Herschel/PACS spectra of 48 evolved stars

We fit the 69um 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 69um band of crystalline olivine (Mg_2-2x_Fe_(2x)_SiO_4_). The 69um 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 69um 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 69um 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 69um 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 (~600K), moderately warm (~200K), or cold (~120K), respectively.