Combined Faculties for the Natural Sciences and for Mathematics of the Ruperto-Carola University of Heidelberg, Germany for the degree of Doctor of Natural Sciences

Asymptotic Giant Branch (AGB) stars are the main distributors of dust into the interstellar medium due to their high mass loss rates in combination with an effective dust condensation. It is therefore important to understand the dust formation process and sequence in their extended atmosphere. The wind of these stars is driven by strong stellar pulsation in combination with radiation pressure on dust. High-resolution mid-IR interferometry is sensitive to the structure of the stellar atmosphere, consisting of the continuum photosphere and overlying molecular layers, as well as to the properties of the dust shell. This work studies the location of molecular layers and newly formed dust as a function of pulsation cycle and chemistry, as well as tries to identify molecules and dust species which cause the diameter of the star to vary across the N-band spectral domain (8 − 13 μm). Mid-IR interferometric data of the oxygen-rich AGB stars R Aql, R Aqr, R Hya and W Hya, and the carbon rich AGB star V Hya were obtained with MIDI/VLTI between April 2007 and September 2009, covering several pulsation cycles. The spectrally dispersed visibility data are modeled by fitting a fully limb-darkened disk in order to analyze the molecular layers, and by fitting a Gaussian in order to constrain the extension of the dust shell. Because uv-coverage was sufficient for R Hya and W Hya, asymmetries could be studied with an elliptical fully limb-darkened disk. The angular diameters of all oxygen-rich stars in the sample appear to be about two times larger in the mid-IR than their photospheric diameters estimated from the near-IR. The overall larger diameter in the mid-IR originates from a warm optically thick molecular layer of H2O, and a detected gradual increase longword of 10 μm can be attributed to the contribution of a spatially resolved, optically thin, close corundum (Al2O3) dust shell. A significant contribution of SiO shortward of 10 μm cannot be ruled out for R Aqr. The circumstellar silicate dust shells of all oxygen-rich stars are found to be very extended except for R Aqr. For the carbon-rich star V Hya, it can only be concluded that amorphous carbon and SiC dust is already present close to the star. The observed angular diameters are smaller at visual minimum than at visual maximum with peakto-peak variations on the oder of 20% to 30% except for W Hya for which the variation is 6%. The diameter periodicity can be explained with the phase-dependent water vapor and corundum dust presence and its temperature sensitivity. Since this variation traces only the location of constituents which are not relevant for the wind formation, no firm conclusions can be drawn concerning the mass loss mechanism. One can only speculate that more dust forms at visual minimum. Cycle-to-cycle variations of the layer traced with MIDI are lower than intracycle variations, and are on the order of 6%. R Hya does not show any deviations from circular symmetry, while an asymmetry of the extended atmosphere of W Hya can be confirmed. These observations of a larger sample of stars than available before confirm previous results, and emphasize the need for dynamic stellar model atmospheres with consistently included dust formation close to the star. It can also be concluded that interferometric observations in the N-band are an irreplaceable tool to resolve close stellar structures and to search for atmospheric constituents.