Be star disc characteristics near the central object

Abstract

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...

Kinematics and geometrical study of the Be stars 48 Persei andψPersei with the VEGA/CHARA interferometer

“…For a disk to be thicker, either additional mechanisms have to be invoked or we must assume that the disk might not be in equilibrium (Bjorkman & Carciofi 2004). Alternatively, hydrostatic equilibrium might not be determined only by gas pressure but also by magnetic fields (Arias et al 2006;Zorec et al 2007), resulting in an increase in their vertical scale height.…”

Spectral synthesis for Be stars

“…An image in a bin of ±2Å around Fe ii 4233Å (multiplet 27, z 4 D o − b 4 P e ) of the disk ρ(R) = 10 −10 (R * /R) 2.5 g cm −3 surrounding a B2V star is shown in Figure 3. Zorec et al (2007) suggest that the optical depths in Fe ii emission lines as compared between pole-on and equator-on Be star systems require enhanced scale heights in the inner disk, above the prediction of gravitational equilibrium. Figure 4 compares observed Fe ii equivalent widths for a sample of B2V stars (Arias et al 2006) with bedisk/beray calculations for a dense disk, ρ(R) = 10 −10 (R * /R) 2.5 g cm −3 in gravitational equilibrium, seen at several inclinations.…”

“…The observed equivalent widths are well-bracketed by models with R disk between 5 and 10 R * . However, Zorec et al (2007) note that such dense disks may by physically unrealistic. beray/bedisk is currently being used to model all H i and Fe ii observations of Arias et al (2006) to search for realistic sets of ρ o , n, and R disk to see if increased inner disk scale heights are required by the data.…”

“…The bedisk code (Sigut & Jones, 2007;Sigut, Jones & McGill, 2009) can self-consistently predict both T and H given a density model for the equatorial plane of the disk. There have been suggestions that the inner density scale heights of Be star disks are enhanced over the prediction of simple vertical hydrostatic equilibrium (Chauville et al 2001;Zorec et al 2007). These results are based on the comparison of (unresolved) H i and Fe ii spectra between nearly pole-on (i ≈ 0 o ) and nearly edge-on (i ≈ 90 o ) systems.…”

Analysis of the Balmer discontinuity behavior of Be stars by the Monte Carlo method

“…To describe the density distribution in the envelope we have considered monotonic decreasing functions with r, as well as distributions that take into account an accumulation of material near the star (Zorec et al 2007). To take into account non spherically simetric matter distribution, decay from equator to pole have been considered.…”