We present 48 Herschel/PACS spectra of evolved stars in the wavelength range of 67−72 μm. This wavelength range covers the 69 μm band of crystalline olivine (Mg_2−2xFe_(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, Ṁ ≥ 10^-5 M_⊙/yr), through post-AGB stars with and without circumbinary disks, to planetary nebulae and even a few massive evolved stars.

The goal of this study is to exploit the spectral properties of the 69 μm 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.

We fit the 69 μm band and use its width and position to probe the composition and temperature of the crystalline olivine.

For 27 sources in the sample, we detected the 69 μm band of crystalline olivine (Mg_{(2 − 2x)}Fe_(2x)SiO₄). The 69 μm 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 μm 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 μm 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 μm 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.