of pl 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) = n0(r0/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.

AB - 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 Strömgren 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 Pl. Namely, it grows sharply when p increases up to pl and it becomes very large (essentially infinite), in comparison with the inner radius r0, for p > pl. 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 < pl - Δp while the envelope is almost completely ionized when p > pl, the value of Δp being much smaller than pl. It is shown also that for sufficiently fast density decrease and p in the vicinity of pl 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) = n0(r0/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.

KW - ISM: HII regions

KW - Stars: circumstellar matter

KW - Stars: emission-line

KW - Stars: mass loss

UR - http://www.scopus.com/inward/record.url?scp=3142608422&partnerID=8YFLogxK

M3 - Article

AN - SCOPUS:3142608422

VL - 306

SP - 527

EP - 536

JO - ASTRONOMY & ASTROPHYSICS

JF - ASTRONOMY & ASTROPHYSICS

SN - 0004-6361

IS - 2

ER -