Radiative transfer in a floating stratus irradiated by a luminous source - I. Basics and a plane flat source

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Abstract

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Radiative transfer in a geometrically thin stratus (i.e. a sheet-like gaseous cloud with finite optical depth), floating above a luminous source such as an accretion disc, is examined under the non-grey frequency-dependent treatment. Emergent intensity and other radiative quantities are analytically obtained under the Eddington approximation for two typical cases: purely scattering and local thermodynamic equilibrium (LTE). In the purely scattering case without emission, the mean intensity is a linear function of the optical depth, the flux is constant, and the emergent intensity has angle dependency as a result of scattering. In this case, the emergent spectra have the same profile as those of the central source. In the LTE case, however, there appears to be a reprocessed thermal component in addition to a scattered one. As a result, the behaviour of the mean intensity and flux depends on the fraction of the thermal component. The emergent intensity shows the usual limb-darkening or limb-brightening effect, as well as the scattering effect. In this case, the emergent spectra have thermal and scattering components, and show complicated profiles. We also show illustrative examples of a stratus floating over a plane flat source: observational images and modified spectra due to reprocessing.

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