1 Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
2 Astronomy and Astrophysics Research Group, Department of Physics and Astrophysics, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
3 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
4 Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
5 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
6 Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium
7 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
8 Herschel Science Centre, European Space Astronomy Centre, Research and Scientific Support Department of ESA, Villafranca del Castillo, 28080 Madrid, Spain
9 University of Vienna, Department of Astrophysics, Türkenschanzstrasze 17, 1180 Wien, Asutria
10 Department of Physics and Astronomy, MS 6900, University of Denver, Denver, CO 80208, USA
11 Flemish Institute for Technological Research (VITO), Boeretang 200, 2400 Mol, Belgium (present address)
12 AlbaNova University Centre, Stockholm University, Department of Astronomy, 106 91 Stockholm, Sweden
13 Stockholm University Astrobiology Centre, 106 91 Stockholm, Sweden
Received: 26 August 2013
Accepted: 10 February 2014
Context. 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 (Mg2−2xFe(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.
Aims. 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.
Methods. We fit the 69 μm 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 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.
Key words: stars: AGB and post-AGB / circumstellar matter / stars: winds, outflows / stars: evolution / dust, extinction
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Appendices are available in electronic form at http://www.aanda.org
Reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/565/A109
© ESO, 2014