Computer Science
Scientific paper
Oct 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt..........h&link_type=abstract
PhDT Thesis, University of Amsterdam, 2002.
Computer Science
15
Agb And Post-Agb Stars, Carbon Stars, Circumstellar Matter, Mass-Loss, Infrared Spectroscopy
Scientific paper
We study the dust in the circumstellar environments of evolved C-rich stars in order to derive the composition of the dust and the mass loss history of evolved stars. We examine the dust using infrared (IR) data obtained with ESA's Infrared Space Observatory. The main advantage of observing in the mid-IR is that we are able to directly determine the dust composition from the vibrational fingerprints of the dust species since these fingerprints are all located in the mid-IR wavelength range.
In my thesis I describe the discovery of some new solid-state components in these environments, one of those is the first ever evidence for iron sulfide grains outside our solar system. We also discovered the mysterious ``21'' micron feature in two planetary nebulae. Our discovery shows that the material that gives rise to the feature is more stable than previously believed.
We present a careful test of the magnesium sulfide identification for the ``30'' micron feature. We find that the differences in shape of this feature between the various sources can be explained by differences in temperature of the magnesium sulfide grains and the grain shape distribution.
We study the emission of polycyclic aromatic hydrocarbon molecules (PAHs) in the surrounding of HII regions, Herbig AeBe and carbon-rich planetary nebulae. From a detailed comparison with newly available laboratory measurements of PAHs we find that the PAHs around C-rich planetary nebulae contain 100-200 C-atoms and have long straight edges while the PAHs in the surroundings of low- and high-mass young stellar objects have more ragged edges.
We constructed a detailed radiative transfer model for the post-AGB star HD 56126 in which we take, for the first time, the detailed composition of the circumstellar envelope into account. We find that this star has undergone a short (<1500 yr), strong (>10^-4 M_sun/yr) mass loss burst, during which the entire envelope was lost. We also find that the 6-9 and 11-17 micron plateau emission features cannot be explained by hydrogenated amorphous carbon grains in radiative equilibrium, because they do not reach a high enough temperature to emit efficiently at 6 micron.
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