Be star disc characteristics near the central object

¹ Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis bd. Arago, 75014 Paris, France e-mail: zorec@iap.fr
² Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque sn, (1900) La Plata, Argentina
³ Instituto de Astrofísica de La Plata (CONICET-UNLP), Paseo del Bosque sn, (1900) La Plata, Argentina

Received: 21 October 2006
Accepted: 13 April 2007

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 and particle density distribution laws with at (R_o is the stellar radius; N_o is the particle density at ). Multi-scattering Monte Carlo simulations show that CD with particle density distributions , where n depends on the distance R and near the star, might account for the near-UV spectroplorarimetry of Be stars. CD with enhanced scale heights could explain the [ Hα, ] 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.