Radiation thermo-chemical models of protoplanetary discs - III. Impact of inner rims on spectral energy distributions

Abstract

We study the hydrostatic density structure of the inner disc rim around HerbigAe stars using the thermo-chemical hydrostatic code ProDiMo. We compare the Spectral Energy Distributions (SEDs) and images from our hydrostatic disc models to that from prescribed density structure discs. The 2D continuum radiative transfer in ProDiMo includes isotropic scattering. The dust temperature is set by the condition of radiative equilibrium. In the thermal-decoupled case the gas temperature is governed by the balance between various heating and cooling processes. The gas and dust interact thermally via photoelectrons, radiatively, and via gas accommodation on grain surfaces. As a result, the gas is much hotter than in the thermo-coupled case, where the gas and dust temperatures are equal, reaching a few thousands K in the upper disc layers and making the inner rim higher. A physically motivated density drop at the inner radius ("soft-edge") results in rounded inner rims, which appear ring-like in near-infrared images. The combination of lower gravity pull and hot gas beyond ~1 AU results in a disc atmosphere that reaches a height over radius ratio z/r of 0.1 while this ratio is 0.2 only in the thermo-coupled case. This puffed-up disc atmosphere intercepts larger amount of stellar radiation, which translates into enhanced continuum emission in the 3- 30 micron wavelength region from hotter grains at ~500 K. We also consider the effect of disc mass and grain size distribution on the SEDs self-consistently feeding those quantities back into the gas temperature, chemistry, and hydrostatic equilibrium computation.