Micron-sized forsterite grains in the pre-planetary nebula of IRAS 17150−3224: Searching for clues to the mysterious evolution of massive AGB stars

We study the grain properties and location of the forsterite crystals in the circumstellar environment of the pre-planetary nebula (PPN) IRAS 17150-3224 in order to learn more about the as yet poorly understood evolutionary phase prior to the PPN. We use the best-fit model for IRAS 17150-3224 of Meixner et al. and add forsterite to this model. We investigate different spatial distributions and grain sizes of the forsterite crystals in the circumstellar environment. We compare the spectral bands of forsterite in the mid-infrared and at 69 mu m in radiative transport models to those in ISOSWS and Herschel/PACS observations. We can reproduce the non-detection of the mid-infrared bands and the detection of the 69 mu m feature with models where the forsterite is distributed in the whole outflow, in the super-wind region, or in the AGB-wind region emitted previous to the super-wind, but we cannot discriminate between these three models. Among the mechanisms explored is a possible selection effect due to radiation pressure based on photon scattering on micron-sized grains.