The pseudo-photosphere model for the continuum emission of gaseous discs

We investigate the continuum emission of viscous decretion discs around Be stars in this paper. The results obtained from non-LTE (local thermodynamic equilibrium) radiative transfer models show two regimes in the disc surface brightness profile: an inner optically thick region, which behaves as a pseudo-photosphere with a wavelength-dependent size, and an optically thin tenuous outer part, which contributes with about a third of the total flux. The isophotal shape of the surface brightness is well described by elliptical contours with an axial ratio b/a = cos i for inclinations i < 75 degrees. Based on these properties, a semi-analytical model was developed to describe the continuum emission of gaseous discs. It provides fluxes and spectral slopes at the infrared within an accuracy of 10 and 5 per cent, respectively, when compared to the numerical results. The model indicates that the infrared spectral slope is mainly determined by both the density radial slope and the disc flaring exponent, being practically independent of disc inclination and base density. As a first application, the density structure of 15 Be stars was investigated, based on the infrared flux excess, and the results compared to previous determinations in the literature. Our results indicate that the decretion rates are in the range of 10(-12)-10(-9) M-circle dot yr(-1), which is at least two orders of magnitude smaller than the previous outflowing disc model predictions. (AU)