Silicate features in Galactic and extragalactic post-AGB discs

Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, 3001, Leuven, Belgium; Max Planck Institut für Astronomie, 69117, Heidelberg, Germany; SUPA, School of Physics and Astronomy, University of St Andrews, KY16 9SS, North Haugh, St Andrews, Fife, UK; Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, Alan Turing Building, The University of Manchester, M13 9PL, Manchester, UK; Department of Physics and Astronomy, University College London, WC1E 6BT, London, UK; Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box, 23-141, Taiwan; Department of Physics and Astronomy, Iowa State University, A313E, 50010, Zaffarano, Ames, IA, USA; Space Telescope Science Institute, 21218, Baltimore, MD, USA; Department of Astronomy, Cornell University, 14853, Ithaca, NY, USA; Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA, Leiden, The Netherlands
Astronomy and Astrophysics (Impact Factor: 5.08). 09/2011; 533. DOI: 10.1051/0004-6361/201117364

ABSTRACT Aims. In this paper we study the Spitzer and TIMMI2 infrared spectra of post-AGB disc sources, both in the Galaxy and the LMC. Using the observed infrared spectra we determine the mineralogy and dust parameters of the discs, and look for possible differences between the Galactic and extragalactic sources. Methods. Modelling the full spectral range observed allows us to determine the dust species present in the disc and different physical parameters such as grain sizes, dust abundance ratios, and the dust and continuum temperatures. Results. We find that all the discs are dominated by emission features of crystalline and amorphous silicate dust. Only a few sample sources show features due to CO 2 gas or carbonaceous molecules such as PAHs and C 60 fullerenes. Our analysis shows that dust grain processing in these discs is strong, resulting in large average grain sizes and a very high crystallinity fraction. However, we do not find any correlations between the derived dust parameters and properties of the central source. There also does not seem to be a noticeable difference between the mineralogy of the Galactic and LMC sources. Even though the observed spectra are very similar to those of protoplanetary discs around young stars, showing similar mineralogy and strong grain processing, we do find evidence for differences in the physical and chemical processes of the dust processing.

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    ABSTRACT: We report barium isotopic measurements in 12 large (7–58 μm) stardust silicon carbide grains recovered from the Murchison carbonaceous chondrite. The C-, N-, and Si-isotopic compositions indicate that all 12 grains belong to the mainstream population and, as such, are interpreted to have condensed in the outflows of low-mass carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. Barium isotopic analyses were carried out on the Sensitive High Resolution Ion Microprobe – Reverse Geometry (SHRIMP-RG) with combined high mass resolution and energy filtering to eliminate isobaric interferences from molecular ions. Contrary to previous measurements in small (<5 μm) mainstream grains, the analyzed large SiC grains do not show the classical s-process enrichment, having near solar Ba isotopic compositions. While contamination with solar material is a common explanation for the lack of large isotopic anomalies in stardust SiC grains, particularly for these large grains which have low trace element abundances, our results are consistent with previous observations that Ba isotopic ratios are dependent on grain size. We have compared the SiC data with theoretical predictions of the evolution of Ba isotopic ratios in the envelopes of low-mass AGB stars with a range of stellar masses and metallicities. The Ba isotopic measurements obtained for large SiC grains from the LS + LU fractions are consistent with grain condensation in the envelope of very low-mass AGB stars (1.25 M⊙) with close-to-solar metallicity, which suggests that conditions for growth of large SiC might be more favorable in very low-mass AGB stars during the early C-rich stages of AGB evolution or in stable structures around AGB stars whose evolution was cut short due to binary interaction, before the AGB envelope had already been largely enriched with the products of s-process nucleosynthesis.
    Geochimica et Cosmochimica Acta 11/2013; 120:628-647. · 3.88 Impact Factor

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