Spitzer Detection of Polycyclic Aromatic Hydrocarbons and Silicate Features in Post-AGB Stars and Young Planetary Nebulae

We have observed a small sample of hot post-asymptotic giant branch (AGB) stars with the Infrared Array Camera (IRAC) and the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The stars were selected from the literature on the basis of their far-infrared (IR) excess (i.e., post-AGB c...

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Bibliographic Details
Published in:The Astrophysical journal 2009-09, Vol.703 (1), p.585-600
Main Authors: Cerrigone, Luciano, Hora, Joseph L, Umana, Grazia, Trigilio, Corrado
Format: Article
Language:English
Subjects:
AROMATICS
Astronomy
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
DUSTS
Earth, ocean, space
ENERGY SPECTRA
Exact sciences and technology
HYDROCARBONS
IONIZATION
NEBULAE
ORGANIC COMPOUNDS
OXYGEN COMPOUNDS
PLANETARY NEBULAE
POLYCYCLIC AROMATIC HYDROCARBONS
SEGREGATION
SILICATES
SILICON COMPOUNDS
SPECTRA
STARS
TELESCOPES
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Summary:We have observed a small sample of hot post-asymptotic giant branch (AGB) stars with the Infrared Array Camera (IRAC) and the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The stars were selected from the literature on the basis of their far-infrared (IR) excess (i.e., post-AGB candidates) and B spectral type (i.e., close to the ionization of the envelope). The combination of our IRAC observations with Two Micron All Sky Survey and IRAS catalog data, along with previous radio observations in the cm range (where available) allowed us to model the spectral energy distributions of our targets and find that in almost all of them at least two shells of dust at different temperatures must be present, the hot dust component ranging up to 103 K. In several targets, grains larger than 1 Delta *mm are needed to match the far-IR data points. In particular, in IRAS 17423-1755 grains up to 100 Delta *mm must be introduced to match the emission in the millimeter range. We obtained IRS spectra to identify the chemistry of the envelopes and found that more than one-third of the sources in our sample have mixed chemistry, showing both mid-IR bands attributed to polycyclic aromatic hydrocarbons (PAHs) and silicate features. The analysis of the PAH features indicates that these molecules are located in the outflows, far away from the central stars. We consider the larger than expected percentage of mixed-chemistry targets as a selection bias toward stars with a disk or torus around them. Our results strengthen the current picture of mixed chemistry being due to the spatial segregation of different dust populations in the envelopes.
ISSN:0004-637X
1538-4357
DOI:10.1088/0004-637X/703/1/585