1996A&A...307..184C

From the energy distributions of the Be stars HR 7739 and et al. Cen, the excess fluxes at IR wavelengths, up to 60 microns, are derived. Both stars have very peculiar far-IR energy distributions, much steeper than the bulk of the Be stars. This may indicate a different structure of the circumstellar disc, as compared to other Be stars. We attribute this difference to the fact that both stars became active as Be stars shortly before the IRAS far-IR data were taken. The structure of the circumstellar disc of each of these two Be stars is discussed and is compared with the disc structure of other Be stars. The excess IR flux is interpreted in terms of free-free and free-bound radiation from a circumstellar disc surrounding the Be star. The disc has a radius R_disc and a density distribution ρ(r)=ρ₀(r/R_*)^–n. The first model we use is the cone model, it has a diverging geometry characterized by an opening angle θ. Previous model calculations made use only of pole-on models. Since et al. Cen is a star that is seen close to edge-on, we reformulate the cone model in terms of an edge-on model. For et al. Cen we also used a slab geometry (`slab model'). The effects of the various disc parameters (e.g. density, radius, temperature and opening angle) on the shape and size of the IR excess are discussed. Comparing the pole-on and edge-on cases we find that both the size of the IR excess (due to the change in the effective radiating surface) and the position of the turnover points (caused by line-of-sight effects) are different. In the pole-on case the opening angle has an effect on the position of the turnover points, whereas the radius of the disc has not. In the edge-on case the opening angle does not have an effect on the position of these points, whereas the disc radius does. When comparing the theoretical IR excesses with observations, we find that a wide range of disc parameters is consistent with the observations. For et al. Cen we computed theoretical Hα profiles, using the range in parameters we found. From comparison with quasi-simultaneously observed Hα profiles, we find that et al. Cen is most likely surrounded by a small, flat, slab-like disc, rather than by a cone-shaped disc. HR 7739 is also most likely surrounded by a small disc, but on the basis of our observations we cannot favour either the cone, or the slab model.