A model reflection nebula in the far Infrared

Abstract

We have considered a model of a reflection nebula with the star within a homogeneous plane-parallel slab of 1 pc thickness. The model is intended to imitate the Merope reflection nebula. The illuminating star is located at 0.35 pc from the front surface of the nebula. The size distribution of the grains has been incorporated. Using the radiation balance equation for input and output powers, we have calculated the grain temperature as a function of size, offset angle, and position of each grain within the nebula. These results have been used to calculate the expected infrared fluxes in the wavelength range 1-100 m with the offset angle as parameter. The results for ice, graphite, and silicate grains show some interestingly distinguishable structures in the flux versus wavelength curves. A rough estimate for the highest flux turns out to be F (5 × 10-^12±1)n_H W m^-2 µm^-1 sr^-1 corresponding to offset angle Ψ = 0°,n_H being the hydrogen number density. The proportionality to n_H occurs because we have assumed a constant ratio of the grain number density to nH. It is not unreasonable to expect observable infrared fluxes in the appropriate wavelength range in the direction toward the Merope reflection nebula. In particular, from the observational point of view, we predict that the best results on the strongest infrared radiation from the Merope reflection nebula can occur near about 40 µm.