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

摘要

Aims. 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. Methods. We use the best-fit model for IRAS 17150?3224 of Meixner et al. (2002, ApJ, 571, 936) 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 μm in radiative transport models to those in ISO-SWS and Herschel/ PACS observations. Results. We can reproduce the non-detection of the mid-infrared bands and the detection of the 69 μm feature with models where the forsterite is distributed in the whole outflow, in the superwind region, or in the AGB-wind region emitted previous to the superwind, but we cannot discriminate between these three models. To reproduce the observed spectral bands with these three models, the forsterite crystals need to be dominated by a grain size population of 2 μm up to 6 μm. We also tested models where the forsterite is located in a torus region or where it is concentrated in the equatorial plane, in a disk-like fashion. These models show either absorption features that are too strong or a 69 μm band that is too weak, respectively, so we exclude these cases. We observe a blue shoulder on the 69 μm band that cannot be explained by forsterite and we suggest a possible population of micron-sized ortho-enstatite grains. We hypothesise that the large forsterite crystals were formed after the superwind phase of IRAS 17150?3224, where the star developed an as yet unknown hyperwind with an extremely high mass-loss rate (?10-3M⊙/yr). The high densities of such a hyperwind could be responsible for the efficient grain growth of both amorphous and crystalline dust in the outflow. Several mechanisms are discussed that might explain the lower-limit of ～2 μm found for the forsterite grains, but none are satisfactory. Among the mechanisms explored is a possible selection effect due to radiation pressure based on photon scattering on micron-sized grains.