IRAS 22272+5435 - A source with 30 and 21 μm features

Abstract

Recent observations have shown that IRAS 22272+ 5435, a post-AGB object with a cool central star, is one of the few sources known which displays strong 21 and 30 μm features. We present a detailed analysis of the spectral energy distribution and discuss the possible nature of the dust grains in the circumstellar envelope around this source. We find that grains of pure magnesium sulfides could be responsible for the observed 30 μm feature provided they have a high enough temperature and an appropriate broad distribution of shapes inside the shell. The absorption properties of such grains at optical and ultraviolet wavelengths are unknown, so at the present time we are unable to estimate how high a temperature they would have in the circumstellar shell; laboratory measurements of the the optical properties are needed. The 30 μm feature cannot be produced by mantles of magnesium sulfide covering an amorphous carbon core as such grains are shown to have two maxima in their absorption cross sections around a minimum at about 30 μm. Since the material(s) responsible for 21 and 30 μm features is (are) still unknown we constructed an empirical opacity function which provides a reasonable fit to the energy distribution at far-infrared wavelengths. We also show that small particles (with radii smaller than 10 Å) having properties very similar to those of PAH molecules are an indispensable part of dust around this source. For these small grains the quantum treatment of heating and cooling processes is important. Assuming that the central star of IRAS 22272+5435 is burning H quiescently in a shell, the parameters of the best fit to the energy distribution of this source indicate a stellar core mass 0.60 M ⊙ which is presently evolving with no (or with very little) mass loss.