Interferometric studies of AGB and post-AGB stars

Several classes of evolved stars contain highly luminous objects, which, more often than not, are characterized by strong mass loss. Two fundamental properties that lie at the basis of their outer appearance and internal evolution, as well as that of the galaxies in which they are embedded. A thorough physical understanding of their evolution, and in particular the feedback they give on their surroundings is thus important, but still lacking.Most stars are found to gravitationally interact with a (close) companion in a stable orbital configuration. If close enough, this companion has a profound impact on the mass loss process, the resulting constitution of the circumstellar environment, and the evolutionary outcome of the system as a whole. Many observational classes of objects in which there is at least one evolved intermediate-mass star exist, but in this thesis only results for two of them are presented: single AGB stars and binary post-AGB stars.Large-amplitude pulsating AGB stars are in the phase in which most of the stellar envelope is being expelled, of which the physical mechanism is believed to be pulsation-enhanced dust-driven mass loss. The most advanced theoretical models are however not capable of simulating outflows of therequired mass-loss rate from first principles. In this thesis, such models are confronted with a time series of high-angular resolution observations withthe aim of identifying the key shortcomings in the input physics.The existence of circumbinary disks around many post-AGB stars, and the correlation with the presence of an unevolved low-mass star in a ~1 AUorbit, is now well established. The formation and outcome of these passive,puffed-up disks is badly understood, as is the position/impact of these systems in the grand scheme of binary evolution. Do they have a role to playin the formation of planetary nebulae, or do they evolve such that a quietdeath of the post-AGB star results? Answers to these questions can only come from a better characterization of the close circumstellar environment for a larger sample of these objects, which requires, due to their distance, the application of multiwavelength interferometric techniques. Here, one of the prototypes of the class is studied in depth, and shown to be not as well understood as previously assumed.Finally, some projects related to symbiotic binaries were initiated, but not yet finalised, during this PhD work. The goal of these projects is to observationally test one of the most basic and widely used theoretical concepts in binary star research: the Roche model. On theoretical grounds it has been proposed that the radiation force needs to be included in the Roche formalism for certain binaries involving a luminous, low-gravity component. In particular symbiotic binaries, consisting of a red giant and a white dwarf, have been proposed as good candidates for which this extension of the Roche modelmight be important. Interferometry seems to be the ideal tool to observationally test this hypothesis.status: publishe