Herschel/PACS observations of the 69 μm band of crystalline olivine around evolved stars

Context. We present 48 Herschel/PACS spectra of evolved stars in the wavelength range of 67 72 m. This wavelength range covers the 69 m band of crystalline olivine (Mg 2 2x Fe(2x)SiO4). The width and wavelength position of this band are sensitive to the temperature and composition of the crystalline olivine. Our sample covers a wide range of objects: from high mass-loss rate AGB stars (OH/IR stars, ˙ M 10 5 M /yr), through post-AGB stars with and without circumbinary disks, to planetary nebulae and even a few massive evolved stars. Aims. The goal of this study is to exploit the spectral properties of the 69 m band to determine the composition and temperature of the crystalline olivine. Since the objects cover a range of evolutionary phases, we study the physical and chemical properties in this range of physical environments. Methods. We fit the 69 m band and use its width and position to probe the composition and temperature of the crystalline olivine. Results. For 27 sources in the sample, we detected the 69 m band of crystalline olivine (Mg(2 2x)Fe(2x)SiO4). The 69 m band shows that all the sources produce pure forsterite grains containing no iron in their lattice structure. The temperature of the crystalline olivine as indicated by the 69 m band, shows that on average the temperature of the crystalline olivine is highest in the group of OH/IR stars and the post-AGB stars with confirmed Keplerian disks. The temperature is lower for the other post-AGB stars and lowest for the planetary nebulae. A couple of the detected 69 m bands are broader than those of pure magnesium-rich crystalline olivine, which we show can be due to a temperature gradient in the circumstellar environment of these stars. The disk sources in our sample with crystalline olivine are very diverse. They show either no 69 m band, a moderately strong band, or a very strong band, together with a temperature for the crystalline olivine in their disk that is either very warm ( 600 K), moderately warm ( 200 K), or cold ( 120 K), respectively.