Spitzer detection of PAH and silicate features in post-AGB stars and young Planetary Nebulae

摘要

We have observed a small sample of hot post-AGB stars with the InfraRed ArrayCamera (IRAC) and the InfraRed Spectrograph (IRS) on-board the Spitzer SpaceTelescope. The stars were selected from the literature on the basis of theirfar-Infrared excess (i.e., post-AGB candidates) and B spectral type (i.e.,close to the ionization of the envelope). The combination of our IRACobservations with 2MASS and IRAS catalog data, along with previous radioobservations in the cm range (where available) allowed us to model the SEDs ofour targets and find that in almost all of them at least two shells of dust atdifferent temperatures must be present, the hot dust component ranging up to1000 K. In several targets grains larger than 1 micron are needed to match thefar-IR data points. In particular, in IRAS 17423-1755 grains up to 100 micronmust be introduced to match the emission in the mm range. We obtained IRS spectra to identify the chemistry of the envelopes and foundthat more than 1/3 of the sources in our sample have mixed chemistry, showingboth mid-IR bands attributed to Polycyclic Aromatic Hydrocarbons (PAH) andsilicate features. The analysis of the PAH features indicates that thesemolecules are located in the outflows, far away from the central stars. Weconsider the larger than expected percentage of mixed-chemistry targets as aselection bias towards stars with a disk or torus around them. Our resultsstrengthen the current picture of mixed chemistry being due to the spatialsegregation of different dust populations in the envelopes.