Context. A recent analysis of visual Fe ii emission lines of 17 classic Be stars using the self-absorption curve (SAC) method revealed that these lines are optically thick and that they form in circumstellar disc (CD) regions within two stellar radii from the central star on average. Aims. The aim of this paper is to study the physical characteristics of CD regions situated close to the central star.Methods. We used the Fe ii emission line optical depths derived for a sample of the above mentioned classic Be stars that are seen either nearly pole-on or equator-on. The disc properties sought are then inferred by reproducing the average pole-on and equator-on Fe ii line optical depths using simple CD models with different density and temperature distributions. Results. We found that the CD regions near the star, which account for the average Fe ii line opacities obtained with the SAC method, have semi-height scales perpendicular to the equatorial plane h > ∼ 0.5R o and particle density distribution lawsn with n < ∼ 1 at R < ∼ 3R o (R o is the stellar radius; N o is the particle density at R = R o ). Multi-scattering Monte Carlo simulations show that CD with particle density distributions N ∼ R −n , where n depends on the distance R and n ∼ 0.5 near the star, might account for the near-UV spectroplorarimetry of Be stars. CD with enhanced scale heights could explain the [Hα, E(J − L)] correlation as they may produce about the same Hα line emission, but larger IR flux excesses than thin discs. The enlarged CD scale heights do not contradict the existing interferometric measurements and should enable us to treat more consistently Balmer line emission formation in Be stars.Key words. stars: emission-line, Be -stars: circumstellar matter -line: formation -polarization Arias et al. (2006) analyzed Fe ii emission lines in the visual spectrum of 17 southern classical Be stars by applying the selfabsorption curve (SAC) method (Friedjung & Muratorio 1987;Muratorio & Friedjung 1988). The self-absorption curve is related to the line-of-sight photon escape probability associated with the model of the line formation region (Kastner 1999) and provides information on the line optical depth regime. This method gives the average optical depth of emission lines that belong to a given reference line-multiplet, τ o , and yields an estimate of the extent of the line emitting region. Arias et al. (2006) concluded that the studied Fe ii lines are optically thick, having an average opacity of τ o = 2.4 ± 0.9 in the central wavelength of multiplet 38 (hereafter m38), and that in most cases they form between 1 and 3 stellar radii. Thus, in this paper we use Fe ii line optical depths to infer some insights into the density and temperature distributions in CD that are close to the central star and discuss the incidence they can have on the polarimetric, photometric, interferometric and spectroscopic data. Introduction Circumstellar disc characteristics "Probe" star+circumstellar discThe goal of the present contribution is to explore th...

Aims. The Fe ii emission lines formed in the circumstellar envelopes (CE) of classical Be stars are studied in order to determine whether they are optically thin or optically thick. We also aim at deriving both average Fe ii line excitation temperatures and the extent of their formation region in the CE.Methods. We simultaneously observed several series of Fe ii emission lines in the λλ4230−7712 Å wavelength interval and the first members of the hydrogen Balmer series of 18 southern classical Be stars. The optical depth regime that controls the formation of the observed Fe ii lines and the physical parameters of their CE formation region were studied using the empirical self-aborption-curve (SAC) method.Results. Our calculations give an average value of τ o = 2.4 ± 0.9 for the optical depth of the studied Fe ii lines, which implies that these lines are optically thick in the CE of Be stars. Qualitative indications that Fe ii emission lines should be formed in circumstellar regions close to the central star are inferred from the correlations between Fe ii emission line widths and V sin i. The application of the SAC method to Fe ii emission lines confirms this result, which gives R e = 2.0R * ± 0.8 for the extension of the line-forming region. The proximity of the line-forming region to the central star is also supported by the behavior of the source function of Fe ii lines, which rapidly decreases with radii. This prevents the lines from being formed over extended regions and/or far from the star. Finally, the correlations of the central depression in the Balmer emission lines with V sin i are consistent with the flattened geometrical shapes of CEs.

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Theoretical profiles of He ii lines in OB stars with an expanding, spherically symmetric atmosphere are computed. The extended atmospheric model is formed by a classical photosphere, characterized by the effective temperature and the surface gravity, and superimposed layers that have different velocity and temperature structure. We solve rigorously the radiative transfer equation, simultaneously with the statistical equilibrium equations for multilevel atoms, by making use of Feautrier's method in the comoving frame. We discuss the influence on the He ii lines of the hydrodynamic and thermodynamic structure of the atmosphere, paying special attention to those configurations that give rise to emission lines. The main conclusions of our work can be summarized as follows: 1. The shape of the profiles is determined by the velocity gradient at the base of the wind whenever a positive temperature gradient occurs. 2. In O-type stars, the emission-line intensity depends quite sensitively on log g, in agreement with the Walborn luminosity criterion. 3. In addition, we are able to produce emission and absorption profiles that are in qualitative agreement with those observed in O and B stars. 4. We also confirm the previous Cidale & Ringuelet and Venero, Cidale, & Ringuelet results that showed that a warm, extended, and rapidly expanding atmosphere is sufficient to give rise to emission components in the line profiles.

We present a method that relates parameters of the observed Gaussian absorption profile formed in an extended atmosphere with local parameters such as excitation temperature, atom columns, and distance from the central star to the line-forming region. It also allows us to make a diagnosis of the extended atmosphere through line-intensity variations, profile of the saturated lines, or variations in the extent of the emission wings.