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We present 48 Herschel/PACS spectra of evolved stars in the wavelength rangeof 67-72 $\mu$m. This wavelength range covers the 69 $\mu$m band of crystallineolivine ($\text{Mg}_{2-2x}\text{Fe}_{(2x)}\text{SiO}_{4}$). The width andwavelength position of this band are sensitive to the temperature andcomposition of the crystalline olivine. Our sample covers a wide range ofobjects: from high mass-loss rate AGB stars (OH/IR stars, $\dot M \ge 10^{-5}$M$_\odot$/yr), through post-AGB stars with and without circumbinary disks, toplanetary nebulae and even a few massive evolved stars. The goal of this studyis to exploit the spectral properties of the 69 $\mu$m band to determine thecomposition and temperature of the crystalline olivine. Since the objects covera range of evolutionary phases, we study the physical and chemical propertiesin this range of physical environments. We fit the 69 $\mu$m band and use itswidth and position to probe the composition and temperature of the crystallineolivine. For 27 sources in the sample, we detected the 69 $\mu$m band ofcrystalline olivine ($\text{Mg}_{(2-2x)}\text{Fe}_{(2x)}\text{SiO}_{4}$). The69 $\mu$m band shows that all the sources produce pure forsterite grainscontaining no iron in their lattice structure. The temperature of thecrystalline olivine as indicated by the 69 $\mu$m band, shows that on averagethe temperature of the crystalline olivine is highest in the group of OH/IRstars and the post-AGB stars with confirmed Keplerian disks. The temperature islower for the other post-AGB stars and lowest for the planetary nebulae. Acouple of the detected 69 $\mu$m bands are broader than those of puremagnesium-rich crystalline olivine, which we show can be due to a temperaturegradient in the circumstellar environment of these stars. continued...

Herschel/PACS observations of the 69 $\mu m$ band of crystalline olivine around evolved stars