Circumstellar grains: radiation pressure and temperature distribution

The ratioβ of the radiation pressure force to the gravitational attraction is calculated for circumstellar grains. Eleven stars of various spectral type are considered and their spectral flux compiled from available data sometimes supplemented by appropriate models. The materials composing the grains are a silicate (obsidian), organic materials (ice tholin, poly-HCN), graphite and glassy carbon. The radius of the grain extends from 0.005 to 25 μm. β exceeds 1 for submicronic grains around the hottest stars with the exception of the obsidian grains. As far as the coldest stars are concerned, the results are not so clear and depend sharply on the illuminating stars as well as the nature of the material of the solid particles. Their temperatures are also studied and two examples are given for obsidian and graphite grains of radii 0.01, 0.1, 1 and 10 μm. Our results are of interest for thin stellar shells such as exo-zodiacal clouds and for the inner region of dense shells where multiple scattering effects are not taking place.